vji 


HEINRI.CH  RIES 

.      JD 


WORKS  OF  PROF.  HEINRICH  RIES 

PUBLISHED    BY 

JOHN  WILEY  &  SONS,  INC. 


Building  Stones  and  Clay  Products 

A  Handbook  for  Architects.  8vo,  xiii  +  415  pages,  59 
plates,  including  full-page  half-tones  and  maps,  20 
figures  in  the  text.  Cloth,  $3.00  net. 

Clays:    Their  Occurrence,  Properties  and  Uses 

With  Especial  Reference  to  Those  of  the  United  States. 
Second  Edition,  Revised.  8vo,  xix+554  pages,  112 
figures,  44  plates.  Cloth,  $5.00  net. 


BY  RIES  AND  LEIGHTON 

>ry   of 
States 

By  Prof.  Heinrich  Ries,  and  Henry  Leighton,  Professor 
of  Economic  Geology,  University  of  Pittsburgh.  8vo, 
viii  +  270  pages,  illustrated.  Cloth,  $2.50  net. 


BY  RIES  AND  WATSON 

Engineering  Geology 

By  Prof.  Heinrich  Ries,  and  Thomas  L.  Watson,  Pro- 
fessor of  Economic  Geology,  University  of  Virginia, 
and  State  Geologist  of  V  irginia.  8vo,  xxvi  +  672  pages, 
225  figures  in  the  text,  and  104  plates,  comprising  175 
figures.  Cloth,  $4.00  net. 


PUBLISHED    BY 

THE   MACMILLAN   CO. 


Economic     Geology,    with    special    reference    to    the 
United  States. 

8vo,  xxxii  +  589  pages,  237  figures,  56  plates.    $3.50  net. 


ENGINEERING 
GEOLOGY 


BY 

HEINRICH   RIES,   PH.  D. 

PROFESSOR   OF  ECONOMIC   GEOLOGY   IN   CORNELL  UNIVERSITY 
AND 

THOMAS   L.   WATSON,    PH.  D. 

PROFESSOR  OF  ECONOMIC   GEOLOGY   IN  THE   UNIVERSITY   OF   VIRGINIA 
AND   STATE    GEOLOGIST   OF  VIRGINIA 


FIRST  EDITION 

SECOND    THOUSAND 


XEW  YORK 

JOHN  WILEY  &  SONS,  INC. 

LOKDOX;    CHAPMAN   &   HALL,    LIMITED 

1914 


COPYRIGHT,  1914, 

BY 
HEIMRICH  RIES  AND  THOMAS  L.  WATSON 


vV 


Stanhope  iprcss 

F.    H.GILSON   COMPANY 
BOSTON,  U.S.A. 


PREFACE 


FOR  some  years  the  authors  of  this  book  have  been  giving  to  students 
of  civil  engineering  in  their  respective  universities  a  special  course  in 
geology  as  applied  to  engineering.  The  method  followed  by  them  has 
met  with  much  success,  and  since  the  plan  adopted  has  gradually  been 
put  into  operation  at  other  universities  it  has  encouraged  them  to 
believe  that  it  might  be  of  service  to  others  to  prepare  the  present  work. 

There  are  probably  but  few  people  of  observation  and  practical 
experience  who  doubt  the  value  of  proper  geological  training  for  the 
engineer,  since  he  must  be  prepared  to  meet  and  often  to  solve  many 
problems  which  involve  geological  principles.  For  such  knowledge  it 
is  necessary  that  the  engineer  should  have  adequate  training  in  at  least 
those  fundamental  principles  of  geology  which  relate  to  engineering 
problems. 

Among  the  important  questions  which  the  engineer  has  to  consider  are 
the  character  of  the  common  rocks  in  their  use  for  building  stone  and 
road  material;  the  structure  of  rocks  in  relation  to  tunneling  opera- 
tions, dam  and  reservoir  foundations,  landslides,  etc.;  the  geological 
conditions  affecting  and  controlling  underground  water  supplies;  the 
relation  of  soils  to  sewage  disposal  and  water  purification,  etc.  More- 
over, some  familiarity  with  such  materials  as  fuels  (coal,  oil  and  gas), 
clays,  cements,  etc.,  is  also  necessary. 

There  may  be  difference  of  opinion  as  to  whether  the  civil  engineer 
should  be  grounded  in  abstract  geological  principles  and  afterwards 
allowed  to  apply  them  in  the  field,  or  whether  the  exposition  of  the 
necessary  principles  should  be  illustrated  in  each  instance  by  actual 
cases,  which  show  the  application  of  the  principle.  The  first  method 
does  not  usually  appeal  to  those  who  have  had  much  practical  experience, 
nor  does  it  find  much  favor  with  the  engineering  student;  moreover,  it 
can  hardly  be  considered  successful  from  the  pedagogic  standpoint. 

The  authors  have  attempted  to  emphasize  throughout  the  book  the 
practical  application  of  the  topics  treated  to  engineering  work,  because 
hitherto  in  many  engineering  courses  of  study  the  subject  of  Geology 
has  not  been  given  the  attention  which  they  think  it  should  receive 
from  both  professors  and  students. 

iii 


iv  PREFACE 

Although  this  book  is  intended  primarily  for  civil  engineers,  it  is 
hoped  that  it  may  be  of  use  to  others  interested  in  applied  geology. 
For  this  reason  certain  parts  of  the  work  contain  more  detail  than  may 
seem  necessary  for  the  actual  requirements  of  the  civil  engineer,  but 
any  one  using  it  for  purposes  of  instruction  will  find  it  convenient  to 
eliminate  as  much  or  as  little  of  the  subject  matter  as  is  desired  to  meet 
the  special  requirements  of  his  course. 

For  permission  to  reproduce  illustrations  from  their  works,  the 
authors  desire  to  make  grateful  acknowledgment  to  Professor  L.  V. 
Pirsson,  for  figures  3,  6,  7,  8,  11,  12,  13,  17,  18,  25,  26,  28,  29,  30,  31,  37, 
38,  40,  from  Rocks  and  Rock  Minerals;  to  Professor  W.  E.  Ford  for 
figure  1  from  Dana's  Manual  of  Mineralogy;  and  to  Professor  E.  S. 
Dana  for  figures  2,  4,  5,  9,  10,  14,  15,  16,  19,  20,  21,  22,  23,  24,  27,  32, 
33,  34,  35,  36,  39,  41  and  42,  from  A  System  of  Mineralogy.  The  authors 
are  similarly  indebted  to  Professor  J.  S.  Grasty  for  the  photographs 
reproduced  as  plates  XCII,  XCIII,  CI  and  GUI,  and  to  the  Macmillan 
Company  for  the  loan  of  cuts  from  Ries'  Economic  Geology.  For  the 
loan  of  other  cuts  acknowledgment  is  made  under  each  illustration. 
Mr.  R.  E.  Somers  gave  much  assistance  in  the  preparation  of  the 
work. 

ITHACA,  N.  Y.,  and  CHARLOTTESVILLE,  VA. 
March  16,  1914. 


CONTENTS 


PAGE 

LIST  OF  PLATES xv 

LIST  OF  FIGURES xxi 

CHAPTER  I 

THE  ROCK-FORMING  MINERALS 1 

Introduction,  1;  Definition  of  a  mineral,  1;  Definition  of  a  crystal,  2; 
Twinning,  2;  General  physical  properties  of  rock-making  minerals,  3; 
Hardness,  3;  Cleavage,  4;  Luster,  4;  Streak,  5;  Color,  5;  Crystal  form,  5; 
Specific  gravity,  6;  Fracture,  6;  Chemical  tests,  7;  Description  of  rock- 
forming  minerals,  7;  Silicates,  7;  Anhydrous  silicates,  7;  Feldspars,  7; 
Feldspathoid  group,  12;  Nephelite,  12;  Sodalite,  12;  Mica  group,  12; 
Pyroxene  group,  15;  Amphibole  group,  17;  Garnet  group,  20;  Olivine 
group,  21;  Epidote  group,  22;  Staurolite,  23;  Tourmaline,  23;  Hydrous 
silicates,  24;  Kaolinite,  24;  Talc,  25;  Serpentine,  26;  Chlorite,  27;  Zeolite 
group,  28;  Oxides,  29;  Quartz,  29;  Corundum,  31;  Iron  ores,  31;  Mag- 
netite, 32;  Ilmenite,  32;  Hematite,  33;  Limonite,  34;  Carbonates,  35; 
Calcite,  35;  Aragonite,  36;  Dolomite,  36;  Sulphates,  37;  Gypsum,  37; 
Anhydrite,  39;  Phosphates,  39;  Apatite,  39;  Sulphides,  40;  Pyrite,  41; 
Marcasite  and  pyrrhotite,  42;  Chalcopyrite,  42;  Galena,  43;  Sphalerite, 
43;  References  on  rock-forming  minerals,  44. 

CHAPTER  H 

ROCKS,  THEIR  GENERAL  CHARACTERS,  MODE  OF  OCCURRENCE,  AND  ORIGIN.  .       46 

Introduction,  46;  Varieties  of  rocks,  47;  Igneous  rocks,  47;  Occurrence 
and  origin,  47;  Mode  of  occurrence,  48;  Intrusive  or  plutonic  rocks,  48; 
Forms  of  intrusives,  48;  Dikes,  50;  Intrusive  sheets,  51;  Laccoliths,  53; 
Necks,  54;  Stocks,  55;  Batholiths,  56;  Extrusive  or  volcanic  rocks,  56; 
Lava  flows  and  sheets,  57;  Composition  of  igneous  rocks,  59;  Chemical 
composition,  60;  Mineral  composition,  63;  Grouping  of  minerals,  64;  Order 
of  crystallization,  65;  Mineralizers,  65;  Texture  of  igneous  rocks,  66;  Kinds 
of  texture,  66;  Porous  structure,  69;  Differentiation  of  igneous  rocks,  69; 
Classification  of  igneous  rocks,  70;  Description  of  igneous  rocks,  74;  Intru- 
sive rocks,  74;  Granite,  74;  Syenite,  77;  Diorite,  78;  Gabbro,  86;  Perido- 
tite,  88;  Pyroxenite,  91;  Volcanic  or  dense  igneous  rocks,  85;  Felsite,  86; 
Basalt,  87;  Glassy  igneous  rocks,  89;  Porphyritic  igneous  rocks  (Porphyries), 
91;  Pyroclastic  or  volcanic  fragmental  rocks,  92;  Sedimentary  rocks,  93; 
Introduction,  93;  General  properties  of  sedimentary  rocks,  94;  Variation 
in  size  of  material,  94;  Texture  of  sedimentary  rocks,  94;  Cementation  of 
sedimentary  material  into  solid  rock,  95;  Quantity  of  cement,  96;  Color 
of  cement,  96;  Durability  of  cement,  96;  Structure  of  sedimentary  rocks, 


VI  CONTENTS 

PAGE 

96;  Composition  of  sedimentary  rocks,  97;  Classification  of  sedimentary 
rocks,  97;  Sedimentary  rocks  of  mechanical  origin,  99;  Breccias,  99; 
Conglomerate,  101;  Sandstone,  103;  Shale,  106;  Clays,  108;  Variation  in 
shale  and  sandstone  deposits,  108;  Wind  deposits,  109;  Loess,  109;  Sand 
deposits  (Dunes),  111;  Sand  dunes,  111;  Sediments  of  chemical  origin 
formed  from  solution,  113;  Sulphates:  gypsum  and  anhydrite,  113; 
Chlorides:  rock  salt,  114;  Siliceous  deposits,  114;  Flint,  115;  Jasper,  115; 
Geyserite,  115;  Diatomaceous  earth,  115;  Ferruginous  rocks  (Iron  ores), 
128;  Carbonate  rocks,  128;  Limestone,  128;  Phosphate  rocks,  121; 
Carbonaceous  rocks,  121;  Metamorphic  rocks,  122;  Introduction,  122; 
Agents  of  metamorphism,  122;  Chemical  composition  of  metamorphic 
rocks,  124;  Mineral  composition  of  metamorphic  rocks,  124;  Texture  and 
structure  of  metamorphic  rocks,  124;  Varieties  of  structure,  125;  Criteria 
for  the  discrimination  of  metamorphosed  igneous  and  sedimentary  rocks, 
125;  Classification  of  metamorphic  rocks,  126;  Gneiss,  127;  Crystalline 
schists,  132;  Quartzite,  135;  Slate  and  phyllite,  137;  Phyllite,  140;  Crys- 
talline limestones  and  dolomites  (Marbles),  140;  Ophicalcite,  serpentine, 
and  soapstone,  142;  Ophicalcite,  143;  Serpentine,  144;  Soapstone,  145; 
References  on  rocks,  146. 

CHAPTER  III 

STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS 147 

Introductory,  147;  Folds,  147;  Introduction,  147;  Dip,  147;  Strike, 
148;  Parts  of  folds,  148;  Kinds  of  folds,  148;  Anticlines,  148;  Synclines, 
149;  Monocline,  149;  Other  types  of  folds,  150;  Folds  modified  by 
erosion,  154;  Relation  of  folding  to  engineering  operations,  155;  Tunnel- 
ing, 155;  Quarrying,  157;  Ore  deposits,  157;  Mining,  157;  Field  observa- 
tions, 158;  Angular  measurements,  158;  Vertical  measurements,  158; 
Determination  of  thickness  of  beds,  158;  Determination  of  depth  of 
beds,  163;  Classification  of  joints,  166;  Joints  in  sedimentary  rocks,  166; 
Joints  in  igneous  rocks,  166;  Joints  in  relation  to  engineering  work,  167; 
Quarrying  operations,  167;  Rock  slides,  167;  Reservoir  construction,  167; 
Water  supply,  167;  Ore  deposits,  168;  Faults,  168;  Definition,  168; 
Significance,  168;  Fault  terms,  168;  Criteria  for  faulting,  169;  General 
classes  of  faults,  171;  Faults  in  stratified  rocks,  171;  Slip,  171;  Shift,  172; 
Throw  and  heave,  173;  Offset,  174;  Classification  of  faults  according  to 
direction  of  movement,  174;  Strike  faults,  174;  Normal  faults,  174; 
Reverse  faults,  174;  Vertical  faults,  174;  Special  classes  of  faults,  174; 
Effect  of  faults  on  the  outcrop,  177;  Topographic  effects,  177;  Geologic 
effects,  178;  Relation  between  faults  and  folds,  180;  Relation  of  faulting 
to  engineering  work,  181;  Tunneling,  181;  Aqueduct,  183;  Earthquakes, 
174;  Coal  mines,  174;  Ore  deposits,  174;  Submarine  cables,  185;  Land- 
slides, 185;  Determination  of  faults,  185;  Dip  of  fault  plane,  186;  Angle 
of  intersection  of  oblique  vertical  plane,  187;  Per  cent  and  angular  in- 
clination, 188;  Form  of  outcrop,  188;  Rock  cleavage,  190;  Definition, 
190;  Original  and  secondary,  190;  Fracture  cleavage,  191;  Flow  cleavage, 
191;  Origin  of  folds,  faults,  joints  and  cleavage,  192;  Introduction,  192; 
Cause  of  folds,  192;  Cause  of  joints,  194;  Cause  of  cleavage,  194;  Uncon- 
formity, 195;  Overlap,  196;  Inliers,  197;  Outliers,  197;  Concretions,  198; 


CONTENTS  Vll 

PAGE 

Form  and  occurrence,  198;  Origin,  199;  Material  forming  concretions,  199; 
Practical  considerations,  199;  Metamorphism  of  rocks,  199;  Introduction, 
199;  Definition,  200;  Agents  of  metamorphism,  203;  Mechanical  move- 
ments and  pressure,  203;  Solutions  (liquids  and  gases),  203;  Heat,  203; 
Zones  of  metamorphism,  204;  Kinds  of  metamorphism,  204;  Contact  or 
local  metamorphism,  206;  Introduction,  206;  Endomorphic  changes,  206; 
Exomorphic  changes,  207;  Contact  metamorphic  effects  on  different  kinds 
of  rocks,  207;  General  or  regional  metamorphism,  208;  Topographic 
and  geologic  map  and  section,  209 ;  Topographic  (base)  map,  209 ;  Geologic 
map,  209;  Conventions  and  symbols,  210;  Sections,  210;  Method  of  con- 
structing geologic  maps  and  sections,  211;  References  on  structural  fea- 
tures and  metamorphism,  213. 

CHAPTER   IV 

ROCK-WEATHERING  AND  SOILS 215 

Introduction,  215;  Importance  of  rock  weathering,  215;  Weathering 
processes,  216;  Definition  of  weathering,  216;  Mechanical  agents,  216; 
Temperature  changes,  216;  Expansion  and  contraction,  216;  Expansion 
due  to  alternate  freezing  and  thawing,  218;  Effects  of  frost  and  tempera- 
ture changes,  219;  Mechanical  abrasion,  219;  Growing  organisms,  221; 
Chemical  agents,  223;  Hydration,  225;  Dehydration,  225;  Oxidation,  227; 
Deoxidation,  228;  Carbonation,  228;  Solution,  230;  Summary  of  chemi- 
cal decay,  231;  Residual  clay  and  sand,  232;  Mineral  resistance,  234; 
Relation  of  structure  to  weathering,  234;  Weathering  of  different  rocks, 
236;  Siliceous  crystalline  (igneous)  rocks,  236;  Sedimentary  rocks,  238; 
Sandstones,  238;  Argillaceous  rocks  (shales  and  slates),  239;  Calcareous 
rocks  (limestones  and  dolomites),  239;  Gypsum,  240;  Soils,  240;  Defi- 
nition, 241;  Formation  of  soils,  241 ;  Classification  of  soils,  241;  Sedentary 
soils,  241;  Transported  soils,  242;  Composition  of  soils,  242;  Mineral 
matter,  242;  Organic  matter,  242;  Soil  areas,  242;  References  on  rock- 
weathering  and  soils,  243. 

CHAPTER  V 

SURFACE  WATERS  (RIVERS) 244 

Introduction,  244;  Stream  flow,  244;  Rainfall  and  run-off,  244;  Factors 
controlling  run-off,  244;  Ratio  of  run-off  to  rainfall,  245;  Run-off  from 
different  water  sheds,  246;  Stream  formation,  248;  Measurement  of  water, 
250;  Stream  measurement,  252;  Unit  of  measurement,  252;  Work  per- 
formed by  rivers  and  its  economic  application,  253;  Work  of  erosion,  253; 
Erosion,  253;  Corrasion,  254;  Corrosion,  254;  Factors  governing  rate  of 
erosion,  254;  Depth  of  erosion,  256;  Character  of  meandering  streams,  257; 
Shoals,  bends,  and  crossings,  259;  Scour,  259;  Eddies  and  currents,  261; 
Erosion  of  banks,  262;  Irregular  hardness  of  river  bed,  263;  Falls  and 
rapids,  264;  Potholes,  264;  Work  of  transportation,  264;  Transportation 
of  sediment,  264;  Amount  of  sediment  transported,  266;  Relation  of  size 
of  particles  to  current  velocity,  267;  Relation  of  sediment  to  cross-section 
and  slope,  268;  Change  of  shape  of  cross-section,  268;  Slope  of  streams, 
268;  Work  of  deposition,  269;  Alluvial  plains,  269;  Deltas,  271;  Struc- 
ture of  deltas,  273;  Conditions  favorable  to  delta  formation,  274;  Extent 


vili  CONTENTS 

PAGE 

of  deltas,  274;  Fossil  deltas,  275;  River  terraces,  275;  Flood-plain  ter- 
races, 275;  Outlets  of  rivers,  277;  Bars  at  mouth  of  rivers,  277;  Drainage 
forms  and  modifications,  277;  Development  of  valleys  and  tributaries,  277; 
Piracy,  278;  Young  and  old  topography,  279;  Formation  of  canyons,  280; 
Buried  channels,  280;  Floods  and  dam  foundations,  282;  Floods  and 
their  regulation,  282;  Ice  gorges,  284;  Dam  foundations,  284;  Bed-rock 
foundations,  284;  Unconsolidated  material,  285;  Composition  of  river 
water,  286;  Statement  of  analyses,  286;  Relation  of  river  water  to  rock 
formation,  288;  River  waters  of  United  States,  290;  Chlorine,  291; 
Carbonates  in  water,  292;  Sulphuric  acid  waters,  292;  References  on 

rivers,  294. 

CHAPTER  VI 

UNDERGROUND  WATERS 295 

Introduction,  295;  Sources  of  underground  water,  295;  Quantity  of 
rainfall,  295;  Disposal  of  meteoric  water,  296;  Evaporation,  297;  Run- 
off, 297;  Absorption,  297;  Ground  water,  298;  Water  table,  298;  Move- 
ment of  groundwater,  299;  Causes  of  fluctuation  of  water  table,  300; 
Natural  causes,  301;  Artificial  causes,  302;  Perched  water  tables,  303; 
Springs,  303;  Seepage  springs,  304;  Tubular  springs,  305;  Fissure 
springs,  306;  Value  of  springs  for  water  supply,  306;  Miscellaneous 
effects  of  underground  waters,  306;  Clay  slides,  306;  Dam  and  reservoir 
foundations,  307;  Limestone  sink  holes  and  caverns,  309;  Tunneling 
operations,  312;  Railway  embankments,  312;  Foundation  work,  312; 
Drainage  by  wells,  312;  Types  of  drainage,  312;  Application  of  drainage  by 
wells,  314;  Pollution  by  drainage  wells,  314;  Artesian  water,  314;  Defi- 
nition, 314;  Water  capacity  of  rocks,  315;  Amount  of  groundwater,  315; 
Rate  of  movement  of  underground  water,  317;  Requisite  conditions  of 
artesian  flow,  317;  Artesian  water  in  stratified  rocks,  318;  Sands  and 
sandstones,  318;  Limestones,  319;  Factors  affecting  artesian  water  sup- 
plies, 320;  Several  aquifers  in  same  section,  320;  Irregularities  of  artesian 
supply,  320;  Interference,  321;  Yield  of  wells,  322;  Source  of  water  in 
aquifers,  322;  Depth  of  aquifers,  322;  Artesian  water  in  glacial  drift,  322; 
Artesian  water  in  crystalline  rocks,  324;  Irregularities  in  the  behavior  of 
wells,  327;  Fluctuations  of  head,  327;  Roiliness  of  well  water,  328;  Blow- 
ing wells,  328;  Breathing  wells,  328;  Freezing  of  wells,  328;  Cause  of 
preceding  phenomena,  329;  Groundwater  provinces  of  the  United  States, 
330;  Glacial-drift  province,  331;  Weathered-rock  province,  331;  Atlantic 
Coastal  plain,  331;  Piedmont  plateau  province,  332;  Appalachian  Moun- 
tains province,  332;  Mississippi-Great  Lakes  basin,  333;  High  Plains, 
province,  334;  Rocky  Mountain  province,  335;  Great  Basin  province, 
335;  Pacific  provinces,  336;  Composition  of  groundwaters,  337;  Intro- 
duction, 337;  Relation  of  rock  material  to  dissolved  matter,  337;  Effect 
of  mineral  ingredients,  338;  Hardness,  338;  Boiler  scale,  338;  Corrosion, 
338;  Potable  water,  339;  Suspended  matter,  339;  Color,  340;  References 
on  underground  waters,  340. 

CHAPTER  VH 

LANDSLIDES  AND  THEIR  EFFECTS 342 

Definition,  342;  Classification  of  landslides,  342;  Type  I,  343;  Type  II, 


CONTENTS  ix 

PAGE 

343;  Type  III,  349;  Type  IV,  350;  Type  V,  351;  Type  VI,  351;  En- 
gineering considerations,  353;  Angle  of  rest,  353;  Angle  of  slide,  353; 
Angle  of  pull,  353;  Excavation  deformation,  354;  Factors  affecting  exca- 
vation deformations,  354;  Slopes  to  minimize  sloughing  and  deformation, 
355;  References  on  landslides,  356. 

CHAPTER  Vm 

WAVE  ACTION  AND  SHORE  CURRENTS;    THEIR  RELATION  TO  COASTS  AND 

HARBORS 358 

Introductory,  358;  Formation  of  waves,  359;  Cause  of  waves,  359; 
Depth  of  wind  disturbance,  359;  Theory  of  wave  motion,  359;  Storm 
waves,  362;  Gaillard's  observations,  362;  Undertow,  363;  Work  per- 
formed by  waves,  363;  Erosion,  363;  Vertical  range  of  wave  action,  365; 
Recession  of  coast,  365;  Wave-cut  topography,  366;  Cliff  and  terrace,  366; 
Coast  outline,  366;  Transportation  by  shore  currents,  368;  Shore  deposi- 
tion topography,  368;  Beach  and  barrier,  388;  Spits,  hooks,  and  bars,  372; 
Problems  of  harbor  and  river-mouth  improvement,  381 ;  Relation  of  wave 
and  shore  current  work  to  harbors,  381;  Case  of  Manasquan  Inlet,  383; 
Relations  of  bars  to  rivers,  385;  Improvement  of  tidal  rivers,  388;  Shore 
drift,  390;  Case  of  the  Columbia  River,  Oregon,  391;  Case  of  Mississippi 
River,  South  Pass,  392;  Conditions  along  coast  of  United  States,  393; 
References  on  waves  and  shore  currents,  394. 

CHAPTER  IX 

LAKES:  THEIR  ORIGIN  AND  RELATION  TO  ENGINEERING  WORK 396 

Definition,  396;  Relation  to  engineering  work,  396;  Types  of  lakes,  396; 
Original  consequent  lakes,  396;  Lakes  of  normal  development,  397;  Ox- 
bow lakes,  397;  Beaches  across  inlets,  397;  Sink-hole  lakes,  399;  Crustal 
movement  lakes,  399;  Lakes  due  to  accident,  399;  Drift-dam  lakes,  400; 
Landslide  lakes,  400;  Lava  dams,  400;  Crater  lakes,  401;  Glacial  dams, 
401;  Lake  waters,  401;  Waves  and  currents,  401;  Wave  and  ice  action, 
401;  Lake  currents,  403;  Variations  in  lake  level,  403;  Gradual  variations, 
403;  Sudden  variations,  404;  Effect  of  strong  wind,  404;  Temperature  of 
lakes,  404;  Composition  of  lake  waters,  408;  Obliteration  of  lakes,  408; 
Obliteration  by  evaporation,  408;  Cutting  down  of  outlet,  410;  Oblitera- 
tion by  filling,  410;  Obliteration  by  lowering  of  groundwater  level,  412; 
Extinct  lakes,  413;  References  on  lakes,  413. 

CHAPTER   X 

GLACIAL  DEPOSITS:   THEIR  ORIGIN,  STRUCTURE,  AND  ECONOMIC  BEARING..     414 

Origin  and  nature  of  glaciers,  414;  Formation  of  snow  fields,  414; 
Change  of  snow  to  ice,  414;  Ice  motion,  415;  Conditions  essential  to  the 
formation  of  glaciers,  415;  Types  of  glaciers,  415;  General  features  of 
glaciers,  415;  Effects  of  advancing  glaciers,  417;  Glacial  erosion,  418; 
Glacial  transportation,  420;  Glacial  deposits,  420;  Surface  moraines,  420; 
Nature  of  glacial  deposits,  421;  Glacial  water  deposits,  422;  Past  glacia- 
tion,  422;  Glacial  drift,  422;  Topography  of  the  drift;  Glaciation  and  en- 


X  CONTENTS 

gineering  problems,  424;  Buried  channels,  424;  Tunneling  and  buried 
channels,  424;  Underground  water  supply,  425;  Dam  sites,  425;  Quarry- 
ing operations,  426;  Water  powers,  426;  Economic  materials  in  glacial 
deposits,  426;  References  on  glaciers,  426. 

CHAPTER   XI 

BUILDING  STONE 429 

Properties  of  building  stone,  429;  Kinds  of  rock  used,  429;  Factors 
governing  the  selection  of  building  stone,  429;  Cost,  429;  Beauty,  430; 
Durability,  430;  Structural  features  of  building  stones,  430;  Joints,  430; 
Stratification,  431;  Durability  of  building  stone,  431;  Structure,  431; 
Texture,  431;  Mineral  composition,  432;  Life  of  a  building  stone,  432; 
Quarry  water,  432;  .Estimated  life  of  a  building  stone,  432;  Injurious 
minerals,  433;  Flint  or  chert,  533;  Mica,  433;  Pyrite,  435;  Tremolite,  435; 
Prolongation  of  life  of  building  stone,  435;  Physical  properties,  437;  Ab- 
sorption, 437;  Relation  of  absorption  to  porosity,  438;  Character  of 
pores,  439;  Amount  of  water  absorbed  under  different  conditions,  439; 
Crushing  strength,  439;  Relative  strength  on  bed  on  edge,  441;  Relative 
strength  wet  and  dry,  441;  Effect  of  intermittent  pressure,  442;  Effect  of 
freezing  on  crushing  strength,  442;  Transverse  strength,  443;  Fire  resist- 
ance, 446;  Expansion  and  contraction  of  building  stone,  450;  Modulus  of 
elasticity,  451;  Abrasive  resistance,  451;  Frost  resistance,  453;  Freezing 
method,  454;  Sulphate  of  soda  test,  455;  Effect  of  atmospheric  gases, 
455;  Chemical  composition  of  building  stone,  456;  Microscopic  examina- 
tion, 456;  Igneous  rocks,  457;  Granites,  457;  Definition,  457;  Properties 
of  granites,  457;  Classification,  462;  Structure  of  granites,  462;  Uses  of 
granite,  464;  Distribution  of  granites  and  granite  gneisses,  464;  Eastern 
crystalline  belt,  464;  Minnesota-Wisconsin  area,  468;  Cordilleran  area, 
468;  Plutonic  rocks  other  than  granite,  469;  Syenite,  469;  Gabbro,  469; 
Diabase,  469;  Volcanic  rocks,  469;  Sandstones  and  quartzites,  470; 
Structural  features,  470;  Properties  of  sandstone,  470;  Distribution  of 
sandstones  and  quartzites,  473;  Geologic  distribution,  473;  Geographic 
distribution,  473;  Limestones,  473;  Structural  features,  473;  Properties 
of  limestones,  474;  Chemical  composition,  476;  Varieties  of  limestone 
and  dolomite,  478;  Distribution  of  limestones  in  the  United  States,  478; 
Marbles,  478;  Crystalline  limestones,  478;  Properties  of  crystalline  lime- 
stones, 478;  Distribution  of  marbles  in  the  United  States,  482;  Onyx 
marble,  482;  Serpentine,  482;  Slate,  483;  Structural  features,  483; 
Properties  of  slate,  484;  Tests  of  slate,  486;  Quarrying  488;  Classifica- 
tion of  slates,  488;  Distribution  of  slates  in  the  United  States,  488;  Ref- 
erences on  building  stone,  490. 

CHAPTER   XII 

LIMES,  CEMENT  AND  PLASTER 493 

Limes  and  calcareous  cements,  493;  Composition  of  limestones,  493; 
Changes  in  burning,  494;  Lime,  494;  Hydraulic  or  silicate  cements,  495; 
Hydraulic  limes,  495;  Grappier  cement,  496;  Feebly  hydraulic  limes,  496; 
Natural  cements,  497;  Portland  cement,  497;  Raw  materials  used,  498; 
Calculation  of  Portland  cement  mixture,  500;  Burning  changes  in 


CONTENTS  XI 

PAGH 

cements,  500;  Economic  considerations,  501;  Puzzolan  cements,  501; 
Cement  tests,  503;  Collos  cement,  503;  Cementation  index,  503;  Distri- 
bution of  lime  and  cement  materials  in  the  United  States,  504;  Gypsum 
plasters,  505;  Properties  and  occurrence,  505;  Chemical  composition,  507; 
Chemistry  of  gypsum  calcination,  507;  Kinds  of  plaster,  508;  Distri- 
bution of  gypsum,  508;  References  on  limes  and  calcareous  cements,  509; 
References  on  gypsum,  509. 


CHAPTER 

CLAY  AND  CLAY  PRODUCTS  ..........................................     510 

Properties  of  clay  510;  Physical  properties,  510;  Plasticity,  510; 
Tensile  strength,  511;  Shrinkage,  511;  Fusibility,  511;  Color,  513; 
Specific  gravity,  513;  Chemical  properties,  514;  Occurrence  of  clay,  516; 
Classification  of  clay  deposits,  516;  Residual  clays,  516;  Transported 
clays,  517;  Uses  of  clay,  519;  Kinds  of  clay,  519;  Engineering  uses  of 
clay,  519;  The  use  of  clay  for  brick,  519;  Methods  of  manufacture,  521; 
Sewer  pipe,  524;  Railroad  ballast,  524;  Road  material,  525;  Puddle,  525; 
Distribution  of  clays  in  the  United  States,  525;  References  on  clay,  526. 

CHAPTER  XIV 

COAL  SERIES  .........................................................     527 

Kinds  of  coal,  527;  Peat,  527;  Lignite,  629;  Subbituminous  coal  or 
black  lignite,  530;  Bituminous  coal,  530;  Cannel  coal,  532;  Semibitumi- 
nous  coal,  532;  Semianthracite  coal,  532;  Anthracite  coal,  532;  Compo- 
sition of  coal,  533;  Proximate  analysis  of  coal,  533;  Structural  features 
of  coal  beds,  536;  Outcrops,  536;  Associated  rocks,  536;  Variations  in 
extent  and  thickness,  537;  Variation  in  quality,  538;  Folding,  539;  Fault- 
ing, 539;  Classification  of  coals,  540;  Origin  of  coal,  543;  Technology  of 
coal,  544;  Calorific  power  of  coals,  544;  Coke,  546;  Use  of  coals  in  gas 
producers,  556;  Coal  briquetting,  551;  Escape  of  gas  from  coal,  554; 
Distribution  of  coal  in  the  United  States,  554;  Geologic  distribution,  555; 
Appalachian  region,  556;  Eastern  Interior  region,  557;  Northern  In- 
terior or  Michigan  region,  559;  Western  Interior  region,  559;  Rocky 
Mountain  region,  559;  Pacific  Coast  region,  560;  References  on  coal,  560. 

CHAPTER  XV 
PETROLEUM,  NATURAL  GAS  AND  OTHER  HYDROCARBONS  ..................     562 

Petroleum  and  natural  gas,  562;  Introductory,  562;  Properties  of 
petroleum,  562;  Properties  of  natural  gas,  563;  Occurrence  of  oil  and 
gas,  564;  Origin  of  oil,  gas  and  asphalt,  566;  Distribution  of  petroleum 
in  the  United  States,  567;  Appalachian  field,  567;  Ohio-Indiana  field,  569; 
Illinois  field,  569;  Mid-continental  field,  569;  Gulf  field,  569;  California 
field,  569;  Distribution  of  natural  gas  in  the  United  States,  571;  Solid  and 
semi-solid  bitumens,  570;  Vein  bitumens,  571;  Asphaltenes,  572;  Albert- 
ite,  572;  Grahamite,  572;  Gilsonite  or  Uintaite,  572;  Glance  Pitch,  573; 
Manjak,  573;  Maltha,  573;  Trinidad  lake  asphalt,  573;  Bituminous  rocks, 
573;  References  on  petroleum  and  natural  gas,  575;  References  on  solid 
and  semi-solid  bitumens,  576. 


xii  CONTENTS 

CHAPTER  XVI  PAGE 

ROAD  FOUNDATIONS  AND  ROAD  MATERIALS 577 

Road  foundations,  577;  Kind  of  rock,  577;  Rock  structure,  578; 
Valley  crossings,  578;  Slope  of  cuts,  578;  Drainage,  579;  Road  ma- 
terials, 579;  Raw  materials  used  for  highway  construction,  579;  Clay,  580; 
Gravel,  580;  Requirements  of  gravel,  581;  Tests  of  gravel,  581;  Tests  of 
gravel  from  different  localities,  582;  Chert  gravel,  582;  Broken  stone, 
582;  Properties  of  crushed  stone,  583;  Resistance  to  wear,  583 ;  Hardness, 
584;  Toughness,  584;  Cementing  value,  584;  Weight  per  cubic  foot,  584; 
Absorption,  584;  Results  of  tests,  585;  Significance  of  tests,  585;  Quali- 
ties of  individual  rock  types,  587;  Trap  and  fine-grained  basic  rocks,  587; 
Fine-grained  volcanics,  587;  Gabbros  and  other  coarse-grained  basic 
igneous  rocks,  587;  Granites  and  other  coarse-grained  acidic  igneous 
rocks,  587;  Slates  and  argillaceous  schists,  587;  Quartzite  and  quartzitic 
conglomerate,  587;  Limestone,  588;  Shales,  588;  Economic  considera- 
tions, 588;  Stone  blocks,  588;  References  on  road  materials,  589. 


CHAPTER  XVII 

ORE  DEPOSITS 590 

Nature  and  occurrence,  590;  Definition  of  ore  deposits,  590;  Ore 
minerals,  590;  Gangue  minerals,  590;  Origin  of  ore  bodies,  591;  Con- 
temporaneous ore  deposits,  591;  Subsequent  ore  deposits,  592;  Mode  of 
concentration,  593;  Source  of  concentrating  waters,  594;  Concentration 
by  meteoric  waters,  594;  concentration  by  magmatic  waters,  594;  De- 
position of  ores,  595;  Cavity  deposition,  595;  Precipitation  of  metals 
from  solution,  595;  Replacement,  596;  Physical  conditions  of  ore  de- 
position, 598;  Pneumatolytic  deposits,  598;  Contact-metamorphic 
deposits,  600;  Deep-seated  gold  and  silver  veins,  600;  Ore  deposits  at 
shallow  depths,  601;  Deposits  formed  at  surface,  602;  Distribution  of 
magmatic  waters,  602;  Hydrothermal  alteration,  602;  Propylitization, 
602;  Sericitization,  603;  Silicification,  603;  Alunitization,  603;  Greiseni- 
zation,  603;  Forms  of  ore  bodies,  603;  Fissure  veins,  603;  Chimney, 
605;  Stock,  606;  Fahlband,  606;  Disseminated  deposits,  606;  Residual 
deposits,  606;  Ore  shoots,  606;  Primary  and  secondary  ores,  607; 
Weathering  and  secondary  enrichment,  607;  Zones  in  an  ore  body,  607; 
Zone  of  weathering,  608;  Secondary  sulphide  zone,  610;  Change  of  ore 
with  depth;  611;  Zone  of  primary  sulphides,  612;  Outcrops  of  ore  bodies, 
612;  Distribution  of  ore  deposits  in  the  United  States,  612;  Coastal 
Plain,  612;  Piedmont  Plateau,  614;  Appalachian  Province,  614;  Allegheny 
Plateau,  614;  Prairie  Plains,  614;  Great  Plains,  615;  Cordilleran  Region, 
615;  Occurrence  of  more  important  ore  types,  615;  Iron  ores,  615;  Iron 
ore  minerals,  616;  Types  of  iron-ore  bodies,  618;  Magnetite,  618;  Hema- 
tite, 618;  Limonite,  621;  Siderite,  621;  Production,  622;  Copper  ores,  622; 
Ore  minerals  of  copper,  622;  Types  of  copper-ore  bodies,  624;  Contact 
metamorphic  deposits,  624;  Disseminated  deposits,  624;  Vein  deposits, 
624;  Lenses  in  schists,  626;  Production,  626;  Lead  and  zinc  ores,  626; 
Ore  minerals  of  zinc,  626;  Ore  minerals  of  lead,  627;  Weathering  of  lead 


CONTENTS  xiii 

PAGE 

and  zinc  ores,  627;  Classification  of  lead  and  zinc  ores,  627;  Mode  of 
occurrence  of  lead  and  zinc  ores,  628;  Production,  629;  Gold  and  silver 
ores,  629;  Ore  minerals,  629;  Occurrence  of  gold  and  silver  ores,  629; 
Quartz  vein  type,  630;  Propylitic  veins,  630;  Auriferous  gravels,  630; 
Production,  631;  References  on  ore  deposits,  631. 

APPENDIX  A.   GEOLOGIC  COLUMN 632 

APPENDIX  B.   GEOLOGICAL  SURVEYS  . .  634 


LIST   OF   PLATES 


PLATE  PAGE 

I.     Fig.  1.   Parallel  dikes  of  diabase  cutting  pegmatite  dike,  near 

Pourpour,  Que 49 

Fig.  2.   Irregular  granite  dikes  cutting  gneiss,  Moose  Moun- 
tain, Ont 49 

II.     Fig.  1.   Volcanic  cone  of  Colima,  Mex 58 

Fig.  2.   Table  Mountain,  Golden,  Colo 58 

III.  Fig.  1.   End  of  aa  flow  of  lava,  Colima,  Mex 61 

Fig.  2.   Basaltic  lava,  near  Mexico  City,  Mex 61 

IV.  Fig.  1.    Banded  felsite,  showing  flow  structure 68 

Fig.  2.    Trachyte  showing  porphyritic  texture 68 

V.     Fig.  1.   Basalt,  showing  vesicular  structure 71 

Fig.  2.    Graphic  granite,  showing  characteristic  intergrowth  of 

quartz  and  feldspar 71 

VI.     Fig.  1.   Dikes  of  pegmatite  in  granite,  Richmond,  Va 76 

Fig.  2.   Volcanic  ash  deposits,  volcano  of  Toluca,  Mex 76 

VII.     Fig.  1.    Photomicrograph  of  section  of  granite 83 

Fig.  2.   Photomicrograph  of  section  of  diabase 83 

VIII.     Columnar  jointing  in  basalt 88 

IX.     Fig.  1.    Breccia  formed  by  crushing  of  marble  by  earth  move- 
ment   101 

Fig.  2.    Talus  breccia  formed  by  disintegration  of  limestone. . . .  101 

X.     Fig.  1.    Medium-grained  sandstone 104 

Fig.  2.    Coarse     conglomerate     with    little     cement,     Frank, 

Alberta 104 

XI.     Fig.  1.   Section  in  hard  sandstone  showing  horizontal  stratifica- 
tion  107 

Fig.  2.   Beds  of  gently-dipping  shale,  overlain  by  hard  much- 
jointed  sandstone 107 

XII.     Fig.  1.    Front  slope  of  advancing  sand  dune 110 

Fig.  2.   General  view  of  sand-dune  area,  showing  planting 1 10 

XIII.  Fig.  1.   Deposit  of  siliceous  sinter,  Steamboat  Springs,  Nev. . . .  117 
Fig.  2.   Cherty  limestone,  west  of  Lexington,  Va 117 

XIV.  Fig.  1.   Weyers   Cave,  Va.,  showing  stalactites  of  lime  car- 

bonate suspended  from  roof 120 

Fig.  2.   Same    cave,    showing    coalescence    of    stalactites    and 

stalagmites 120 

XV.     Fig.  1.    Pisolitic  structure 123 

Fig.  2.   Fossilif erous  limestone 123 

xv 


XVI 


LIST  OF  PLATES 


PLATE  PA0B 

XVI.     Fig.  1.   Hornblende  gneiss  showing  irregular  banding 130 

Fig.  2.    Biotite  gneiss  showing  folding  of  the  bands 130 

XVII.     Fig.  1.   Magnetite  gneiss  showing  distinct  banding 131 

Fig.  2.    Gneiss  quarry  near  Lynchburg,  Va 131 

XVIII.     Beds  of  slate  showing  cleavage,  overlain  by  quartzite 136 

XIX.     Fig.  1.   Ideal  section  of  an  upright  normal  anticlinorium 152 

Fig.  2.    Ideal  section  of  an  upright  normal  synclinorium 152 

Fig.  3.   Generalized   fan   fold   of    the    central    massif    of    the 

Alps 152 

Fig.  4.   General  section  of  "roof  structure" 152 

XX.     Fig.  1.   Contorted  strata  in  Chickamauga  limestone,  near  Ben" 

Hur,  Va 153 

Fig.  2.   Folded  quartzite,  Eagle  Mtn.,  Va 153 

XXI.     Types  of  folds 156 

XXII.     Fig.  1.   Limestone   showing   horizontal   bedding   and   vertical 

joints 165 

Fig.  2.   Faulted  pegmatite  dike  in  granite,  near  Boulder,  Colo.  165 

XXIII.  Fig.  1.   Fault  in  Ordovician  slate 170 

Fig.  2.   View  from  Mount  Stephen  near  Field,  B.  C.,  looking 

towards  pass  at  Hector 170 

XXIV.  Fig.  1.   Diagram  illustrating  trough  faults 176 

Fig.  2.   Strike-fault  section,  hading  with  dip 176 

Fig.  3.   Fault  showing  change  of  dip 176 

Fig.  4.   Faulting  in  an  unconformable  series  of  beds,  showing 

age  of  fault 176 

Fig.  5.     Strata  repeated  by  faulting 176 

XXV.     Diagram  showing  effects  of  different  kinds  of  faults  on  block 

with  monoclinal  structure,  and  one  coal  bed 179 

XXVI.     Sections  to  illustrate  development  of  overthrust  fold  and  faulting  182 

XXVII.     Fig.  1.   Siderite  concretions  in  clay 201 

Fig.  2.   Lime  carbonate  concretions  in  clay 201 

XXVIII.     Fig.  1.    Much  contorted  and  metamorphosed  argillaceous  and 
calcareous  beds,  filled  with  contact  silicates  due  to 

granitic  intrusions 202 

Fig.  2.   Fractures  in  limestone  produced  by  folding,  and  filled 

with  calcite 202 

XXIX.     Fig.  1.   Granite  quarry  near  Woodstock,   Md.,  showing  hori- 
zontal joints .  . 205 

Fig.  2.   Slate  quarry  Penrhyn,  Pa.,  showing  folded  beds,  and 

cleavage 205 

XXX.     Fig.  1.   Quartzite  broken  by  temperature  changes,  frost  and 

plant  roots,  Monroe,  N.  Y 217 

Fig.  2.    Concretionary  sandstone  weathered  by  solution  and 

wind  action,  Snake  Island,  near  Nanaimo,  B.  C 217 

XXXI.     Fig.  1.   Talus  of  weathered  schist.     French  Pyrenees 220 

Fig.  2.    Weathered  diabase  dike,  Virginia 220 

XXXII.     Fig.  1.   Granite  boulders,  produced  by  disintegration  and  de- 
composition, Faith,  N.  C 222 


LIST  OF  PLATES 


XV11 


Fig.  2.   Granite  boulders  of  disintegration,   near  Winchester, 

Cal 222 

XXXIII.  Fig.  1.   Dike  cutting  granite 224 

Fig.  2.    Millstone  cracked  by  growth  of  tree 224 

XXXIV.  Fig.  1.   Granite  quarry,  Manchester,  Va.     Shows  sheeted  struc- 

ture of  granite  and  covering  of  residual  clay 226 

Fig.  2.   Stone   Mountain,   Wilkes  County,   N.   C.     A  granite 
dome,  which  has  resisted, the  weather  better  than  the 

surrounding  rocks 226 

XXXV.     Fig.  1.   Elongated  boulders  of  granite,  produced  by  weathering 

along  the  joints,  Woodstock,  Md 229 

Fig.  2.   Granite     boulder     showing     concentric     weathering, 

Oglesby,  Ga 229 

XXXVI.     Fig.  1.   Diabase  showing  boulders  produced  by  weathering,  sur- 
rounded by  concentric  shells  of  decayed  rock 233 

Fig.  2.   Same  as  Fig.  1,  but  showing  one  of  the  boulders  in  more 

detail 233 

XXXVII.     Fig.  1.   Limestone  " chimneys,"  separated  by  hollows  caused  by 

solution  along  vertical  joint  planes 235 

Fig.  2.    Pinnacled  surface  of  limestone  bedrock,  after  residual 

clay  has  been  removed,  Limonite  pits,  Ivanhoe,  Va.     235 
XXXVIII.     Fig.  1.   Residual   clay   derived  from  schist,   but  showing   no 

traces  of  structure  of  parent  rock 237 

Fig.  2.   Residual  clay  derived  from  gneiss.     Banded  structure 

of  parent  rock  preserved 237 

XXXIX.     Fig.  1.   Hillside  gullied  by  erosion,  near  Milledgeville,  Ga 249 

Fig.  2.    Gravelly  character  of  material  carried  by  swiftly  flowing 

stream 249 

XL.     Diagrams  showing  volume  of  discharge  of  several  rivers 251 

XLI.     Fig.  1.    Patuxent  River,  Maryland 258 

Fig.  2.   Saskatchewan  River,  near  Medicine  Hat,  Alberta ....       258 
XLII.     Plan  and  sections  showing  typical  features  of  a  meandering 

river 260 

XLIII.     Fig.  1.   Waterfalls    over    vertically    dipping    limestone    beds, 

Tomasopo  Canyon,  Mex 265 

Fig.  2.   Basalt  flow  overlying  stream  gravels,  central  France .  . .     265 
XLIV.     Fig.  1.   A  flood  plain.     View  along  Danube  River,  in  Servia.  .  .     270 

Fig.  2.     Section  of  ancient  delta,  near  Fishkill,  N.  Y 270 

XLV.     Plan  of  Mississippi  delta 272 

XL VI.     Fig.  1.    High  river  terrace,  Orizaba,  Mex 276 

Fig.  2.    View  of  Hudson  River  valley,  looking  north  from  West 

Point,  N.  Y 276 

XL VII.     Fig.  1.    View  looking  west  down  Fall  Creek  gorge,  Ithaca,  N.  Y.     281 
Fig.  2.    View  looking  east  up  Fall  Creek  gorge,  Ithaca,  N.  Y.. .     281 
XLVIII.     Fig.  1.   Street   in   Staunton,    Va.,    showing  sewer-pipe   whose 
break  started  the  caving,  and  holes  formed  in  pave- 
ment       310 

Fig.  2.    Large  hole  formed  along  line  of  caving,  Staunton,  Va.     310 


XV111 


LIST  OF  PLATES 


PLATE  PAGE 

XLIX.     View  looking  along  line  of  limestone  cavern,  Staunton,  Va 311 

L.     Fig.  1.   Slide  of  clay  caused  partly  by  undermining  action  of 

stream  and  partly  by  clay  becoming  water-soaked. . .     346 
Fig.  2.   View  of  Turtle   Mountain,    Frank,   Alberta,   showing 
place  from  which  rock  fell,  and  a  portion  of  slide  in 

foreground 346 

LI.     General  view  of  clay  slide,  Riviere  Blanche,  Que 347 

LII.     Section  across  Turtle  Mountain,  Frank,  Alberta 352 

LIII.     Changes   at   Hereford  Inlet,    and  Five-mile   Beach,    1771   to 

1901 367 

LIV.     Fig.  1.   Cliffs  formed  by  wave  action,  Sydney,  N.  S 369 

Fig.  2.   Beach  and  sand  dunes,  formed  by  wave  and  wind  action 

across  harbor  of  Inverness,  N.  S 369 

LV.     Map  showing  barrier  beach  and  partly-filled  lagoon  behind. ...     371 
LVI.     Hooked  spit  of  sand,  mouth  of  Ausable  River,  Lake  Champlain, 

N.  Y 373 

LVII.     Shows  simplification  of  shore  line  by  deposition 375 

LVIII.     A  coast  line  showing  both  erosion  and  deposition 377 

LIX.     Development  of  coastal  irregularities  through  processes  which 

will  ultimately  result  in  coastal  simplification 379 

LX.     Map  showing  changes  in  the  shore-line  between  Brigantine  and 

Little  Egg  Harbor  Inlets,  N.  J.,  between  1840  and  1904 382 

LXI.     Map  of  mouth  of  Manasquan  Inlet,  N.  J.,  Sept.,  1878 384 

LXII.     Maps  showing  changes  at  the  mouth  of  Manasquan  Inlet,  in 

the  year  1907 386 

LXIII.     Survey  and  plan  for  the  improvement  of  Manasquan  Inlet,  N.  J.     387 
LXIV.     Fig.  1.    Gravelly  beach  formed  by  wave  action,  Kootenay  Lake, 

British  Columbia : 398 

Fig.  2.   Lake  formed  by  barrier  of  lava,  central  France 398 

LXV.     Fig.  1.   Lakelet  held  in  by  terminal  moraine  and  glacier 402 

Fig.  2.    Crater  lake,  volcano  of  Toluca,  Mexico 402 

LXVI.     Fig.  1.   General  view  of  an  alpine  glacier,  the  Asulkan,  near 

Glacier,  B.  C 416 

Fig.  2.    General  view  of  Lake  Louise,  Alberta,  from  the  Victoria 

glacier 416 

LXVII.     Fig.  1.   View  of  lower  end  of  Asulkan  glacier,  in  1908 419 

Fig.  2.    The  same  glacier,  from  same  viewpoint  in  1910,  showing 

recession 419 

LXVIII.     Fig.  1.   Weathered    sandstone,    second    story,    County    Court 

House,  Denver,  Colo 434 

Fig.  2.    Roughened    surface    of    limestone    after    exposure    to 

weather 434 

LXIX.     Fig.  1.    View  in  limestone  quarry  showing   solvent  action  of 

water  along  joint  planes 436 

Fig.  2.  Weathered  outcrop  of  a  limestone  conglomerate.  The 
silicified  pebbles  and  quartz  veins  show  greater  resist- 
ance to  the  weather 436 

LXX.    Fire  tests  on  3-inch  cubes  of  limestone,  Newton,  N.  J 448 


LIST  OF  PLATES 


XIX 


PLATE  PAGE 

LXXI.     Fig.  1.   Moderately  fine-grained  granite,  Hallowell,  Me 458 

Fig.  2.   Very  coarse-grained  granite,  St.  Cloud,  Minn 458 

LXXII.     Fig.  1.   Port  Deposit,  Maryland,  gneissic  granite,  with  face  cut 

at  right  angles  to  banding 460 

Fig.  2.   Same  with  face  cut  parallel  to  banding 460 

LXXIII.     Fig.  1.   Diorite  from  Ferris,   CaL,   showing  contrast  between 

light  and  dark  minerals 461 

Fig.  2.    Boulder  quarry,  Richmond,  Va 461 

LXXIV.     Fig.  1.   Granite  quarry  at  North  Jay,  Me 463 

Fig.  2.    Granite  quarry,  Hardwick,  Vt 463 

LXXV.     Orbicular  gabbro  from  North  Carolina 465 

LXXVI.     Map  showing  distribution  of  igneous  rocks  and  gneisses  in  the 

United  States 466 

LXXVII.     Fig.  1.    General  view  of  Stone  Mountain,  Ga 467 

Fig.  2.   Quarry  of  granite  along  base  of  Stone  Mountain,  Ga. . .  467 

LXXVIII.     Fig.  1.   Horizontally  stratified  limestone,  Milwaukee,  Wis 475 

Fig.  2.    Quarry  in  calcareous  tufa,  Tivoli,  Italy 475 

LXXIX.     Map  showing  limestone  areas  of  United  States 478 

LXXX.     Slabs  of  marble,  showing  plain  white,  and  streaks,  bands,  and 

mottlings  produced  by  mica 479 

LXXXI.     Fig.  1.   Quarry  of  Vermont  Marble  Company,  Proctor,  Vt 481 

Fig.  2.   Slate  quarry,  Penrhyn,  Pa 481 

LXXXII.     Fig.  1.   Sculping  slate 485 

Fig.  2.   Splitting  slate 485 

LXXXIII.     Map  showing  slate-producing  districts  of  the  United  States. . . .  489 

LXXXIV.     Fig.  1.   Quarry  in  natural  cement  rock,  Milwaukee,  Wis 502 

Fig.  2.   Shell  marl  outcropping  along  James  River,  Va 502 

LXXXV.     Map  showing  distribution  of  cement  plants  in  United  States . . .  506 
LXXXVI.     Bricklets  of  different  clays,  all  fired  at  the  same  temperature  to 

show  their  different  fusibilities 512 

LXXXVII.     Fig.  1.   Deposit  of  stony  glacial  clay 518 

Fig.  2.   Stratified  marine  clay,  from  Athens,  Texas 518 

LXXXVIII.     Fig.  1.   Section  showing  fire  clay  underlying  coal  seam 520 

Fig.  2.   Shale  used  for  paving  blocks,  Veedersburg,  Ind 520 

LXXXIX.     View  of  a  peat  bog  and  peat-excavating  machine 528 

XC.     Fig.  1.   Outcrop  of  lignite,  Williston,  N.  Dak 531 

Fig.  2.    Culm  pile  in  Pennsylvania  anthracite  region 531 

XCI.     Fig.  1.   Subbituminous  coal,  showing    the    irregular  checking 

produced  in  drying 542 

Fig.  2.   Bituminous  coal,  showing  prismatic  structure 542 

XCII.     Washing  plant  for  bituminous  coal,  Searles,  Ala 545 

XCIII.     Beehive  coke  ovens,  Brookwood,  Ala 548 

XCIV.     By-products  coke  ovens 549 

XCV.     Fig.  1.   Lignite  briquettes  at  beginning  of  weathering  test 552 

Fig.  2.   Same  after  9  days 552 

XCVI.     Fig.  1.   Same  as  in  Plate  XCV,  after  226  days  weathering 553 

Fig.  2.   The  same  after  286  days  weathering 553 

XCVTI.     Map  of  coal  fields  of  the  United  States 556 


XX  LIST  OF  PLATES 

PLATE  PAQE 

XCVIII.     Outcrop   of   thick    "vein"    Freeport   coal    (bituminous)    near 

Pittsburg 558 

XCIX.     Map  showing  oil  and  gas  fields  of  the  United  States 568 

C.     Photomicrographs  of  polished  surfaces  showing: 

Fig.  1.   Intergrowths  of  bornite  and  chalcocite 599 

Fig.  2.   Intergrowths  of  bornite  and  chalcocite 599 

Fig.  3.   Secondary  chalcocite  in  fractures  in  bornite 599 

CI.     General  view  of  a  residual  limonite  deposit  at  Ironton,  Ala. .  .  .  617 

CII.     General  view  of  Mountain  Iron  mine,  Mesabi  Range,  Minn. . .  620 

GUI.     General  view  of  a  blast-furnace  plant  for  making  iron 623 

CIV.     Map  showing  distribution  of  copper  ores  in  the  United  States . .  625 


LIST  OF  FIGURES 


FIG.  PAGE 

1.  Jolly  balance 6 

2,  3.   Feldspar  crystals 9 

4,  5.   Twinned  feldspar  crystals 9 

6,  7,  8.    Multiple  twinning  in  feldspar 10 

9,  10.   Pyroxene  crystals 16 

11,  12.   Cross  sections  of  pyroxene  prisms 16 

13.  Prismatic  cleavage  of  pyroxene 16 

14,  15,  16.   Amphibole  crystals 18 

17.  Prismatic  cleavage  of  amphibole 18 

18,  19,  20.    Garnet  crystals 20 

21,  22,  23.   Crystals  of  staurolite. . . " 23 

24,  25,  26.    Crystals  of  tourmaline 24 

27,  28.   Quartz  crystals.  : 29 

29,  30,  31.   Magnetite  crystals 32 

32,  33,  34,  35,  36.    Crystal  forms  of  calcite 35 

37.  Cleavage  rhomb  of  calcite 35 

38.  Dolomite 37 

39.  40.    Gypsum  crystals 38 

41,  42.   Apatite  crystals 40 

43,  44,  45,  46.    Pyrite  crystals 41 

47.  Section  through  dike  more  resistant  to  weathering  than  the  inclosing  rock, 

marking  the  position  of  a  ridge 50 

48.  Section  through  dike  less  resistant  to  weathering  than  the  inclosing  rock, 

marking  the  position  of  a  valley 50 

49.  Section  through  dike  and  inclosing  rock,  showing  no  topographic  ex- 

pression from  weathering 51 

50.  Section  through  extrusive  and  intrusive  sheets,  and  conduit 52 

51.  Section  of  intrusive  sheet,  breaking  across  the  strata  and  continuing  in 

the  same  general  direction  at  a  higher  level 52 

52.  Section  through  laccolith  showing  associated  sheets  and  dikes 53 

53.  Section  through  partly-eroded  laccolith  showing  different  outline  from 

Figure  52 53 

54.  Section  through  volcanic  neck  or  plug 54 

55.  Plan  of  volcanic  neck  or  plug 54 

56.  Section  through  stock  or  boss 55 

57.  Plan  of  stock  or  boss 55 

58.  Section  through  a  batholith 56 

59.  Section  through  a  series  of   interbedded  lava   flows,   and   fragmental 

materials 57 

60.  Granite  cut  by  pegmatite  dikes 74 

61.  Sketch  of  a  breccia 95 

xxi 


xxii  LIST  OF  FIGURES 

FIG.  PAGE 

62.  Sketch  of  a  conglomerate 95 

63.  Section  of  cliff  illustrating  talus  slope  at  base 100 

64.  Section  showing  a  fault  breccia 100 

65.  Section  showing  bedding  and  lamination 105 

66.  Section  illustrating  cross  bedding 106 

67.  Section  of  a  sand  dune 112 

68.  Section  showing  relation  of  cleavage  to  stratification 138 

69.  Diagram  illustrating  strike  and  dip 148 

70.  Diagram  of  A,  anticline,  and  B,  syncline 149 

71.  Section  showing  anticline  and  syncline 149 

72.  Monoclinal  fold 150 

73.  a.   Section  showing  monoclinal  attitude  of  beds;    6.   Perspective  model 

of  same 150 

74.  Plan  and  section  of  quaquaversal  fold 150 

75.  Plan  and  section  of  centroclinal  fold 150 

76.  A.   Isoclinal  folds,  upright;    B.    Isoclinal  folds,  inclined;    C.   Isoclinal 

folds,  recumbent;   D.   Fan  structure,  upright 151 

77.  Stretch  thrust  developed  from  an  overturned  fold 154 

78.  Tilted  folds 154 

79.  Monocline  showing  thinning  of  beds  in  the  fold 155 

80.  Eroded  fold 155 

81.  Ideal  section  of  bent  rock  stratum,  showing  fracturing  along  convex 

surface  and  compression  along  concave  surface 155 

82.  Section  showing  relation  of  tunnel  to  anticlinal  fold 157 

83.  Diagram  illustrating  determination  of  thickness  of  beds  by  trigonometric 

method 159 

84.  Diagram  for  use  in  determination  of  thickness  of  beds  by  graphic  method  161 

85.  Diagram  illustrating  determination  of  thickness  of  beds  by  construction 

method 162 

86.  Diagram  illustrating  determination  of  depth  of  beds  by  trigonometric 

method 163 

87.  Section  showing  "horse"  developed  by  faulting 169 

88.  Faulting  accompanied  by  brecciation 171 

89.  Normal  faulting  showing  distortion  of  shale 171 

90.  Faulted  block  with  parts  named 172 

91.  Normal  faulting  developed  in  horizontal  beds 173 

92.  Normal  fault  hading  against  beds 174 

93.  Normal  fault  hading  with  dip  of  beds 174 

94.  Section  showing  distributive  or  step  faults 174 

95.  Section  showing  reverse  fault 174 

96.  Section  showing  development  of  either  normal  or  reverse  fault 175 

97.  Plan  illustrating  shifting  of  beds  by  faulting 178 

98.  A.   Plan  of  strike  fault  showing  repetition  of  beds  at  surface;  B,  section 

along  line  ab  normal  to  strike  fault 178 

99.  Diagram  showing  effects  of  faulting  on  the  outcrops  of  a  syncline 180 

100.  Step-fold  showing  break  in  massive  limestone  bed  which  determines  the 

plane  of  the  break  thrust 180 

101.  Fold  passing  into  a  fault 180 


LIST  OF  FIGURES  xxiii 

FIQ.  PAGE 

102.  Relation  of  tunnel  to  fault  zone 181 

103.  Section  showing  case  of  bedded  ore  body,  cut  off  by  a  fault 185 

104.  Diagram  illustrating  determination  of  dip  of  fault  plane 186 

105.  Diagram  illustrating  determination  of  angle  of  intersection  of  fault  plane 

with  vertical  plane  oblique  to  strike  of  fault 187 

106.  Diagram  illustrating  form  of  outcrop 189 

107.  Sliced  feldspar  in  micaceous  and  chloritic  schist  from  southern  Appa- 

lachians    191 

108.  Section  showing  erosive  unconformity  (aa)  with  concordant  dips 195 

109.  Section  showing  unconformity  (aa)  with  discordant  dips 196 

1 10.  Igneous  unconformity  between  (a)  granite  and  (6)  sedimentary  rocks .  . .  196 

111.  Igneous  unconformity  between  extrusive  lava  sheets 196 

112.  Section  along  contact  of  Piedmont  crystalline  rocks  and  Coastal  Plain 

sediments  showing  overlap 196 

113.  Section  showing  an  inlier  formed  at  summit  of  an  anticline  by  erosion. .  197 

114.  Section  showing  inlier  formed  by  faulting 197 

115.  Section  of  outlier  formed  by  erosion 197 

116.  Plan  and  section  of  inlier  and  outlier 198 

117.  Outlier  formed  by  faulting 198 

118.  Section  through  a  contact-metamorphic  zone 206 

119.  Section  of  slate  containing  bed  of  quartzite 208 

120.  Symbols  used  in  geologic  sections 211 

121.  Geologic  map,  showing  dips  and  strikes  along  line  AB,  and  structure 

section  along  same 214 

122.  Section  showing  formation  of  residual  clay  from  granite 236 

123.  Section  showing  residual  clay  from  shale 239 

124.  Section  showing  residual  clay  derived  from  limestone «. 240 

125.  Map  of  Salton  Sink  and  Imperial  Valley,  California 255 

126.  River  curve  indicating  place  of  greatest  erosion  on  bend 257 

127.  Sketch  of  a  suction  eddy 261 

128.  Sketch  of  a  pressure  eddy 262 

129.  Sections  showing  successive  stages  of  bank  erosion 263 

130.  Section  of  a  delta 273 

131.  Stream  piracy 279 

132.  Sections  across  the  Hudson  River  Valley 280 

133.  Chart  showing  variations  in  hardness  of  Allegheny  River  water  during 

a  year 292 

134.  Chart  showing  variation  in  acidity  of  Monongahela  River  water  during 

different  years 293 

135.  Map  showing  mean  annual  rainfall  of  the  United  States 296 

136.  Section  showing  relation  of  water  table  to  surface  irregularities 293 

137.  Map  showing  position  of  water  table  by  contours,  lines  of  motion  of 

groundwater,  and  surface  streams 299 

138.  Map  showing  the  deltas  or  fans  of  disappearing  streams  as  they  leave 

their  mountain  canyons 30C 

139.  Section  illustrating  conditions  governing  movement  of  water  away  from 

streams  or  lakes -. 302 

140.  Section  showing  lowering  of  water  table  by  pumping 304 


xxiv  LIST  OF  FIGURES 

FIG.  PAGH 

141.  Seepage  spring  fed  from  unconfined  waters  in  porous  sands 304 

142.  Diagram  showing  possibility  of  pollution  of  wells  and  springs  by  material 

conducted  from  cesspool  through  tubular  water  passage  in  till 305 

143.  Tubular  springs  in  limestone,  the  passages  connecting  with  sinkholes 305 

144.  Section  showing  occurrence  of  a  fissure  spring 306 

145.  Plan  and  section  of  Nashville  reservoir,  showing  cause  of  break 308 

146.  Conditions  illustrating  the  drainage  of  wells  into  a  saturated  stratum  of 

lower  head 313 

147.  Conditions  encountered  by  wells  sunk  through  perched  water  tables.  ...  313 

148.  Pond  held  in  impervious  basin  above  the  water  table 313 

149.  Section  of  an  artesian  basin 318 

150.  Section  showing  relation  of  tide  to  level  of  water  table 318 

151.  Section  in  water-bearing  gravel,  with  intake  too  low  to  cause  water  to 

rise  to  surface 319 

152.  Section  illustrating  conditions  of  flow  forming  solution  passages  in  lime- 

stone       319 

153.  Section  illustrating  the  thinning  out  of  a  porous  water-bearing  bed 320 

154.  Section  illustrating  the  transition  of  a  porous  water-bearing  bed,  A,  into 

a  close-textured  impervious  one 320 

155.  Section  across  Michigan,  showing  cover  of  glacial  drift  yielding  flowing 

wells 323 

156.  Map  of  artesian  field  of  Wapsipinicon  River,  Iowa,  and  of  buried  channel 

of  Bremer  River 323 

157.  Section    illustrating    artesian    conditions    in    jointed    crystalline    rocks 

without  surface  covering 325 

158.  Location  of  flowing  or  nearly  flowing  wells  of  Maine 325 

159.  Section  illustrating  conditions  of  flow  from  vesicular  trap 327 

160.  Conditions  governing  freezing  in  a  cased  well  with  escape  of  air  at 

bottom 328 

161.  Conditions  governing  freezing  in  wells  with  leaky  casings  and  porous 

walls 328 

162.  Conditions  governing  freezing  in  limestone  wells 328 

163.  Geologic  and  water-supply  districts  of  eastern  United  States 330 

164.  Wisconsin  outcrop  of  Potsdam  and  St.  Peter  sandstones 333 

165.  Barton's  map  of  catchment  area  of  the  Dakota  sandstone  and  the  Dakota 

artesian  basin 334 

166.  Section  from  Black  Hills  to  eastern  South  Dakota,  showing  structure  of 

artesian  basin 335 

167.  Sections  across  Owens  Valley,  Cal.,  showing  unconsolidated  beds  in  which 

the  groundwater  accumulates 336 

168.  Section  showing  position  of  Miihlthal  tunnel  and  creep  material  on 

Brenner  Railroad 344 

169.  Map  of  Slide  on  Lievre  River,  Que 345 

170.  Ideal  profile  of  landslides  on  the  northern  side  of  Lookout  Mountain, 

Wash 349 

171.  Section  showing  structural  conditions  likely  to  produce  rock  slides  along 

joint  or  stratification  planes 350 

172.  Series  of  particles  in  their  orbits 360 


LIST  OF  FIGURES  XXV 

FIG.  PAGE 

173.  The  same  with  orbits  doubled  in  size,  phasal  difference  45  degrees 360 

174.  The  same  as  Fig.  172,  with  phasal  difference  reduced  45  degrees 360 

175.  The  same  as  Fig.  172,  with  phasal  difference  increased  to  90  degrees. . .  360 

176.  The  same  as  Fig.  175,  with  orbits  sufficiently  reduced  in  size  to  prevent 

breaking 360 

177.  The  same  as  Fig.  176,  with  orbits  still  further  reduced 360 

178.  Diagram  showing  relative  directions   of  wave,   undertow,    and  shore 

current 363 

179.  Section  of  wave-cut  terrace  in  gentle  slope 366 

180.  Section  of  wave-cut  terrace  on  steeply  sloping  coast 368 

181.  Section  of  a  beach  ridge 370 

182.  Section  of  a  barrier  beach 370 

183.  Section  of  a  barrier  beach  which  has  moved  inland  part  way  across  a 

marshy  lagoon 370 

184.  Sketch  map  showing  the  development  of  a  hooked  spit 372 

185.  Sketch  map  showing  a  bay  inclosed  by  a  pair  of  overlapping  spits 372 

186.  Section  of  a  bar 380 

187.  Curves  of  temperature  during  twelve  months,  in  Lake  Cochituate,  Mass.  406 

188.  Section  showing  relation  of  outwash  plain  to  terminal  moraine 421 

189.  Section  through  glacial  drift  and  bed  rock,  showing  how  the  deposition 

of  morainal  material  has  made  the  surface  more  irregular 423 

190.  Section  showing  how  the  deposition  of  glacial  drift  has  reduced  surface 

irregularities 423 

191.  Sections  across  Rondout  Valley,  N.  Y.,  showing  pre-Glacial  valleys  which 

have  been  filled  with  drift 425 

192.  Section  through  Tongore  dam  site,  tested  for  Catskill  aqueduct 427 

193.  Section  through  Olive  Bridge  dam  site  tested  for  Catskill  aqueduct 427 

194.  Sandstone  broken  by  transverse  strain,  caused  by  settling  of  the  building  443 

195.  Effect  of  fire  on  granite  columns 446 

196.  Section  in  slate  quarry  with  cleavage  parallel  to  bedding 483 

197.  Section  showing  relation  of  cleavage,  false  cleavage,  and  bedding 484 

198.  Map  showing  distribution  of  gypsum  in  the  United  States 508 

199.  Section  showing  passage  of  fully  formed  residual  clay  on  the  surface  into 

the  solid  bed  rock  below 516 

200.  Diagram  showing  how  plants  fill  depressions  from  the  sides  and  top,  to 

form  a  peat  deposit 527 

201.  Peaty  deposit  with  cypress  stumps,  covered  by  sandy  clays  due  to  sinking 

of  land  below  sea  level,  Chesapeake  Bay,  Md 529 

202.  Sections  of  Clarion  coal,  Foxburg  quadrangle,  Pa. . 537 

203.  Section  showing  irregularities  hi  a  coal  bed 538 

204.  Section  in  coal  basins  of  Pennsylvania,  showing  several  beds  in  the  same 

section,  and  also  intense  folding 539 

205.  Section  of  faulted  coal  seam 539 

206.  Coalbreaker  in  Pennsylvania  anthracite  region 556 

207.  Generalized  section  of  Michigan  coal  field 559 

208.  Section  showing  association  of  oil  and  gas  with  anticline 565 

209.  Diagrammatic  section  of  sands  in  the  central  Appalachian  region 566 

210.  Map  of  asphalt  and  bituminous  rock  deposits  of  the  United  States 574 


XXVI  LIST  OF  FIGURES 

FIG.  PAGE 

211.  Section  of  Red  Mountain,  Birmingham,  Ala.,  showing  a  bedded  ore  de- 

posit of  contemporaneous  origin 592 

212.  Vein  filling  a  fault  fissure 596 

213.  Photomicrograph  of  a  section  of  quartz  conglomerate,  showing  replace- 

ment of  quartz  by  pyrite 596 

214.  Photomicrographs  of  thin  sections  of  sulphide  ore  from  Austinville,  Va. .     597 

215.  Section  through  the  Tuscon  shaft,  Leadville,  Colo.,  showing  replacement 

ore  bodies 597 

216.  Sketch  of  a  fissure  vein  indicating  how  deposition  may  take  place  on 

the  walls  of  the  fissure  or  by  replacement  of  the  wall  rock 604 

217.  Section  across  veins  of  Pennsylvania,  Rarus,  Mountain  View  and  West 

Colusa  mines,  Butte,  Mont 605 

218.  Vertical  section  showing  structure  of  a  residual  deposit  of  brown  ore 

from  Reed  Island,  Va 606 

219.  Section  through  Copper  Queen  mine,  Bisbee,  Ariz.,  showing  variable 

depth  of  weathering 607 

220.  Section  of  an  ore  body  showing  the  several  zones  that  may  be  developed 

by  weathering  and  secondary  enrichment 608 

221.  Section  of  ore  showing  precipitation  of  secondary  chalcocite  on  pyrite.  . .  610 

222.  Map  of  United  States  showing  physiographic  provinces 613 

223.  Map  showing  distribution  of  hematite  and  magnetite  in  the  United 

States 619 

224.  Map  showing  distribution  of  limonite  and  siderite  in  the  United  States.  .     621 

225.  Map  showing  distribution  of  lead  and  zinc  ores 628 


ENGINEERING  GEOLOGY 


CHAPTER  I 
THE  ROCK-FORMING  MINERALS 

Introduction.  —  Of  the  seventy-odd  elements  known  to  the  chemist 
only  sixteen  enter  largely  into  the  composition  of  the  outer  solid  portion 
of  the  earth  so  far  as  it  is  accessible  to  observation.  It  has  been  esti- 
mated that  98  per  cent  of  the  earth's  crust  is  made  up  of  eight  elements 
(Scott) .  Arranged  in  their  order  of  abundance  the  percentages  of  these 
elements,  as  calculated  by  Professor  F.  W.  Clarke,  are: 

Oxygen 47 . 17     Calcium 3 . 42 

Silicon 28.00     Potassium 2.49 

Aluminum 7.84     Sodium 2.43 

Iron 4 . 44     Magnesium 2 . 27 

Titanium,  carbon,  sulphur,  hydrogen,  chlorine,  phosphorus,  man- 
ganese and  barium,  are  much  less  abundant,  but  of  importance.  With 
only  few  exceptions  these  elements  occur  combined  with  each  other  form- 
ing compounds  called  minerals. 

All  rocks,  with  the  exception  of  the  glassy  igneous  ones,  are  composed 
of  minerals,  and  since  these  minerals  not  only  make  up  the  rocks  but 
vary  in  their  resistance  to  weather,  it  is  necessary  that  we  have  a  good 
knowledge  of  the  characters  and  properties  of  the  important  rock- 
forming  ones,  in  order  to  be  able  to  identify  rocks  and  judge  their  value. 
The  present  chapter  is  devoted  first,  to  an  account  of  the  general  proper- 
ties of  the  common  rock-forming  minerals  that  are  of  use  in  their  meg- 
ascopic determination,  and  second,  to  individual  descriptions  of  the 
more  important  rock-forming  minerals. 

Definition  of  a  mineral.  —  A  mineral  may  be  defined  as  any  natural 
inorganic  substance  of  definite  chemical  composition.  It  is  usually  a 
solid,  generally  having  definite  crystalline  structure,  and  may  or  may 
not  occur  bounded  by  crystal  faces.  As  a  rule  external  form  (crystal 
faces)  is  not  developed  in  minerals  as  they  occur  in  rocks,  but  usually 
as  crystalline  grains  marked  by  irregular  boundaries  or  outlines,  because 

1 


2  ENGINEERING  GEOLOGY 

of  interference  with  one  another  during  growth.  Crystalline  grains  are 
commonly  referred  to  as  anhedrons,  signifying  absence  of  crystal  faces. 
Altogether  about  a  thousand  definite  kinds  of  minerals  are  known;  but 
the  more  common  rock-forming  minerals  number  less  than  thirty. 

Definition  of  a  crystal.  —  A  crystal  may  be  defined  as  a  solid  bounded 
by  flat  and  somewhat  smooth  surfaces,  called  faces,  symmetrically 
grouped  about  imaginary  lines  as  axes.  By  axes  are  meant  imaginary 
lines  which  connect  the  centers  of  opposite  faces,  edges,  or  solid  angles, 
and  which  intersect  at  some  point  within  the  crystal.  Such  a  polyhedral 
form  results  when  the  molecules  of  that  particular  substance  of  definite 
chemical  composition  possess  such  freedom  of  movement  as  to  arrange 
themselves  according  to  mathematical  laws,  which  result  in  internal 
crystalline  structure  and  the  outward  expression  of  plane  surfaces  or 
faces.  Under  such  conditions  the  minerals  will  usually  crystallize  with 
outward  crystal  form,  such  as  cubes,  octahedrons,  prisms,  etc.  In  the 
formation  of  rocks  the  conditions  are  sometimes  present  which  permit  of 
definite  arrangement  of  the  molecules,  and  one  or  more  of  the  minerals 
assume  outward  crystal  form,  as  shown  in  certain  types  of  igneous  and 
metamorphic  rocks. 

The  number  of  crystal  forms  is  large  and  yet  when  they  are  grouped 
in  their  relations  to  the  crystallographic  axes  they  fall  into  six  systems. 
The  names  usually  given  to  the  six  systems  of  crystal  forms  and  their 
axial  relations  are: 

I.  Isometric  system  having  three  axes  of  equal  lengths  and  inter- 
secting one  another  at  right  angles. 

-  II.  Tetragonal  system  having  three  axes  intersecting  at  right  angles, 
the  two  lateral  axes  being  of  equal  lengths,  while  the  vertical  axis  is 
longer  or  shorter  than  the  two  lateral  ones. 

III.  Hexagonal  system  having  four  axes,  the  three  laterals  being  of 
equal  length  and  intersecting  at  angles  of  60°,  while  the  vertical  axis 
is  perpendicular  to  and  longer  or  shorter  than  the  three  laterals. 

IV.  Orthorhombic  system  having  three  axes  intersecting  at  right 
angles  and  of  unequal  lengths. 

V.  Monoclinic  system  having  three  axes  of  unequal  lengths,  the  two 
lateral  ones  at  right  angles  to  each  other,  while  the  vertical  axis  is 
oblique  to  one  of  the  laterals. 

VI.  Triclinic  system  having  three  axes  of  unequal  lengths  making 
oblique  intersections  with  one  another. 

Twinning.  —  Crystals  frequently  appear  not  to  be  simple  or  single 
forms  but  compound,  in  which  one  or  more  parts  regularly  arranged  are 
in  reverse  position  with  reference  to  the  other  part  or  parts  (Dana). 


THE  ROCK-FORMING   MINERALS  3 

This  peculiar  grouping  is  known  as  twinning,  the  different  members  of 
such  a  crystal  appearing  as  if  revolved  180°  about  a  line  known  as  the 
twinning  axis.  The  plane  normal  to  the  twin  axis  is  called  the  twinning 
plane,  and  the  plane  of  union  of  the  two  parts  is  called  the  composition 
plane.  Many  minerals  frequently  exhibit  twinning,  and  in  some  it 
serves  as  an  important  means  in  determining  them.  Feldspars  very 
often  show  several  kinds  of  twinning,  two  of  which  are  of  importance 
in  megascopic  determinations,  namely,  Carlsbad  and  albite  (multiple) 
twins  (see  Figs.  4  to  8,  pages  9  and  10).  Multiple  twinning  is  character- 
istic of  the  plagioclase  or  soda-lime  feldspars,  and  affords  the  surest 
means  of  distinguishing  them  from  orthoclase  (see  under  feldspar  group) . 
Carlsbad  twinning  may  be  developed  in  any  variety  of  feldspar,  but  is 
generally  more  frequent  in  orthoclase  than  in  plagioclase. 

General  Physical  Properties  of  Rock-making  Minerals 

The  important  physical  properties  of  rock-making  minerals  which 
are  of  value  in  their  megascopic  determination  are  hardness,  cleavage^ 
luster,  streak,  color,  crystal  form,  and  specific  gravity.  These  have  not 
equal  weight  in  determining  minerals.  The  behavior  of  minerals 
before  the  blowpipe  and  with  chemical  reagents  is  an  important  means 
of  determining  them  and  is  comprised  under  that  division  of  the 
subject  known  as  determinative  mineralogy. 

Hardness.  —  Hardness  is  an  important  property  of  minerals  and  is 
of  great  value  in  their  rapid  determination.  It  may  be  denned  as  the 
resistance  of  a  mineral  to  abrasion  or  scratching.  The  hardness  of 
minerals  is  usually  determined  by  comparing  with  Moh's  scale,  which 
comprises  ten  minerals  arranged  in  the  order  of  increasing  hardness, 
as  follows : 

1.  Talc  6.    Feldspar 

2.  Gypsum  7.   Quartz 

3.  Calcite  8.   Topaz 

4.  Fluorite  9.    Corundum 

5.  Apatite  10.   Diamond 

In  testing  the  hardness  of  a  mineral  care  must  be  taken  to  select  a 
fresh  fragment,  and  not  mistake  a  scratch  for  a  mark  left  by  a  soft 
mineral  on  the  surface  of  a  hard  one.  If  an  unknown  mineral  scratches 
and  in  turn  is  scratched  by  a  member  of  the  scale,  its  hardness  is  the 
same  as  that  of  the  scale  member.  Again  if  the  unknown  mineral 
scratches  fluorite  its  hardness  is  greater  than  4,  but  if  it  does  not  scratch 
apatite  and  is  scratched  by  it,  its  hardness  is  between  4  and  5,  approxi- 
mately 4.5. 


4  ENGINEERING  GEOLOGY 

In  the  absence  of  a  scale,  the  hardness  of  a  mineral  may  be  approxi- 
mated by  use  of  the  following  materials:  The  finger  nail  will  scratch 
gypsum  (2),  but  not  calcite;  a  copper  coin  will  just  scratch  calcite(3); 
and  the  blade  of  an  ordinary  pocket  knife  will  scratch  apatite  (5). 

Minerals  sometimes  show  different  degrees  of  hardness,  depending 
upon  the  direction  in  which  they  are  tested.  Thus  cyanite  shows  a 
hardness  of  4-5  when  scratched  in  one  direction,  and  of  7  at  right  angles 
to  this  direction. 

Cleavage.  —  When  properly  tested  most  minerals  exhibit  more  or 
less  readiness  to  part  or  cleave  along  one  or  more  definite  planes.  In 
most  minerals  possessing  crystalline  structure  the  molecules  are  so 
arranged  that  the  force  of  cohesion  is  less  along  a  particular  direction 
or  directions  than  along  others.  This  property  is  called  cleavage.  It 
is  a  fairly  constant  property  of  minerals  and  is  of  great  value  in  deter- 
mining them.  Cleavage  does  not  occur  at  random  in  a  mineral,  but 
is  always  parallel  to  possible  crystal  faces,  and  is  so  described.  Thus  we 
have  cubic  cleavage  (galena),  octahedral  cleavage  (fluorite),  rhombo- 
hedral  cleavage  (calcite),  prismatic  cleavage  (amphibole),  basal  cleav- 
age (mica).  All  minerals  do  not  possess  cleavage,  and  comparatively 
few  exhibit  it  in  an  eminent  degree.  Quartz  and  garnet  do  not  show 
cleavage,  but  such  minerals  as  feldspar,  amphiboles,  pyroxenes,  and 
calcite  are  distinguished  chiefly  by  their  cleavage.  Such  terms  as 
perfect,  imperfect,  good,  distinct,  indistinct  and  easy  are  frequently  used 
in  accordance  with  the  manner  and  ease  with  which  cleavage  is  obtained. 

Luster.  —  The  luster  of  a  mineral  is  the  appearance  of  its  surface 
in  reflected  light,  and  is  an  important  aid  in  the  determination  of  min- 
erals. Two  kinds  of  luster  are  recognized:  Metallic  luster,  the  luster 
of  metals,  most  sulphides,  and  some  oxides,  all  of  which  are  opaque  or 
•  nearly  so ;  nonmetallic  luster,  the  luster  of  minerals  that  are  transparent 
on  their  thin  edges,  and  in  general  of  light  color,  but  not  necessarily  so. 
The  more  common  nonmetallic  lusters  are  described  as  follows :  Vitreous, 
the  luster  of  glass;  example  quartz.  Resinous,  the  appearance  of  resin; 
example  sphalerite.  Greasy,  the  appearance  of  oil;  example  some 
sphalerite  and  quartz.  Pearly,  the  appearance  of  mother-of-pearl; 
example  talc.  Silky,  the  appearance  of  silk  (satin),  due  to  a  fibrous 
structure;  example,  satin  spar  and  asbestos.  Adamantine,  the  brilliant, 
shiny  luster  of  the  diamond.  Dull,  as  in  chalk  or  kaolin. 

Streak.  —  By  the  streak  of  a  mineral  is  meant  the  color  of  its 
powder.  It  is  frequently  one  of  the  most  important  physical  properties 
to  be  applied  in  the  determination  of  minerals,  such  as  hematite  and 
limonite.  The  color  of  a  mineral  in  mass  may  vary  greatly  from  that 


THE  ROCK-FORMING  MINERALS  5 

of  its  powder  (streak,  which  is  frequently  fairly  constant),  and  is  usually 
much  lighter.  The  streak  of  a  mineral  may  be  determined  by  crushing, 
filing,  or  scratching,  but  the  most  satisfactory  method  is  to  rub  the 
sharp  point  of  a  mineral  over  a  piece  of  white,  unglazed  porcelain. 
Small  plates,  known  as  streak  plates,  are  made  especially  for  this 
purpose. 

Streak  is  of  most  value  in  distinguishing  between  the  dark-colored 
minerals  like  the  metallic  oxides  and  sulphides,  and  is  of  less  value  in 
discriminating  between  the  light-colored  silicate  and  carbonate  minerals. 

Color.  —  Color  is  one  of  the  most  important  properties  of  minerals, 
and,  when  used  with  proper  precaution,  it  is  of  great  help  in  their  rapid 
determination.  The  color  of  metallic  minerals  is  a  constant  property; 
but  it  may  vary  greatly  in  many  of  the  nonmetallic  minerals,  due  to 
the  presence  of  pigments  or  impurities,  which  may  be  either  chemically 
combined  or  mechanically  admixed.  Even  the  metallic  minerals,  such 
as  the  sulphides  (pyrite,  marcasite  and  chalcopyrite)  whose  color  is 
constant,  are  susceptible  to  tarnish  (alteration),  and  a  fresh  surface 
should  always  be  examined  in  noting  the  color. 

The  color  of  minerals  is  dependent  upon  their  chemical  composition, 
in  which  case  it  may  be  natural,  or  it  may  be  due  to  some  foreign  sub- 
stance distributed  through  them  and  acting  as  a  pigment,  and  their 
color  may  then 'be  termed  exotic  (Pirsson).  Precaution  should  be  used, 
therefore,  in  the  latter  case  when  color  is  employed  in  the  determination 
of  minerals. 

When  pure,  the  acid  radicles,  silica  and  carbon  dioxide,  and  the  oxides 
alumina,  lime,  magnesia,  soda,  and  potash  are  colorless.  Hence,  when 
these  combine  to  form  the  corresponding  compounds,  silicates  and 
carbonates,  they  are  colorless  or  white.  Thus  quartz,  feldspar,  enstatite, 
tremolite,  calcite  and  dolomite,  when  pure,  are  colorless  or  white.  The 
introduction  of  the  metallic  oxides,  the  commonest  one  of  which  is  iron, 
will  influence  the  color,  and  according  to  its  quantity  the  mineral  will 
ordinarily  exhibit  some  shade  of  green,  brown,  or  even  black.  Examples 
among  the  silicate  minerals  are  the  iron-bearing  members  of  the  amphi- 
bole,  pyroxene,  and  mica  groups. 

Exotic  color,  as  previously  stated,  may  result  (1)  from  the  presence 
of  a  very  small  amount  of  some  compound  in  chemical  combination, 
such  as  manganese  oxide  in  quartz  imparting  an  amethyst  color;  or 
(2)  mechanically  admixed  impurities  such  as  small  amounts  of  hematite 
in  quartz  producing  the  red  variety  jasper. 

Crystal  form.  —  As  stated  in  a  preceding  paragraph  minerals  are 
usually  developed  in  rocks  as  crystalline  grains  without  definite  shape 


6 


ENGINEERING  GEOLOGY 


or  outward  crystal  form.  To  this  statement,  however,  there  are  fre- 
quent exceptions,  especially  in  the  group  of  porphyritic  rocks,  where  the 
conspicuously-developed  mineral  or  minerals  (phenocrysts)  frequently 
exhibit  crystal  boundaries.  When  minerals  exhibit  definite  shapes  crys- 
tal form  becomes  an  important  aid  for  their  determination.  Because 
of  the  fact,  however,  that  minerals  composing  rocks  are  more  often 
developed  without  crystal  boundaries,  crystal  form  is  less  important  as 
an  aid  in  determining  them  than  other  physical  properties. 

Specific  gravity.  —  The  specific  gravity  (density)  of  a  mineral  is 
its  weight  compared  with  that  of  an  equal  volume  of  water.  In  a  pure 
mineral  of  given  composition,  it  is  a  constant  factor,  and  is  an  important 
aid  in  identification.  The  specific  gravity  of  most  silicate  minerals 
lies  between  2.25  and  3.5;  of  minerals  with  metallic  luster  usually 
between  4.5  and  10;  and  of  natural-occurring  metals  as  high  as  23 
(iridium) . 

As  ordinarily  carried  out  in  the  laboratory,  the  determination  of  the 
specific  gravity  of  a  mineral  is  as  follows:  The  fresh  mineral  is  first 
weighed  in  air,  which  value  we  may  call  x.  It  is  then 
immersed  in  water  and  weighed  again,  and  the  value 
may  be  called  y.  Then  x  —  y  equals  the  loss  of  weight 
in  water,  or  the  weight  of  an  equal  volume  of  water. 
We  then  have 


G  = 


x-y 


,  G  being  the  specific  gravity. 


The  determination  of  specific  gravity  may  be  car- 
ried out  on  several  different  kinds  of  balances,  but 
one  of  the  most  convenient  forms  is  the  Jolly  balance, 
shown  in  Fig.  1.  The  time  required  for  the  whole 
determination  on  this  balance  should  not  exceed  sev- 
eral minutes. 

Fracture.  —  When  a  mineral  breaks  irregularly 
without  regard  to  definite  direction  it  is  described  as 
fracture.  The  appearance  of  a  fracture  surface  is 
somewhat  characteristic  and  is  commonly  designated 
by  the  following  terms:  Conchoidal,  when  the  surface 
presents  a  somewhat  shelly  appearance;  fibrous  or 
splintery,  when  the  surface  shows  fibers  or  splinters;  hackly,  when  the 
surface  is  irregular  with  sharp  edges;  uneven,  when  the  surface  is  rough 
and  irregular. 

Other   physical   properties   of  minor   importance   but   nevertheless 


FIG.  1. 


THE  ROCK-FORMING  MINERALS  7 

useful  at  times  in  the  determination  of  minerals  are  taste,  odor,  feel  or 
touch,  and  magnetism. 

Chemical  Tests.  —  Since,  from  the  definition  of  a  mineral,  chemical 
composition  is  its  most  fundamental  property,  chemical  tests  with  dry 
and  wet  reagents  form  the  safest  and  most  satisfactory  means  of  identi- 
fication. The  common  rock-forming  minerals,  however,  can  usually 
be  readily  and  quickly  determined  by  their  physical  properties,  and 
since  the  equipment  of  a  laboratory  is  not  available  in  the  field,  it  is 
essential  that  a  thorough  knowledge  of  the  physical  properties  of  min- 
erals be  obtained.  Tables  employing  both  physical  and  chemical  tests 
for  the  determination  of  minerals  are  to  be  found  hi  a  number  of  excellent 
manuals  on  determinative  mineralogy. 

Description  of  Rock-forming  Minerals 

The  number  of  known  minerals  is  large;  but  only  a  few  are  of  impor- 
tance as  rock-makers.  The  principal  ones  from  the  geological  stand- 
point may  be  grouped  under  silicates,  oxides,  carbonates,  sulphates, 
and  sulphides,  under  which  in  the  order  named  the  individual  minerals 
are  treated. 

SILICATES 

The  silicates  are  the  most  important  rock-forming  minerals,  since 
they  compose  the  largest  part  of  the  earth's  crust.  They  are  salts  of 
silicic  acids,  the  three  important  ones  being  orthosilicic  acid  (HaSiOi), 
metasilicic  acid  (H2SiO3),  and  polysilicic  acid  (H^isOg).  Many  of  the 
silicates  are  complex  in  composition,  and  the  chemical  formulae  for  some 
of  them  are  still  in  doubt.  The  silicates  that  are  of  most  importance 
as  rock-forming  minerals  are  the  feldspar,  feldspathoid,  pyroxene, 
amphibole,  mica,  olivine,  garnet,  tourmaline,  and  epidote  groups.  A 
few  less  common  ones  that  at  times  are  important  are  also  considered 
in  this  chapter. 

For  convenience  of  treatment  the  silicates  described  in  this  book  may 
be  divided  into  two  large  groups  as  follows:  A.  Anhydrous  silicates 
and  B.  Hydrous  silicates. 

A.   ANHYDROUS  SILICATES 
Feldspars 

Introduction.  —  Feldspar  is  a  family  name  and  not  that  of  a  single 
mineral.  It  constitutes  one  of  the  most,  if  not  the  most,  important 
group  of  rock-forming  minerals,  nearly  60  per  cent  of  the  earth's  crust 


8 


ENGINEERING  GEOLOGY 


being  composed  of  feldspar.     The  members  of  this  group  play  a  funda- 
mental role  in  the  classification  of  igneous  rocks. 

Composition.  —  The  species  included  under  the  group  name  are 
essentially  silicates  of  alumina  together  with  potash,  soda,  or  lime,  or 
their  mixtures.  The  rock-forming  feldspars  are  orthoclase  (microcline), 
albite,  and  anorthite,  together  with  their  mixtures.  These  may  be 
tabulated  as  follows: 

1.  Orthoclase   (microcline)    (KAlSi308),   a  silicate  of  alumina  and 
potash. 

2.  Albite  (NaAlSi308),  a  silicate  of  alumina  and  soda. 

3.  Anorthite  (CaAl2Si208),  a  silicate  of  alumina  and  lime. 
Mixtures  of  these  are: 

Alkalic  feldspar  ((KNa)AlSi308),  mixtures  of  1  and  2. 
Plagioclase  feldspar  (NaAlSi3O8(Ab)  +  CaAl2Si2O8(An)),  mixtures  of 
2  and  3. 

The  series  of  plagioclase  (soda-lime)  feldspars  includes  a  number  of  species  that 
are  isomorphous  mixtures  of  the  two  end  members  albite  (pure  soda  feldspar) 
NaAlSiaOs  (designated  Ab)  and  anorthite  (pure  lime  feldspar)  CaAl2Si2O8  (designated 
An). 

The  intermediate  members  of  this  series  are  mixtures  in  varying  proportions  of 
the  two  molecules  Ab  and  An,  as  shown  in  the  annexed  table. 

Plagioclase  Feldspars 

Albite AbiAno  to  AbeAni  Labradorite AbiAni  to  AbiAns 

Oligoclase. . .  Ab6Ani  to  Ab3Ani  Bytownite AbiAna  to  AbiAn6 

Andesine . . .  Ab3Ani  to  AbiAni  Anorthite AbiAne  to  Ab0Ani 

The  percentages  of  the  various  oxides  in  each  feldspar  variety  are  shown  in  the 
following  table: 

PERCENTAGES  OF  OXIDES  IN  THE  FELDSPARS  OF  THE   PLAGIOCLASE  GROUP 


Si02. 

A1203. 

Na20. 

CaO. 

Abi  Ano  .... 

68.7 

19.5 

11.8 

0.0 

AbeAni  

64.9 

22.1 

10.0 

3.0 

AbsAni  

62.0 

24.0 

8.7 

5.3 

AbiAm  

55.6 

28.3 

5.7 

10.4 

AbiAiia 

49  3 

32  6 

2  8 

15  3 

AbiAne 

46  6 

34  4 

1  6 

17  4 

Ab0  An  i  .  . 

43  2 

36  7 

0.0 

20  1 

The  potash  varieties  of  feldspar,  orthoclase  and  microcline,  repre- 
sented by  the  formula  KAlSi3O8  or  K2O.A12O3.6  SiO2,  can  not  be  dis- 
tinguished from  each  other  with  the  naked  eye,  and  may  be  regarded 


THE  ROCK-FORMING   MINERALS 


9 


as  identical,  and  the  two  designated  as  orthoclase.  In  most  cases  the 
feldspars  are  either  mixtures  (intimate)  of  orthoclase  and  albite  in 
varying  proportions,  with  the  former  usually  greatly  in  excess,  desig- 
nated as  alkalic  feldspar;  or  mixtures  of  albite  and  anorthite,  designated 
as  plagioclase  or  soda-lime  feldspar. 

Form.  —  The  feldspars  may  be  either  monoclinic  (orthoclase)  or 
triclinic  (microcline  and  plagioclase  group)  in  crystallization.  The 
crystals  may  be  stout  and  thick  (Fig.  2),  or  thin  and  tabular  (Fig.  3) 


FIG.  3. 


in  habit;  sometimes  long  and  columnar.  They  often  exhibit  a  tendency 
to  assume  crystal  form,  yet  perfect  crystals  are  rarely  observed  except 
when  developed  as  phenocrysts  in  porphyritic  igneous  rocks  (see  Chap- 
ter II) .  They  are  commonly  developed  in  rocks  as  formless  grains  with- 
out crystal  boundaries. 

Twinning.  —  Twinning  is  very  common  in  the  feldspars   (Figs.  4 
to  8)  and  is  an  important  means  of  distinguishing  between  the  potash 


FIG.  4. 


FIG.  5. 


(orthoclase)  and  soda-lime  (plagioclase)  varieties  with  the  unaided  eye. 
Carlsbad  twins  (Figs.  4  and  5),  the  name  being  derived  from  Carlsbad 
in  Bohemia  where  specimens  of  great  perfection  have  been  found,  are 
the  most  commonly-occurring  forms  in  orthoclase.  Multiple  (polysyn- 


10 


ENGINEERING  GEOLOGY 


thetic  or  albitic)  twinning  (Figs.  6  to  8),  which  results  in  the  cleavage 
surface  of  the  twinned  feldspar  being  marked  by  parallel  striations,  is 
characteristic  of  the  soda-lime  (plagioclase)  series,  and  when  visible  to 
the  unaided  eye  it  affords  the  surest  proof  that  the  feldspar  belongs  to 
the  plagioclase  group.  This  form  of  twinning  is  crystallographically 
impossible  in  orthoclase.  If  present  and  visible  to  the  naked  eye,  the 
striations  are  readily  observed  by  turning  the  crystal  or  grain  in  the 


FIG.  6. 


FIG.  7. 


sunlight,  so  as  to  catch  the  reflection  from  the  cleavage  face.  Other 
forms  of  twinning  in  feldspars  occur;  but  are  of  little  or  no  importance 
in  their  megascopic  determination. 

Cleavage.  —  All  species  of  feldspar  possess  good  cleavage  in  two  direc- 
tions, which  intersect  either  at  90°  as  in  orthoclase,  or  at  about  86°  as 
in  the  plagioclase  series.  The  difference,  however,  in  angle  of  intersec- 
tion of  the  cleavages  is  too  small  to  be  of  use  in  distinguishing  between 
plagioclase  and  orthoclase  by  the  unaided  eye,  unless  accurately  meas- 
ured. If  the  feldspar  grains  as  developed  in  rocks  are  of  sufficient  size, 
the  cleavages  can  be  readily  observed  by  reflected  light. 

Physical  properties.  —  Fracture  of  feldspars  in  directions  other 
than  those  of  cleavage  is  uneven,  usually  poorly  developed.  Brittle. 
Hardness  6.  Specific  gravity  varies  with  chemical  composition :  Ortho- 
clase =  2.55,  albite  =  2.62,  anorthite  =  2.76;  the  other  species  (mix- 
tures) vary  between  these  limits.  Luster  vitreous;  on  cleavage  faces 
often  pearly.  Streak  white,  not  characteristic.  The  feldspars  exhibit 
a  variety  of  color.  Colorless,  sometimes  transparent  and  glassy,  white, 
gray,  red,  and  green.  In  rocks,  colorless  and  glassy  feldspars  are  limited 
to  the  fresh  and  recent  lavas.  Some  shade  of  red  is  common  to  ortho- 
clase and  the  alkalic  feldspars,  while  the  plagioclase  or  soda-lime  feld- 
spars are  commonly  gray  or  white.  Feldspar  is  frequently  the  dominant 
coloring  mineral  in  granites. 


THE  ROCK-FORMING  MINERALS  11 

Chemical  tests.  —  Orthoclase  and  albite  are  insoluble  in  ordinary  acids,  but 
with  increase  in  lime  in  the  plagioclase  group  they  become  slowly  soluble  (labradorite 
to  anorthite).  The  lime-rich  varieties  fuse  more  easily  than  do  albite  and  orthoclase. 

Alteration.  —  Feldspars  commonly  alter  to  kaolin  in  the  belt  of 
weathering,  when  acted  on  by  water  containing  carbon  dioxide,  with 
the  separation  of  free  silica  and  alkaline  carbonates.  Alteration  of  the 
lime-bearing  species  is  apt  to  be  accompanied  by  the  formation  of 
calcite.  Under  conditions  of  dynamic  metamorphism  (see  Chapter  III)  , 
or  in  the  presence  of  hot  waters,  potash  feldspar  commonly  alters  to 
muscovite  (sericite).  Alteration  of  feldspars  involving  the  formation 
of  kaolin1  is  of  much  importance  in  the  formation  of  soils.  The  process 
is  described  as  kaolinization,  and  is  first  noted  in  feldspars  by  the  loss 
of  luster,  and  the  mineral  becoming  dull  and  chalky  or  earthy  in  appear- 
ance. Usually  in  the  feldspar-bearing  rocks  used  for  building  and 
ornamental  purposes,  it  has  been  observed  that  the  lime-soda  feldspars 
are  more  susceptible  to  alteration  than  orthoclase.  Both  orthoclase 
and  plagioclase  are  less  durable  than  quartz,  with  which  they  are  fre- 
quently associated,  but  they  are  not  to  be  regarded  as  unsafe  on  this 
account. 

The  changes  involved  in  the  alteration  of  feldspar  to  kaolin  and 
muscovite  have  been  expressed  chemically  as  follows  (Pirsson)  : 

Orthoclase  Water        Garb.  diox.        Kaolin  Quartz        Potas.  carb. 

2KAlSi3O8    +     2H2O     +     CO2     =       BUAlgSijO,   +     4  SiO2     +    K2CO3 

Orthoclase  Water         Carb.  diox.      Muscovite  Quartz         Potas.  carb. 

H2O       +     CO2     =  H2K(AlSiO4)3  +      5  SiO2     +      K2CO3 


Other  forms  of  alteration  of  feldspars  are  known. 

Occurrence.  —  The  feldspars  are  probably  more  widely  distributed 
than  any  other  group  of  rock-forming  minerals.  They  occur  in  most  of 
the  igneous  rocks,  such  as  granites,  syenites,  and  lavas  ;  in  certain  sand- 
stones and  conglomerates  among  sedimentary  ones;  and  in  gneisses  of 
the  metamorphic  rocks.  Hence  feldspar  is  an  important  constituent  of 
many  building  stones. 

Determination.  —  The  two  cleavages  of  90°  or  nearly  so,  hardness, 
luster,  and  color  usually  serve  to  distinguish  the  feldspars  from  other 
minerals  which  they  closely  resemble.  When  observed,  the  striations  on 
good  cleavage  surfaces  are  the  surest  means  of  distinguishing  plagio- 
clase or  soda-lime  feldspars  from  orthoclase.  It  is  not  safe,  however, 
in  all  cases  to  conclude  that  a  feldspar  which  does  not  exhibit  striations 

1  Kaolin  may  sometimes  be  formed  in  other  ways.  See  Ries:  "Clays,  their 
Occurrence,  Properties,  and  Uses." 


12  ENGINEERING  GEOLOGY 

is  orthoclase,  for  the  twinning  is  frequently  so  fine  that  the  lines  cannot 
be  detected  even  with  the  aid  of  a  good  pocket  lens. 

Feldspathoid  Group 

Like  the  feldspars  the  members  of  the  feldspathoid  group  are  silicates  of  alumina 
with  soda,  potash,  and  lime.  Unlike  the  feldspars  they  are  greatly  restricted  in 
occurrence  and  are  comparatively  rare,  being  found  only  in  certain  kinds  of  igneous 
rocks,  such  as  nephelite  syenites.  Nephelite  and  sodalite  are  the  two  most  important 
members  of  the  group.  These  are  briefly  described  below. 

Nephelite 

Composition.  —  Sodium-aluminum  silicate,  chiefly 'NaAlSiO4,  with  a  few  per  cent 
of  potash  present  replacing  soda;  sometimes  also  lime. 

General  properties.  —  Hexagonal  in. crystallization;  commonly  without  crystal 
form  as  shapeless  grains  and  masses.  Cleavage  sometimes  distinct,  usually  not 
good.  Fracture  somewhat  conchoidal.  Brittle.  Hardness,  5.5-6.  Specific  gravity 
2.55-2.65.  Luster  vitreous  to  greasy.  White,  gray,  and  yellowish,  sometimes  red- 
dish. Streak  light.  Fusible  before  the  blowpipe.  Is  readily  soluble  in  dilute  acid, 
and  on  evaporation  yields  gelatinous  silica.  It  easily  alters  into  various  minerals, 
similar  to  the  feldspars.  It  occurs  in  some  lavas  and  in  certain  kinds  of  syenite. 

Sodalite 

Composition.  —  Na4(AlCl)Al2(SiO4)3  or  3  NaAlSi04  •  NaCl. 

General  properties.  —  Isometric  in  crystallization;  crystals  rare;  usually  occurs 
in  rocks  as  shapeless  grains.  Cleavage  dodecahedral,  but  of  little  value  in  mega- 
scopic determination.  Fracture  uneven.  Hardness  5.5-6.  Specific  gravity,  2.15-2.3. 
Luster  vitreous,  sometimes  greasy.  Color  usually  blue,  also  white,  gray,  green. 
Streak  white.  Fusible  before  the  blowpipe  to  a  colorless  glass.  Soluble  in  dilute  acids, 
and  on  evaporation  yields  gelatinous  silica.  Nitric  acid  solution  with  silver  ni- 
trate, gives  a  white  precipitate  of  silver  chloride.  It  is  a  comparatively  rare  rock 
mineral,  being  restricted  in  occurrence  to  nephelite-syenites,  trachytes,  and 
phonolites. 

Mica  Group 

Composition.  —  Of  the  many  species  included  in  the  mica  group  the 
more  important  ones  are: 

Muscovite  H^KAlsCSiO^s.     Potash  mica. 
Lepidolite  KLi[A1.2(OH,F)]Al(SiO3)3.     Lithia  mica. 
Biotite  (H,K)2(Mg,Fe)2Al2(Si04)3.     Iron-magnesia  mica. 
Phlogopite  H2KMg3Al(SiO4)3(?).     Magnesia  mica. 
Lepidomelane  (H,K)2Fe3(Fe, Al)  4(SiO4)  6(?) .     Iron  mica. 

As  illustrated  in  the  above  tabulation,  the  micas  form  a  group  of 
complex  silicates  (orthosilicates)  of  aluminum  with  potassium  and 
hydrogen,  magnesium,  iron,  and  lithium.  Other  species  belonging  to  the 


THE  ROCK-FORMING  MINERALS  13 

mica  group  but  not  listed  above  show  the  presence  of  other  elements, 
such  as  sodium,  manganese,  chromium,  etc.  For  megascopic  study  the 
micas  may  be  conveniently  classified  into  (a)  light  colored  micas  (mus- 
covite)  and  related  varieties,  and  (6)  dark  colored  micas  (biotite)  and 
related  varieties. 

Form.  —  The  micas  form  an  isomorphous  series  crystallizing  in  the 
monoclinic  system.  The  crystals  are  tabular  in  form,  often  of  hexagonal 
outline,  with  flat  bases.  Crystals  are  sometimes  observed  in  rocks, 
but  the  micas  more  often  occur  as  flecks,  scales,  or  shreds,  without 
crystal  boundaries. 

Physical  properties.  —  All  micas  are  characterized  by  very  perfect 
basal  cleavage,  which  allows  them  to  be  split  into  extremely  thin  elastic 
plates  or  laminae,  that  are  tough  and  flexible.  This  property  combined 
with  transparency,  toughness,  and  flexibility,  makes  the  large  sheets 
of  muscovite  of  much  value  for  use  in  stove  windows,  lamp  chimneys, 
electrical  work,  etc. 

The  micas  have  a  wide  range  of  color,  dependent  chiefly  on  chemical 
composition.  Colorless,  white,  gray,  green,  violet  or  lilac  to  red, 
yellowish  to  brown,  and  black.  Muscovite  is  colorless,  white  to  gray, 
sometimes  greenish  to  light  brown;  lepidolite  is  usually  of  pink  or  lilac 
color;  biotite  is  usually  brown  to  black,  sometimes  dark  green;  phlogo- 
pite  is  pale  brown,  often  coppery;  and  lepidomelane  is  black  to  greenish 
black.  The  color  of  mica  frequently  exerts  an  important  effect  on  build- 
ing and  ornamental  stones  containing  it.  Luster  vitreous  to  pearly  or 
silky;  sometimes  splendent  in  the  dark-colored  varieties.  Streak 
uncolored.  Hardness  2-3;  easily  scratched  with  the  knife.  Specific 
gravity  2.7-3.2. 

Chemical  tests.  —  Before  the  blowpipe  the  micas  vary  from  easily  (lepidolite)  to 
difficultly  (biotite)  fusible.  They  yield  little  or  no  water  when  heated  in  a  closed 
glass  tube,  which  aids  in  distinguishing  them  from  other  micaceous  minerals,  such 
as  the  chlorites.  Most  of  the  micas  are  insoluble  in  hydrochloric  acid,  but  when 
boiled  in  sulphuric  acid  the  dark-colored  ones  (biotite,  etc.)  are  decomposed  and  give 
milky  solutions. 

Alteration.  —  Most  micas  are  susceptible  to  alteration  when  sub- 
jected to  the  action  of  weathering  processes.  Some  alter  more  readily 
than  others,  dependent  upon  their  chemical  composition.  Muscovite  is 
very  resistant,  being  often  tunes  the  product  of  alteration  from  other 
minerals,  especially  the  feldspars  (then  in  minute  scales  of  silvery  white 
color  and  silky  luster,  and  called  sericite),  but  it  ultimately  loses  its 
elasticity  and  is  probably  changed  to  clay.  Biotite  on  account  of  its 
high  iron  content  is  more  liable  to  decompose  on  exposure  to  weather. 


14  ENGINEERING  GEOLOGY 

Because  of  this  fact  the  alteration  of  biotite  in  some  building  stones  may 
cause  unsightly  discoloration  at  times  from  the  liberation  of  free  iron 
oxide.  This  is  frequently  observed  in  the  natural  outcrops  of  many 
granites,  and  it  not  infrequently  happens  that  on  the  opening  of  a  new 
quarry  failure  to  strip  the  stone  below  the  depth  of  oxidation,  an  in- 
ferior rock  (sappy  granite)  has  been  placed  on  the  market.  The  com- 
monest alteration  of  biotite,  however,  is  to  chlorite  (see  p.  27),  when  it 
loses  its  elasticity,  becomes  soft  and  of  a  green  color.  Other  members  of 
the  mica  group  alter  under  similar  conditions  into  different  mineral 
products,  according  to  their  composition. 

Occurrence.  —  The  commoner  micas,  muscovite  and  biotite,  have 
wide  distribution  in  rocks.  They  are  abundant  constituents  of  both 
igneous  and  metamorphic  rocks,  and  are  components  of  some  sedimen- 
tary ones,  especially  sandstones.  Muscovite  is  a  common  constituent  of 
granites  and  some  syenites,  and  especially  pegmatites,  where  it  is  found 
in  blocks  and  sheets  of  large  enough  size  to  be  used  for  the  purposes 
mentioned  above.  It  is  abundant  in  the  metamorphic  rocks,  especially 
in  mica  schists,  often  being  the  main  constituent,  and  in  gneisses. 
Muscovite  is  frequently  a  secondary  mineral,  often  called  sericite  and 
having  silky  luster,  derived  from  feldspars  and  minerals  of  similar 
composition.  The  alteration  process  is  called  sericitization  (see  Ore- 
Deposits.) 

Biotite  occurs  in  many  kinds  of  igneous  and  metamorphic  rocks.  It 
is  a  much  less  frequent  constituent  of  sedimentary  rocks  because  of 
its  ready  susceptibility  to  alteration  on  account  of  its  iron  content.  It 
occurs  in  many  granites,  diorites,  gabbros,  syenites,  and  peridotites, 
as  well  as  in  their  fine-grained  equivalents.  In  metamorphic  rocks  it 
is  a  common  mineral  in  schists  and  gneisses,  and  is  frequently  developed 
in  contact  metamcrphic  zones  (see  Chapter  III). 

The  other  varieties  of  mica  are  less  abundant  and  are  more  restricted 
in  distribution.  Lepidolite  occurs  chiefly  in  granite  pegmatites ;  phlogo- 
pite  principally  in  crystalline  limestones;  and  lepidomelane  is  found 
in  granites  and  syenites,  especially  their  pegmatite  equivalents. 

The  kind,  quantity,  and  mode  of  distribution  of  mica  in  building 
stones,  exert  an  important  influence  on  their  durability  and  work  ability. 
When  present  in  abundance  and  the  shreds  have  parallel  arrangement, 
the  rock  may  split  readily  along  this  direction.  In  quantity  mica  is  an 
undesirable  component  of  marble  since  it  is  apt  to  weather  out  and  leave 
a  pitted  surface.  It  also  interferes  at  times  with  the  production  of  a 
good  polish.  Although  some  building  stones,  such  as  granite,  etc.,  are 
rarely  free  from  mica,  it  is  not  an  injurious  constituent  unless  present 


THE  ROCK-FORMING  MINERALS  15 

in  large  quantity,  or  segregated  into  large  and  small  areas  through  the 
stone  as  "  knots"  rendering  the  rock  unsightly  and,  therefore,  undesir- 
able for  some  uses. 

Determination.  —  Megascopically,  the  micas  may  be  generally  dis- 
tinguished from  other  minerals  by  their  very  perfect  basal  cleavage, 
yielding  very  thin  elastic,  tough,  and  flexible  laminae;  by  their  luster 
and  hardness. 

Pyroxene  Group 

Composition.  —  The  pyroxene  group  includes  a  number  of  related 
species  that  are  important  as  rock-making  minerals.  They  are  meta- 
silicates,  salts  of  metasilicic  acid,  H2SiO3,  in  which  hydrogen  (H2)  is 
replaced  by  calcium,  magnesium,  and  ferrous  iron  as  the  important 
bases;  sometimes  manganese  and  zinc.  Certain  other  molecules  con- 
tain the  alkalies,  and  aluminum  and  ferric  iron. 

RSiO3  with  R  =  Ca,  Mg,  Fe;  also  Mn,  Zn. 
RR2SiO6  with  R  =  Mg,  Fe;   R  =  Fe,  Al. 
RR(SiO3)2  with  R  =  Na,  Li;   R  =  Al,  Fe. 
v- 

The  more  important  varieties  of  pyroxenes  as  rock-making  minerals 

are: 

Orthorhombic  section  : 

Enstatite,  MgSiO3. 

(Bronzite). 
Hyper  sthene,  (Mg,Fe)SiO3. 

Monoclinic  section: 


^.       ., 
D^ops^de, 

Auaite      HCa(Mg,Fe)(Si03)2], 
U(Mg,Fe)(Al,Fe)2Si06], 
sometimes  Na(Al,Fe)(SiO3)2. 
Aegirite,      NaFe(SiO3)2,  mostly. 
(Acmite). 

Members  of  the  triclinic  section  are  of  no  importance  megascopically 
as  rock-forming  minerals. 

Form.  —  Pyroxenes  belong  to  three  systems  of  crystallization, 
orthorhombic,  monoclinic,  and  triclinic,  but  only  members  of  the  ortho- 
rhombic  and  monoclinic  systems  are  of  importance  megascopically  as 
rock-making  minerals.  They  all  agree  in  general  crystal  habit,  a  prism 
with  an  angle  of  about  93°  and  87°;  usually  short,  stout,  prismatic,  or 


16 


ENGINEERING  GEOLOGY 


columnar  (Figs.  9  and  10).  A  cross  section  of  the  prism  form  is  usu- 
ally octagonal  in  outline  as  shown  in  Fig.  11.  (Compare  with  cross 
section  of  hornblende,  p.  18.)  As  rock-forming  minerals  pyroxenes 
are  commonly  developed  in  shapeless  grains  and  masses. 


FIG.  9. 


FIG.  10. 


Physical  properties.  —  The  cleavage  is  usually  very  good,  developed 
in  two  directions  parallel  to  the  prism  faces,  intersecting  at  an  angle  of 
87°  (Fig.  13).  It  is  a  fundamental  property  and  serves  to  distinguish 
pyroxenes  from  the  amphiboles.  Parting  in  other  directions  is  often 
developed  in  some  varieties.  Fracture  uneven.  Brittle.  Hardness 
5-6.  Specific  gravity,  3.2-3.6. 


FIG.  11. 


FIG.  12. 


The  color  varies  according  to  the  amount  of  iron  present;  white  to 
gray  and  pale  green  in  enstatite  and  diopside;  dark  brownish  green, 
greenish  black,  and  brown  in  bronzite;  various  shades  of  green  to  black 
in  augite;  black  and  greenish  black  in  aegirite.  Luster  vitreous  to 
resinous,  sometimes  pearly.  Streak  varies  from  white  and  uncolored 
to  brownish  gray  and  grayish  green. 

Chemical  tests.  —  Fusibility  and  solubility  vary  with  the  amount  of  iron  present. 
Enstatite  is  almost  infusible,  other  varieties  much  more  fusible.  They  are  but 
slightly  acted  upon  by  acids,  the  iron-rich  varieties  usually  being  most  affected. 

Alteration.  —  The  pyroxenes  alter  more  or  less  readily  into  different 
mineral  products,  dependent  partly  upon  the  kind  of  process  and 
partly  upon  their  composition.  Under  the  action  of  weathering  ser- 
pentine and  chlorite  are  common  alteration  products  of  the  magnesium- 
and  iron-bearing  varieties,  often  accompanied  by  carbonates  and  iron 
oxides  (limonite).  Another  form  of  alteration  of  the  pyroxenes  that 


THE  ROCK-FORMING  MINERALS  17 

is  of  very  great  geologic  importance  is  into  amphiboles,  which  takes 
place  under  metamorphism  (especially  regional). 

Occurrence.  —  The  pyroxenes  are  chiefly  found  in  igneous  rocks, 
occurring  only  sparingly  in  the  quartzose  ones,  but  become  more  abun- 
dant in  the  less  siliceous  ferromagnesian  kinds,  such  as  the  basaltic 
lavas,  gabbros,  and  peridotites  (see  Chapter  II).  They  are  less  common 
in  metamorphic  rocks,  several  varieties  being  noted  in  some  crystalline 
limestones  and  gneisses.  They  are  also  found  in  contact  zones  associ- 
ated with  garnet,  but  are  rarely  if  ever  found  hi  sedimentary  beds. 
They  are  not  very  important  in  the  common  building  stones,  and  when 
present  in  quantity  and  of  the  brittle  variety  they  may  interfere  with 
the  production  of  a  smooth  polish. 

Determination.  —  Crystal  form  and  habit  when  in  well-defined 
crystals,  outline  (octagonal)  of  cross  section  of  prism  form,  and  good 
cleavage  in  two  directions  intersecting  at  87°,  are  the  most  important 
megascopic  properties  by  which  pyroxenes  may  be  distinguished  from 
those  minerals  they  may  closely  resemble.  They  may  be  compared 
with  hornblende,  tourmaline,  and  epidote.  In  fine-grained  igneous  rocks 
it  is  usually  impossible  to  distinguish  between  pyroxene  and  amphibole 
megascopically.  When  of  sufficient  size  the  following  points  should  be 
observed:  Crystal  form  when  hi  distinct  crystals,  outline  of  cross 
section  of  the  prismatic  form,  angle  made  by  intersection  of  the  two 
prismatic  cleavages;  also  perfection  of  cleavage  which  is  usually  less 
perfect  in  pyroxene  than  in  hornblende.  Pyroxene  commonly  occurs 
in  short,  stout  prismatic  forms  or  grains,  while  hornblende  is  developed 
in  needles  or  long  bladed  forms.  Lack  of  cleavage,  triangular  outline 
of  cross  section  of  prism,  superior  hardness,  and  high  luster,  distinguish 
tourmaline  from  pyroxene.  Epidote  can  usually  be  distinguished  by 
unequal  cleavage  development  in  two  directions,  one  perfect,  the  other 
good,  by  its  characteristic  yellow-green  color,  and  by  its  greater  hardness. 

Amphibole  Group 

Composition.  —  The  amphiboles  form  a  strikingly  parallel  group 
of  minerals  to  the  pyroxenes,  the  two  groups  having  similar  chemicd 
compositions  and  physical  properties.  Like  the  pyroxenes  the  am- 
phiboles are  salts  of  metasilicic  acid  (H2SiO3)  in  which  hydrogen  (H2) 
is  replaced  by  certain  metals  and  radicles. 

RSi03  with  R  =  Ca,  Mg,  Fe,  chiefly;    also  Mn,  Na-j,  K2,  and  H2. 


with      =  A1  and  Fe> 
RR(SiO3)2,  with  R  =  Na,  and'R  =  Al,  Fe. 


18 


ENGINEERING  GEOLOGY 


For  megascopic  purposes  the  important  varieties  of  amphibole  are: 

Tremolite,  CaMg3(Si03)4. 

Actinolite,  Ca(Mg,  Fe)3(SiO3)4. 

(Ca(Mg,Fe)2(SiO3)3,  with 
(Na2Al2(SiO3)4and  (Mg,Fe)  (Al,Fe)2Si06. 
,  Na8(Ca,Mg)3(FeJMn)i4(Al;Fe)2Si2i045. 


Hornblende, 


Form.  —  In  crystallization,  amphiboles  like  pyroxenes  are  ortho- 
rhombic,  monoclinic,  and  triclinic.  Of  these  three  systems,  however, 
only  the  monoclinic  varieties  of  amphiboles  are  of  megascopic  impor- 
tance as  rock-making  minerals.  All  amphiboles  agree  in  general  habit 


FIG.  15. 


and  in  having  a  prismatic  cleavage  of  55  and  125  degrees.  They  gen- 
erally occur  in  long  and  bladed  forms,  sometimes  fibrous  and  columnar 
(Figs.  14,  15,  and  16),  and  as  shapeless  grains  and  masses.  The  outline 
of  a  cross  section  of  a  prism  form  is  usually  hexagonal  as  shown  in  Fig. 
17. 


FIG.  16. 


FIG.  17. 


Physical  properties.  —  Amphiboles  have  two  directions  of  perfect 
cleavage  parallel  to  the  prism  faces  which  intersect  at  angles  of  125  and 
55  degrees  as  shown  in  Fig.  17.  The  cleavage  angle  is  one  of  the  most 
distinguishing  characteristics  of  the  group.  Compare  Fig.  17  show- 
ing cleavage  of  amphibole  with  Fig.  13  which  shows  the  cleavage  of 


THE  ROCK-FORMING  MINERALS  19 

pyroxene.  Fracture  uneven.  Hardness  5-6.  Specific  gravity  2.9-3.5, 
according  chiefly  to  the  amount  of  iron  present. 

The  color  of  amphiboles  varies,  according  to  the  amount  of  iron 
present,  from  white  or  gray  in  tremolite  to  bright  green  or  grayish  green 
in  actinolite,  to  dark  green  and  black  in  hornblende,  and  black  in  arfved- 
sonite.  Luster  vitreous  to  pearly  on  cleavage  faces;  often  silky  in 
fibrous  varieties.  Streak  uncolored  or  grayish  to  gray-green  and 
brownish. 

Chemical  tests.  —  The  amphiboles  fuse  rather  easily  before  the  blowpipe,  but  are 
only  slightly  acted  on  by  ordinary  acids.  The  iron-rich  varieties  are  the  most  easily 
fusible  and  show  the  greatest  solution  effect  from  acids. 

Alteration.  —  Since  the  amphiboles  have  the  same  chemical  com- 
position as  the  pyroxenes  they  show  similar  alteration  under  the  action 
of  weathering  agencies.  The  commonest  changes  being,  according  to 
composition,  into  serpentine  or  chlorite,  or  both,  usually  accompanied 
by  carbonates,  quartz,  and  epidote.  Eventually  they  may  break  down 
into  carbonates,  iron  oxides,  and  quartz. 

Occurrence.  —  Amphiboles  have  abundant  and  wide  distribution  in 
igneous  and  metamorphic  rocks;  some  varieties  being  wholly  metamor- 
phic  or  secondary.  Tremolite  and  actinolite  are  secondary  or  meta- 
morphic minerals;  the  former  occurring  chiefly  in  impure  crystalline 
limestones  and  in  contact  zones,  the  latter  in  crystalline  schists.  They 
also  occur  as  common  products  of  alteration  in  igneous  rocks.  Owing 
to  its  tendency  to  decompose  tremolite  is  a  detrimental  mineral  in 
crystalline  limestones.  Hornblende  occurs  both  hi  igneous  and  meta- 
morphic rocks.  In  igneous  rocks  it  is  a  common  constituent  in  granite, 
syenite,  diorite,  some  varieties  of  peridotite,  and  in  many  of  the  fine- 
grained igneous  types.  It  is  often  a  secondary  mineral  derived  from 
pyroxene  by  metamorphic  processes  when  it  is  known  as  the  variety 
uralite.  In  the  metamorphic  rocks  it  occurs  in  gneisses  and  schists. 
Arfvedsonite  is  more  restricted  in  distribution,  being  found  chiefly  in 
nepheline  syenites  and  related  rare  porphyries. 

Determination.  —  The  most  characteristic  megascopic  properties  of 
amphibole  are  crystal  form  and  habit,  two  good  prismatic  cleavages 
making  angles  of  125°,  and  outline  (hexagonal)  cross  section  of  the 
prism  form.  Amphiboles  may  be  confused  megascopically  with  py- 
roxene, tourmaline,  and  epidote.  The  distinction  from  pyroxene  is 
given  under  the  latter  mineral  (page  17).  It  may  be  readily  dis- 
tinguished from  tourmaline  by  good  cleavage  and  outline  (hexagonal) 
of  the  prism  cross  section;  from  epidote  by  two  good  cleavages,  color, 
and  inferior  hardness. 


20  ENGINEERING  GEOLOGY 

Garnet  Group 

Composition.  —  Garnets  are  orthosilicates  corresponding  to  the 
general  formula  fcS^SiOi)*  °r  3RO.R2O3.3SiO2,  in  which  R  =  Ca, 
Mg,  Fe,  Mn;  and  R  =  Al,  Fe,  Mn,  Cr,  Ti.  The  group  has  been 
divided  into  a  number  of  varieties  which  vary  considerably  in  composi- 
tion, but  the  most  common  ones  that  are  of  importance  as  rock  minerals 
are: 

Grossularite,  Ca3Al2(Si04)3. 

Pyrope,  Mg3Al2(SiO4)3. 

Almandite  (common  garnet),  Fe3Al2(Si04)3. 

Andradite  (melanite),  Ca3Fe2(Si04)3. 

Form.  —  Garnets  crystallize  in  the  isometric  system  commonly  as 
rhombic  dodecahedrons  or  icositetrahedrons  (Figs.  18  and  19),  rarely  as 
octahedrons;  sometimes  in  combination  of  the  first  two  (Fig.  20).  They 
very  often  occur  in  rocks  without  crystal  boundaries  as  grains  and  gran- 
ular aggregates  having  rounded  or  irregular  outlines. 


FIG.  18.  FIG.  19.  FIG.  20. 

General  properties.  —  The  cleavage  is  generally  poorly  developed 
and  of  no  value  as  a  megascopic  feature.  Fracture  subconchoidal  to 
uneven.  Brittle.  Hardness  6.5-7.5.  Specific  gravity  3.15-4.3,  varying 
with  the  composition,  common  garnet  being  4.0.  Color  is  variable  ac- 
cording to  the  composition.  Grossularite,  colorless  to  white,  pale  shades 
of  pink,  yellow,  green,  and  brown;  pyrope,  deep  red  to  nearly  black; 
almandite,  deep  red  to  brownish  red;  melanite,  a  variety  of  andradite,  is 
black.  Streak  white.  Luster  vitreous,  sometimes  inclining  to  resinous. 

Chemical  tests.  —  The  garnets  fuse  readily  before  the  blowpipe.  They  are  only 
slightly  acted  upon  by  acids,  except  andradite  which  is  attacked  quite  strongly. 
When  evaporated  the  acid  solution  yields  gelatinous  silica. 

Alteration.  —  Some  garnets  are  quite  resistant  to  atmospheric 
agencies.  Dependent  upon  composition  they  may  alter  to  chlorite  or 
serpentine,  less  frequently  to  hornblende.  Of  the  different  known 


THE  ROCK-FORMING  MINERALS  21 

minerals  into  which  common  garnet  alters,  chlorite  is  the  commonest. 
Alteration  of  those  varieties  containing  iron  may  be  accompanied  by 
limonite  as  one  of  the  products. 

Occurrence.  —  Garnets  have  wide-spread  distribution  as  accessory 
constituents  of  metamorphic  and  sometimes  igneous  rocks.  The  dif- 
ferent varieties  are  unequally  distributed  as  rock  minerals,  some  being 
more  restricted  than  others.  Grossularite  is  chiefly  found  in  crystalline 
limestones  resulting  both  from  contact  and  regional  metamorphism. 
Pyrope  occurs  in  some  basic  igneous  rocks,  peridotites  and  the  serpen- 
tines derived  from  them.  Almandite  is  especially  found  in  schists  and 
gneisses,  sometimes  in  pegmatites,  rarely  in  granites,  and  in  zones  of 
contact  metamorphism.  Andradite,  variety  melanite,  is  restricted  hi 
distribution  to  certain  types  of  igneous  rocks.  It  is  a  common  mineral 
in  contact  metamorphic  ore-deposits.  It  is  not,  however,  a  very  im- 
portant megascopic  mineral. 

Determination.  —  The  more  important  megascopic  characters  of  the 
garnets  by  which  they  may  be  recognized  from  other  minerals  are: 
Crystal  form,  lack  of  cleavage,  luster,  color,  and  hardness. 

Olivine  Group 

Composition.  —  Olivine  (chrysolite)  is  an  orthosilicate  correspond- 
ing to  the  general  formula  R2SiO4,  in  which  R  =  Mg,  Fe.  It  may  be 
regarded  as  a  variable  mixture  of  magnesium  orthosilicate  (Mg2Si04) 
forsterite  and  the  ferrous  orthosilicate  (Fe2SiO4)  fayalite.  It  is  the  only 
member  of  the  group  that  is  of  importance  as  a  rock  mineral. 

General  properties.  —  Orthorhombic  in  crystallization,  but  crystal 
form  is  not  an  important  megascopic  property,  since  olivine  usually 
occurs  as  a  rock  constituent  in  formless  grains  and  granular  masses,  and 
rarely  in  distinct  crystals.  Cleavage  not  distinct.  Fracture  conchoidal. 
Hardness  6.5-7.  Specific  gravity  3.27-3.37,  according  to  the  amount 
of  iron  present.  Color  green,  varying  from  olive-green  to  yellow-green; 
bottle-green  very  common.  Luster  vitreous.  Streak  uncolored,  rarely 
yellowish. 

Chemical  tests.  —  Before  the  blowpipe  olivine  varies  from  nearly  infusible  to 
fusible  according  to  whether  little  or  very  much  iron  is  present.  It  is  soluble  in  acids 
yielding  gelatinous  silica  on  evaporation. 

Alteration.  —  The  commonest  form  of  olivine  alteration  is  into 
serpentine  and  iron  oxide.  The  alteration  begins  from  the  outer  surface 
and  cracks  developing  serpentine  fibers  normal  to  the  surfaces.  The 
separated  iron  oxide  is  deposited  along  the  cracks.  Other  kinds  of 
alteration  of  olivine  occur  but  are  of  less  importance. 


22  ENGINEERING  GEOLOGY 

Occurrence.  —  Olivine  occurs  chiefly  as  a  characteristic  mineral  of 
the  less  siliceous  igneous  rocks,  such  as  gabbros,  peridotites,  and  basaltic 
lavas.  It  also  occurs  in  metamorphosed  magnesian  limestones  and  in 
some  schists. 

Determination.  —  General  appearance  and  association,  green  color, 
lack  of  good  cleavage,  and  superior  hardness  usually  distinguish  olivine 
from  those  minerals  it  may  resemble. 

Epidote  Group 

Composition.  —  Epidote,  the  most  important  rock-making  member 
of  the  group,  is  a  basic  orthosilicate  of  calcium  and  aluminum  with 
variable  iron,  corresponding  to  the  formula  Ca2  (A1OH)  (Al,Fe)2  (SiO4)s. 
Proportions  of  aluminum  to  iron  vary  from  6:1  to  3:2. 

Form.  —  Monoclinic  in  crystallization,  but  usually  crystal  form 
is  of  little  value  in  megascopic  determination.  Crystal  habit  of  epidote 
is  prismatic,  sometimes  in  slender,  needle-like  forms,  often  in  aggregates. 
Its  common  occurrence  in  rocks  is  in  formless  grains  and  aggregates  of 
grains. 

General  properties.  —  Cleavage  unequally  developed  in  two  direc- 
tions, one  perfect  parallel  to  c,  the  other  imperfect  parallel  to  a.  Frac- 
ture uneven.  Brittle.  Hardness  6-7.  Specific  gravity  3.3-3.5.  Color 
usually  some  shade  of  green,  pistachio-green  or  yellowish-green  being 
the  most  characteristic.  Luster  vitreous.  Streak  uncolored,  or  grayish. 

Chemical  tests.  —  Before  the  blowpipe  epidote  fuses  with  intumescence  to  a  black 
mass.  It  is  partly  soluble  in  hydrochloric  acid.  Yields  water  in  closed  tube  on 
strong  ignition.  When  fused  and  dissolved  the  solution  gives  gelatinous  silica  on 
evaporation. 

Occurrence.  —  Epidote  occurs  abundantly  as  a  secondary  mineral 
in  igneous  rocks  derived  from  the  alteration  of  ferromagnesian  min- 
erals and  lime-soda  feldspars,  and  commonly  accompanies  chlorite.  It 
has  a  similar  occurrence  in  crystalline  schists  and  gneisses.  It  is  a  com- 
mon constituent  of  metamorphic  rocks  rich  in  lime  derived  both  by 
regional  and  contact  metamorphism.  In  some  cases  the  mineral  has 
been  reported  as  an  original  constituent  of  igneous  rocks. 

Determination.  —  The  peculiar  yellowish-green  color,  superior  hard- 
ness, and  two  unequally-developed  cleavages,  one  perfect,  the  other 
poor,  are  usually  sufficient  to  distinguish  epidote  megascopically  from 
those  minerals  with  which  it  might  be  confused. 


THE  ROCK-FORMING  MINERALS 


23 


Staurolite 

Composition.  —  Variable,  but  chiefly  a  ferrous  iron-aluminum 
silicate  corresponding  to  the  formula  HFeAl5Si20i3  or  (A1O)4  (A10H) 
Fe  (Si04)j. 

Form.  —  Staurolite  is  orthorhombic  in  crystallization,  usually  in  dis- 
tinct crystals  of  prismatic  habit.  (Fig.  21.)  Crystals  are  commonly 
short  and  stout,  less  often  long  and  slender.  Cruciform  twins  are  very 
common  (Figs.  22  and  23). 


TO, 


FIG.  21. 


FIG.  22. 


FIG.  23. 


General  properties.  —  Cleavage  distinct  but  interrupted.  Fracture 
subconchoidal.  Hardness  7-7.5.  Specific  gravity  3.65-3.75.  Color 
reddish-brown  to  brownish-black.  Luster  resinous  to  vitreous,  dull  to 
earthy  when  altered  or  impure. 

Chemical  tests.  —  Staurolite  is  practically  infusible  before  the  blowpipe  and 
insoluble  in  acids,  but  on  intense  ignition  in  a  closed  tube  it  yields  a  little  water. 

Occurrence.  —  Staurolite  occurs  in  metamorphic  rocks,  especially 
the  crystalline  schists  (mica  schists  chiefly),  in  slates,  and  sometimes  in 
gneiss. 

Tourmaline 

Composition.  —  Tourmaline  is  a  complex  silicate  of  boron  and 
aluminum  with  hydroxyl  and  fluorine,  magnesium,  iron,  and  sometimes 
the  alkalies. 

Form.  —  Tourmaline  crystallizes  in  the  rhombohedral  division  of 
the  hexagonal  system,  the  faces  being  in  threes  or  multiples  of  threes. 
(Figs.  25  and  26.)  The  crystals  are  commonly  prismatic,  ranging  from 
short  and  thick  (Fig.  24)  to  slender  and  acicular.  The  prism  faces  are 
often  vertically  striated.  Outline  of  cross  section  of  prisms  is  character- 
istically trigonal  like  a  spherical  triangle,  three-sided  or  nine-sided.  This 


24 


ENGINEERING  GEOLOGY 


triangular  cross  section  (Figs.  25  and  26)  is  very  characteristic  of  rock- 
making  tourmaline.  Tourmaline  is  less  often  developed  in  shapeless 
grains  and  masses. 


FIG.  24. 


FIG.  25. 


FIG.  26. 


General  properties.  —  Cleavage  not  noticeable.  Fracture  sub- 
conchoidal  to  uneven.  Brittle.  Hardness  7-7.5.  Specific  gravity 
2.98-3.20.  Color  variable,  but  that  of  the  common  rock-making  variety 
is  black.  Luster  vitreous.  Streak  uncolored. 

Chemical  tests.  —  Tourmaline  is  difficultly  fusible  before  the  blowpipe  and  is 
insoluble  in  acids. 

Occurrence.  —  Tourmaline  is  widely  distributed  as  a  constituent  of 
crystalline  schists  and  in  the  more  acid  igneous  rocks,  such  as  granites 
and  their  accompanying  pegmatites.  It  also  occurs  in  gneiss  and  clay 
slates,  and  is  a  common  mineral  of  contact  metamorphic  zones.  As  in- 
dicated by  its  composition  tourmaline  is  one  of  the  most  common  and 
characteristic  minerals  formed  by  pneumatolytic  action  (see  Chap.  III). 

Determination.  —  Characteristic  triangular  cross  section,  crystalline 
form,  black  color,  absence  of  cleavage,  and  hardness  are  the  more 
important  megascopic  properties  by  which  it  can  usually  be  identified. 

B.   HYDROUS  SILICATES 

The  hydrous  silicates  that  are  of  most  importance  as  rock-making 
minerals  are  kaolinite,  talc,  serpentine,  chlorite,  and  the  zeolites.  These 
are  entirely  of  secondary  origin,  and  may  be  formed  either  by  weather- 
ing or  by  heated  circulating  waters  or  vapors  acting  on  rock  masses. 
They  are  of  most  importance  in  sedimentary  and  metamorphic  rocks, 
and  are  of  no  importance  in  fresh  igneous  rocks.  They  occur  as  con- 
stituents in  the  wall  rock  of  many  ore-deposits  formed  by  the  alteration 
of  the  original  silicate  minerals  by  varying  geologic  processes  (see  Chapter 
on  Ore-Deposits). 

Kaolinite 

Composition.  —  Kaolinite  is  a  hydrous  aluminum  silicate  corre- 
sponding to  the  formula  H^Si-A  or  A12O3.2  Si02.2  H20. 


THE  ROCK-FORMING  MINERALS  25 

Form.  —  Kaolinite  crystallizes  in  the  monoclinic  system  as  minute 
scales  or  plates  with  sometimes  hexagonal  outlines,  but  the  crystal  form 
is  of  no  importance  in  megascopic  determinations.  It  may  occur  in 
clay-like  masses,  or  scattered  irregularly  through  feldspathic  rocks. 

General  properties.  —  Color  white;  often  variously  colored  by 
impurities.  Luster  usually  dull  earthy.  Hardness  2-2.5.  Specific 
gravity  2.6-2.63.  Neither  hardness  nor  gravity  is  serviceable  for 
practical  tests.  It  usually  has  an  unctuous,  greasy  feel,  and  is  plastic. 

Chemical  tests.  —  Kaolinite  is  infusible  before  the  blowpipe,  and  is  insoluble  in 
acids.  When  moistened  with  cobalt  nitrate  and  ignited  it  becomes  blue.  Heated 
in  the  closed  tube  it  gives  water. 

Occurrence.  —  Kaolinite  is  of  widespread  occurrence.  It  is  a  com- 
mon constituent  of  clay,  and  is  always  a  secondary  mineral,  formed 
usually  by  the  weathering  of  aluminous  silicate  minerals,  chiefly  feld- 
spars. Derivation  of  kaolinite  from  orthoclase  by  weathering  may  be 
represented  as  follows  : 

Orthoclase  Water        Garb.  diox.        Kaolin  Potas.  carb.         Quartz 

2KAlSi308       +       2H2O    +      CO2     =      H4Al2Si2O9      +      K2CO3     +      4SiO2. 


This  process  is  referred  to  as  kaolinization  and  the  reaction  is  de- 
scribed under  feldspars  (page  11).  By  it  rock-masses  are  decomposed 
and  soils  formed.  Extensive  deposits  often  result  from  the  alteration 
of  aluminous  rocks  and  when  not  discolored  by  iron  oxide  and  other 
impurities  form  the  sources  of  china  and  white  wrare  clays  (see  Chapter 
on  Clays).  Deposits  of  clay  of  variable  thickness  and  extent,  showing 
all  degrees  of  admixture  with  sand,  etc.,  and  variously  discolored  by 
different  impurities,  occur.  Other  hydrous-aluminous  silicates  may  be 
present  in  clays,  but  they  are  difficult  to  recognize  by  the  unaided  eye. 
Masses  of  sericite  are  sometimes  mistaken  for  kaolinite. 

Talc 

Composition.  —  Talc  is  an  acid  metasilicate  of  magnesium, 
H2Mg3(SiO3)4  or  3Mg0.4Si02.H20,  containing  SiO2  =  63.5,  MgO  = 
31.7,  H20  =  4.8. 

Form.  —  The  crystal  form  is  doubtful,  probably  orthorhombic  or 
monoclinic,  but  it  is  of  no  importance  in  megascopic  work  since  it  is 
rare.  It  commonly  occurs  in  foliated  masses,  sometimes  in  stellate 
groups,  compact,  and  fibrous. 

Two  varieties  of  talc  are  usually  recognized,  namely:  (1)  Foliated 
talc  having  light  green  to  white  color,  a  pronounced  greasy  feel,  and 
foliated  structure.  (2)  Steatite  or  soapstone,  a  somewhat  impure  form 


26  ENGINEERING  GEOLOGY 

of  talc,  usually  some  shade  of  green  in  color,  and  fine-  to  coarse-granular 
massive  in  structure.  Frequently  impure  from  the  presence  of  such 
minerals  as  mica,  chlorite,  tremolite,  etc.  Extensively  used  for  sinks, 
laundry  tubs,  etc. 

General  properties.  —  Talc,  like  mica  has  perfect  basal  cleavage, 
the  laminae  being  flexible  but  inelastic.  Characteristic  greasy  feel. 
Hardness  1.  Specific  gravity  2.7-2.8.  Color  silvery-white  to  apple- 
green,  sometimes  gray  to  dark  green.  Luster  pearly  on  cleavage  sur- 
faces. Streak  light-colored. 

Chemical  tests.  —  Talc  is  difficultly  fusible  and  not  acted  on  by  acids.  Yields 
water  in  closed  tube  only  on  intense  ignition. 

Occurrence.  —  Talc  is  a  secondary  mineral  derived  by  alteration 
from  non-aluminous  magnesian  silicates,  such  as  olivine,  enstatite, 
tremolite,  etc.  Its  derivation  from  enstatite  may  be  represented 
chemically  as  follows: 

Enstatite  Water        Garb.  diox.  Talc  Magnesite 

4MgSiO3      +      H2O      +      CO2     =      H2Mg3(SiO3)4      +      MgCO3 

It  is  found  as  an  alteration  product  of  igneous  rocks,  especially  the 
peridotites  and  pyroxenites,  but  it  is  commonest  in  the  crystalline 
schists  forming  an  important  constituent  in  several  varieties,  such  as 
the  talc  schists,  etc.  (See  under  Metamorphic  Rocks.)  In  some  meta- 
morphic  rocks  like  soapstone,  talc  may  form  practically  the  entire  rock- 
mass. 

Important  occurrences  of  talc  and  soapstone  are  found  in  the  crystal- 
line rocks  of  the  eastern  United  States,  extending  from  Vermont  to 
Georgia,  and  large  deposits  of  soapstone  are  quarried  in  the  Albemarle- 
Nelson  counties  belt  in  Virginia. 

Serpentine 

Composition.  —  Serpentine  is  a  hydrous-magnesium  silicate, 
H4Mg3Si2O9  or  2H2O.3MgO.2SiO2,  containing  Si02  =  44.1,  MgO  = 
43.0,  ^20  =  12.9. 

Form.  —  Optically  serpentine  is  probably  monoclinic,  but  it  occurs 
only  in  pseudomorphic  crystals.  It  is  usually  compact  or  granular 
massive,  often  fibrous,  the  fibers  of  which  are  flexible  and  can  be  easily 
separated  from  each  other. 

Varieties.  —  Several  varieties  of  serpentine  are  recognized. 

Ordinary  serpentine.  —  Massive,  opaque,  and  of  various  shades  of 
green. 


THE  ROCK-FORMING  MINERALS  27 

Chrysotile.  —  Fibrous  (asbestif orm)  variety,  usually  occurring  in  seams 
in  the  massive  variety.  This  is  the  asbestos  of  commerce  in  most 
part. 

Precious  serpentine.  —  Massive,  dark  green  in  color,  and  translucent. 
The  spotted  green  and  white  varieties  are  called  ophiolite  or  ophicaldte. 
In  these  the  white  areas  are  calcite  and  the  green  usually  serpentine, 
sometimes  with  a  core  of  pyroxene  (?)  as  at  Moriah,  N.  Y.  (See 
under  Marbles.) 

General  properties.  —  The  cleavage  is  basal  sometimes  distinct,  but 
of  no  importance  as  a  megascopic  property.  Hardness  2.5-5.0,  usually 
4.  Specific  gravity  variable,  fibrous  2.2-2.4,  massive  2.5-2.7.  Color 
is  usually  some  shade  of  green  or  yellow,  with  various  shades  of  black, 
red,  or  brown  noted;  not  apt  to  be  uniform,  but  variegated  showing 
mottling  in  lighter  and  darker  shades  of  green.  Luster  is  greasy  and 
wax-like  in  the  massive  varieties,  and  silky  in  the  fibrous.  Feel  smooth 
or  greasy.  Streak  white.  Fracture  conchoidal  or  splintery  in  massive 
varieties.  Translucent  to  opaque. 

Chemical  tests.  —  Serpentine  fuses  with  difficulty  before  the  blowpipe,  is  decom- 
posed by  hydrochloric  acid,  and  in  the  closed  tube  yields  water  on  ignition. 

Occurrence.  —  Serpentine  is  a  secondary  mineral  formed  as  an 
alteration  product  from  non-aluminous  magnesian  silicates,  such  as 
olivine,  pyroxene,  and  amphibole  in  igneous  and  metamorphic  rocks. 
Its  derivation  from  olivine  may  be  shown  chemically  as  follows : 

Olivine  Water        Garb.  diox.         Serpentine  Magnesite 

2  MgaSiO,     +  2  H2O  +         CO2  =  IMgsSi^  +  MgCO3. 

Serpentine  may  also  be  derived  from  the  above  minerals  by  the 
action  of  heated  waters.  It  is  a  common  and  important  constituent  of 
the  serpentine  or  verd  antique  marbles  used  as  an  ornamental  stone, 
and  in  these  it  occurs  mixed  with  calcite  or  dolomite  (see  Chapter  on 
Building  Stones,  also  under  Metamorphic  Rocks). 

Chlorite 

Chlorite  is  the  name  of  a  group  of  hydrous  silicates,  so  named  on 
account  of  their  green  color,  but  because  of  the  difficulty  to  distinguish 
them  from  each  other  megascopically  they  are  included  under  the  group 
name  chlorite.  They  are  secondary  minerals  and  closely  resemble  the 
micas  in  crystal  form  and  cleavage,  but  are  distinguished  from  them  by 
their  folia  being  soft  and  inelastic. 

Composition.  —  The  chlorites  are  hydrous  silicates  of  aluminum 
with  magnesium  and  ferrous  iron.  Clinochlore,  the  most  common 


28  ENGINEERING  GEOLOGY 

member  of  the  group,  has  the  formula  H8(Mg,Fe)5Al2Si3Oi8  or 
4H2O.5(Mg,Fe)O.Al2O3.3  Si02. 

Form.  —  The  chlorites  are  monoclinic  in  crystallization,  forming 
six-sided  tabular  crystals,  but  since  distinct  crystals  are  rare,  crystal 
form  is  not  an  important  megascopic  property.  Chlorite  commonly 
occurs  in  irregular  flakes  and  scales. 

General  properties.  —  Like  mica,  chlorite  has  perfect  basal  cleavage, 
the  folia  of  which  are  flexible  and  tough,  but  unlike  mica  are  inelastic. 
Color  green  of  various  shades,  usually  dark  green.  Luster  of  cleavage 
surface  somewhat  pearly.  Hardness  2-2.5.  Specific  gravity  2.65-2.96. 
Streak  white  to  pale  green. 

Chemical  tests.  —  The  chlorites  are  infusible  or  difficultly  so  before  the  blow- 
pipe, and  are  insoluble  in  hydrochloric  acid,  but  are  decomposed  by  boiling  sulphuric 
acid,  giving  a  milky  solution.  They  yield  water  in  closed  tube  on  ignition. 

Occurrence.  —  Chlorite  is  a  common  and  widespreadhnineral  and  is 
of  secondary  origin.  It  is  a  common  constituent  of  the  crystalline 
schists,  and  in  some  (chlorite  schist)  it  is  the  predominant  mineral. 
It  occurs  as  a  secondary  mineral  in  igneous  rocks  derived  from  the 
alteration  of  pyroxenes,  amphiboles,  micas,  etc.  The  green  color  of 
many  igneous  rocks  and  many  metamorphic  ones  such  as  schists  and 
slates,  is  due  to  chlorite.  The  green  slates  owe  their  color  to  the  finely 
disseminated  particles  of  chlorite  as  the  coloring  matter.  Chlorite  also 
occurs  as  a  common  product  of  hydrothermal  action  along  some  ore- 
bodies,  especially  those  associated  with  volcanic  rocks  (see  Chapter  on 
Ore-Deposits) . 

Determination.  —  The  chlorites  are  characterized  by  their  green 
color,  perfect  basal  cleavage,  and  inferior  hardness.  They  resemble 
most  closely  the  micas  from  which  they  can  be  distinguished  by  their 
inelastic  folia. 

Zeolite  Group 

Composition.  —  The  zeolites  form  a  large  group  of  hydrous  silicates  of  aluminum 
with  calcium  and  sodium,  rarely  potassium,  as  the  important  bases.  They  show 
close  similarities  not  only  in  composition  but  in  their  association  and  mode  of  occur- 
rence as  well.  The  name  is  derived  from  two  Greek  words  meaning  to  boil  and  stone. 

Among  the  more  common  members  of  the  group  are: 

Natrolite,  Na2Al2Si3Oio  +  2  H2O.     Orthorhombic. 

Analtite,  NaAl(SiO3)2  +  H2O.     Isometric. 

Stilbite,  (Na2,Ca)Al2Si6Oi6  +  6  H2O.     Monoclinic. 

Heulandite,  H4CaAl2(SiO3)6  +  3  H2O.     Monoclinic. 

General  properties.  —  The  zeolites  are  usually  well  crystallized,  four  of  the  six 
crystal  systems  being  represented  by  members  of  the  group.  They  are  usually 


THE  ROCK-FORMING  MINERALS 


29 


colorless  or  white,  sometimes  yellow  or  red.  Luster  vitreous.  Hardness  3.5-5.5, 
and  can  be  scratched  with  the  knife.  Specific  gravity  2-2.4. 

The  members  of  the  group  behave  similarly  before  the  blowpipe,  most  of  them 
fusing  readily  with  intumescence,  hence  the  name.  They  dissolve  in  hydrochloric 
acid,  some  yielding  gelatinous  silica  on  evaporation. 

Occurrence.  —  The  zeolites  are  secondary  minerals,  occurring  chiefly  in  cavities 
and  fissures  of  igneous  rocks,  derived  from  the  alteration  of  feldspars  and  felds- 
pathoids,  by  circulating  waters  and  steam.  They  are  especially  common  in  the 
basaltic  lavas  filling  cavities  and  coating  joint-planes,  and  are  often  associated  with 
quartz  and  calcite.  The  amygdules  of  lavas  are  frequently  composed  entirely  or 
partly  of  zeolites,  giving  rise  to  the  amygdaloidal  structure  of  such  rocks. 

Determination.  —  The  zeolites  are  characterized  by  their  light  color,  low  specific 
gravity,  moderate  hardness,  decomposition  by  hydrochloric  acid,  and  ready  fusi- 
bility with  intumescence.  Crystal  form  is  also  an  important  aid  at  tunes  in  dis- 
tinguishing the  individual  species. 

OXIDES 

The  oxides  that  are  of  importance  as  rock-making  minerals  include 
quartz  (SiO2) ,  corundum  (A12O3) ,  and  the  iron  ores  belonging  to  the  group 
of  oxides,  both  anhydrous  and  hydrous. 

Quartz 

Composition.  —  Silicon  dioxide,  Si02.  Oxygen  =  53.3,  silicon  = 
46.7  when  pure;  often  contains  various  impurities. 

Form.  —  Quartz  crystallizes  in  the  hexagonal  system,  a  common  form 
being  a  hexagonal  prism  terminated  by  a  six-sided  pyramid  (Fig.  27). 
The  prism  and  pyramid  faces  are  frequently  unequally  developed;  at 
times  the  prism  faces  are  entirely  absent.  Often,  however,  the  crystals 
are  elongated  with  a  marked  development  of  the  prism  faces  (Fig.  28). 


FIG.  28. 


Except  when  formed  in  cavities,  or  as  phenocrysts  in  some  porphyries, 
crystal  form  is  not  often  observed  in  rock-making  quartz.  Its  usual 
occurrence  in  rocks  is  as  shapeless  grains  and  masses. 

General  properties.  —  Megascopically  quartz  may  be  said  not  to 
possess  cleavage,  which  generally  serves  to  distinguish  it  from  feldspar. 


30  ENGINEERING  GEOLOGY 

Fracture  conchoidal.  Hardness  7.  Specific  gravity  2.66.  Color  varies 
widely  from  colorless  or  white  through  gray  and  brown  to  black,  some- 
times yellow,  red,  pink,  amethyst,  green,  and  blue.  Luster  vitreous, 
sometimes  greasy.  Streak  white.  Transparent  to  opaque.  Brittle 
to  tough. 

Chemical  tests.  —  Infusible  before  the  blowpipe  and  insoluble  in  acids  except 
hydrofluoric  acid.  It  is  very  resistant  to  weathering  processes,  being  altered  chiefly 
by  physical  (disintegration)  rather  than  by  chemical  forces  (decomposition). 

Occurrence.  —  Quartz  is  the  most  common  of  minerals,  having 
widespread  occurrence  in  igneous,  sedimentary,  and  metamorphic  rocks. 
It  is  an  important  constituent  of  the  acid  igneous  rocks,  such  as  granites, 
rhyolites,  pegmatites,  etc.,  and  it  may  occur  as  phenocrysts  as  well  as 
in  the  groundmass  of  the  acid  porphyries.  In  metamorphic  rocks  it 
occurs  in  gneisses  and  schists,  and  is  the  predominant  constituent  in 
quartzites,  many  of  which  are  composed  almost  entirely  of  it.  It  is 
common  in  sedimentary  rocks,  forming  the  principal  mineral  in  sand- 
stones. It  crystallizes  from  aqueous  solutions,  both  hot  and  cold, 
being  deposited  in  fissures  or  other  cavities,  and  forms  the  most  common 
vein  and  gangue  mineral  of  ore  deposits.  It  is  associated  in  rocks 
chiefly  with  feldspar. 

Varieties.  —  In  addition  to  the  crystalline  anhydrous  form  of  quartz,  many  dif- 
ferent forms  of  silica  occur  to  which  varietal  names  are  given,  dependent  upon  color, 
structure,  and  other  properties.  These  represent  amorphous  or  cryptocrystalline 
silica,  and  have  probably  formed  in  most  cases  on  evaporation  of  solutions  containing 
soluble  silica.  They  are  not  important  as  megascopic  constituents  of  igneous  and 
metamorphic  rocks,  but  are  of  some  importance  in  sedimentary  ones. 

Some  of  the  more  important  varieties  are: 

(a)  Chalcedony.     Amorphous  quartz  of  variable  color  with  waxy  luster,  usually 
found  lining  or  filling  cavities  in  rocks. 

(b)  Agate.     A  variegated  chalcedony,  in  which  the  different  colors  are  usually 
arranged  in  parallel  bands,  but  sometimes  irregularly  distributed. 

(c)  Onyx.1     A  banded  chalcedony  like  agate. 

(d)  Flint.     Resembles  chalcedony  somewhat,  but  dull  and  often  dark  in  color, 
breaking  with  pronounced  conchoidal  fracture.     The  irregular  nodules  or  concretions 
and  layers  of  flint  occurring  in  many  limestones  are  called  chert. 

(e)  Jasper.     Opaque  quartz  usually  colored  red  from  hematite. 

(f)  Siliceous  sinter:    geyserite.     Somewhat  porous  or  cellular  silica  formed  by 
deposition  through  evaporation  or  algae  from  waters  containing  soluble  silica  (Plate 
XIII,  Fig.  1) .     The  sinter  deposits  of  the  Yellowstone  National  Park  are  typical. 

1  The  onyx  marble  of  commerce  is  not  silica,  but  calcium  carbonate.     (See 
Chap.  XI.) 


THE  ROCK-FORMING  MINERALS  31 

Corundum 

Composition.  —  Aluminum  oxide,  A12O3  =  oxygen  47.1,  aluminum 
52.9. 

Form.  Corundum  is  hexagonal  (rhombohedral)  in  crystallization. 
The  crystals  are  usually  prismatic,  or  tapering  hexagonal  pyramids, 
often  rounded  into  barrel  shapes.  The  barrel-shaped  forms  are  common 
in  some  syenites.  Corundum  also  occurs  as  grains  and  shapeless 
masses. 

General  properties.  —  Parting,  resembling  perfect  cleavage,  occurs 
parallel  to  the  base  and  in  three  other  directions  (rhombohedral) ,  which 
gives  a  laminated  structure  to  the  mineral  in  large  pieces.  Fracture 
uneven  to  conchoidal.  Hardness  9  (next  to  diamond  in  hardness). 
Specific  gravity  3.95^4.10.  Color  of  rock-making  corundum  is  usually 
dark  gray  to  bluish-gray  or  smoky.  Luster  adamantine  to  vitreous, 
sometimes  greasy.  Translucent  to  opaque.  Brittle,  sometimes  very 
tough. 

Chemical  tests.  —  Infusible  before  the  blowpipe  and  insoluble  in  acids.  Moistened 
with  cobalt  nitrate  and  intensely  ignited  it  assumes  a  blue  color  (aluminum). 
Corundum  is  a  resistant  mineral  to  weathering  processes  but  it  may  alter  into  a  variety 
of  aluminous  minerals,  such  as  margarite,  muscovite,  gibbsite,  etc. 

The  varieties  usually  recognized  are:  Ordinary  (rock-making)  corun- 
dum, gem  corundum,  and  emery. 

Occurrence.  —  Corundum  occurs  as  an  important  constituent  of 
some  igneous  rocks  rich  in  alumina,  such  as  syenites  and  nepheline 
syenites,  and  peridotites,  and  to  a  less  extent  in  some  other  types. 
It  occurs  in  crystalline  schists,  in  metamorphosed  limestones,  and  in 
zones  of  contact  metamorphism.  Magnetite  corundum  known  as 
emery  occurs  in  veins  or  lenses  in  metamorphic  rocks,  and  like  ordinary 
corundum  has  somewhat  extended  use  as  an  abrasive. 

Determination.  —  Corundum  is  characterized  chiefly  by  crystal 
form  when  present,  its  great  hardness,  luster,  and  specific  gravity. 

Iron  Ores  (Oxides) 

The  iron  ores  belonging  to  the  group  of  oxides  that  have  value  as 
rock-making  minerals  are:  (a)  Anhydrous,  including  magnetite,  il- 
menite,  and  hematite;  (6)  hydrous,  limonite.  These  minerals  have 
wide  distribution  and  are  frequent  constituents  of  rocks,  although  from 
the  standpoint  of  rock-making  species  they  occur  chiefly  as  accessory 
minerals,  and  as  such  do  not  play  so  important  a  role  as  the  more 


32 


ENGINEERING  GEOLOGY 


common  silicate  minerals,  such  as  feldspar,  mica,  amphibole,  pyroxene, 
etc.  They  frequently  form  large  bodies  concentrated  by  geologic 
processes,  and  excepting  ilmenite,  constitute  the  sources  of  ore  for  the 
metal  iron. 

Magnetite 

Composition.  —  Iron  ferrate,  Fe3O4  or  FeO.Fe203  =  oxygen  27.6, 
iron  72.4  (FeO  =  31.0,  Fe2O3  =  69.0).  Ferrous  iron  sometimes  re- 
placed by  magnesium.  Titanium  oxide  occurs  in  variable  amounts 
up  to  25  per  cent,  r 

General  properties.  —  Isometric  in  crystallization;  commonly  in 
octahedrons  (Fig.  29) ,  also  in  dodecahedrons  (Fig.  30) ,  sometimes  in  com- 
binations of  these  forms  (Fig.  31).  Magnetite  sometimes  occurs  in 
rocks  in  such  small  crystals  that  the  form  is  indeterminate  megascopically. 


FIG.  29. 


FIG.  30. 


FIG.  31. 


Cleavage  not  distinct;  parting  octahedral,  sometimes  well  developed. 
Fracture  subconchoidal  to  uneven.  Brittle.  Hardness  5.5-6.5.  Specific 
gravity  5.16-5.18.  Opaque.  Luster  metallic.  Color  iron-black.  Streak 
black.  Strongly  magnetic.  Infusible  and  slowly  soluble  in  hydrochloric 
acid.  Alters  principally  to  hematite  and  limonite,  sometimes  to  siderite. 
Occurrence.  —  Magnetite  is  a  very  common  and  widely  distributed 
accessory  mineral  in  rocks  of  all  classes;  especially  in  the  crystalline 
metamorphic  and  igneous  rocks.  It  occurs  as  a  contact  mineral;  in 
ore-bodies  due  to  magmatic  segregation;  in  lenses  inclosed  in  metamor- 
phic rocks,  especially  schists  and  gneisses;  and  as  a  constituent  of  the 
so-called  black  sands.  It  is  less  common  in  non-metamorphosed  sedi- 
ments, and  is  of  little  importance  in  building  stones.  Magnetite  is  an 
important  ore  of  iron.  It  is  distinguished  chiefly  by  its  strong  magnet- 
ism, its  black  color  and  streak,  and  its  hardness. 

Ilmenite 

Composition.  —  Ferrous  titanate,  FeTi03  or  FeO.Ti02  =  oxygen 
31.6,  titanium  31.6,  iron  36.8  (FeO  =  47.3,  Ti02  =  52.7).  It  is  fre- 
quently not  pure,  but  mixed  with  more  or  less  hematite  (Fe2O3)  and 
magnetite. 


THE  ROCK-FORMING  MINERALS  33 

General  properties.  —  Ilmenite  is  hexagonal  (rhombohedral)  in 
crystallization,  but  crystals  in  rocks  are  not  often  observed  by  the 
unaided  eye,  hence  crystal  form  is  not  of  great  importance.  It  usually 
occurs  in  grains  and  masses,  and  often  in  thin  plates.  Cleavage  not 
developed;  parting  is  sometimes  shown.  Fracture  conchoidal.  Brittle. 
Hardness  5-6.  Specific  gravity  4.5-5.  Opaque.  Luster  metallic  to 
submetallic.  Color  iron-black.  Streak  black  to  brownish-red.  Some- 
times magnetic.  Infusible  and  not  acted  on  by  acids.  After  fusion 
with  sodium  carbonate,  dissolved  in  hydrochloric  acid,  and  the  solution 
boiled  with  tin,  it  assumes  a  violet  color  (titanium).  Ilmenite  alters 
chiefly  into  leucoxene  (titanite). 

Occurrence.  —  Ilmenite  is  a  common  mineral  in  igneous  and  meta- 
morphic  rocks  (gneisses  and  schists) ,  and  its  mode  of  occurrence  in  these 
is  similar  to  that  of  magnetite.  Unless  it  occurs  in  crystals  with  definite 
boundaries,  or  in  grains  of  sufficient  size  to  be  tested  chemically,  it  is 
difficult  and  sometimes  impossible  to  distinguish  from  magnetite. 
Luster  may  sometimes  serve  to  distinguish  the  two  minerals,  which  are 
associated  in  some  occurrences.  The  most  important  occurrence  of 
ilmenite  as  a  megascopic  mineral  is  as  segregation  bodies  of  varying  size 
in  gabbros  and  anorthosites.  Its  high  titanium  content  precludes  its 
use  as  an  ore  of  iron,  but  it  is  used  to  some  extent  as  a  source  of  titanium 
in  the  manufacture  of  ferro-titanium  alloys. 

Hematite 

Composition.  —  Iron  sesquioxide,  Fe2O3  =  oxygen  30,  iron  70. 
Sometimes  contains  titanium  and  magnesium. 

General  properties.  —  Hematite,  like  ilmenite,  is  hexagonal  (rhom- 
bohedral) in  crystallization,  but  as  a  rock  mineral  it  is  so  rarely  found 
in  definite  crystals,  that  crystal  form  is  of  little  value  in  its  deter- 
mination. It  is  found  in  a  variety  of  forms,  but  as  a  rock  mineral 
specular  or  micaceous  hematite,  and  common  red  hematite  are  the  varieties 
of  chief  importance.  Rhombohedral  parting  resembling  cleavage  is 
sometimes  developed.  Fracture  conchoidal  to  uneven.  Brittle  in 
compact  form.  Hardness  5.5-6.5.  Specific  gravity  4.8-5.3.  Opaque, 
but  translucent  red  in  thin  scales.  Luster  metallic  to  dull.  Color  iron 
black  to  deep  red.  Streak  cherry-red  to  reddish-brown.  Difficultly 
fusible.  Slowly  soluble  in  concentrated  hydrochloric  acid.  Becomes 
magnetic  when  heated  in  the  reducing  flame.  It  alters  principally  into 
limonite  on  exposure  to  weather. 

Occurrence.  —  Hematite  is  one  of  the  most  widely  distributed  of 
minerals.  It  occurs  in  igneous,  sedimentary,  and  metamorphic  rocks, 


34  ENGINEERING  GEOLOGY 

both  as  a  primary  constituent  and  as  an  alteration  product.     It  is  a 
common  alteration  product  of  most  iron-bearing  minerals. 

It  is  the  principal  ore  of  iron,  and  supplies  more  than  70  per  cent  of 
the  total  annual  production  of  iron  ores  in  the  United  States.  The 
streak  is  one  of  its  most  distinctive  megascopic  properties. 

Limonite 

Composition.  —  Limonite  is  the  hydrous  sesquioxide  of  iron, 
Fe403(OH)6  or  2Fe2O3.3H2O,  and  contains  when  pure  oxygen  =  25.7,- 
iron  =  59.8,  water  14.5.  Often  impure  and  is  frequently  admixed  with 
other  hydrous  oxides  of  iron. 

Form.  —  Noncrystalline.  Occurs  in  earthy  masses  in  rocks,  and 
in  deposits  in  mammillary  and  stalactitic  forms  with  frequently  radiating 
fibrous  structure;  also  concretionary,  and  in  earthy  deposits. 

General  properties.  —  Limonite  has  no  cleavage.  Luster  sub- 
metallic  to  dull.  Hardness  5-5.5  in  the  compact  mineral.  Specific 
gravity  3.6-4.0.  Color  is  usually  some  shade  of  brown,  brownish- 
yellow  to  very  dark  opaque.  Streak  yellow-brown,  very  characteristic 
and  serves  to  distinguish  it  from  hematite. 

Chemical  tests.  —  It  is  difficultly  fusible  before  the  blowpipe,  becoming  strongly 
magnetic  after  heating  in  the  reducing  flame.  Slowly  soluble  in  hydrochloric  acid, 
and  yields  much  water  when  heated  in  closed  tube. 

Occurrence.  —  Limonite  is  a  secondary  mineral  formed  by  weather- 
ing and  alteration  from  other  iron-bearing  compounds.  It  is  frequently 
noted  in  igneous  and  metamorphic  rocks  as  small  yellowish  earthy 
masses  derived  from  other  iron-bearing  minerals,  such  as  pyrite,  etc., 
by  oxidation  and  hydration.  It  forms  an  essential  part  of  the  gossan  or 
"iron  hat"  of  many  sulphide  veins,  as  accumulations  in  beds  and 
irregular  bodies  forming  residual  deposits  from  iron-bearing  rocks, 
especially  ferruginous  limestones,  and  in  porous  earthy  form  known  as 
bog-iron  ore  deposited  on  the  bottom  of  swamps,  bogs,  and  other  shallow 
water  bodies  through  oxidation  of  iron  carbonate  chiefly  (FeH2(CO3)2), 
and  also  from  iron  sulphate.  Admixed  with  more  or  less  clay  it  forms 
yellow  ocher,  and  may  then  be  of  value  as  a  mineral  pigment.  It  occurs 
as  a  pigment  or  stain  in  various  rocks  and  is  a  common  cement  of  many. 

Limonite  is  an  important  ore  of  iron  and  ranks  next  to  hematite  in 
importance  in  the  United  States;  Alabama,  Virginia,  Tennessee,  and 
Georgia  being  the  principal  producers.  Other  hydrous  oxides  of  iron 
are  frequently  admixed  with  limonite. 

Determination.  —  Its  color,  streak,  and  structure  usually  suffice 
to  distinguish  it  from  other  minerals. 


THE  ROCK-FORMING  MINERALS 


35 


CARBONATES 

The  carbonates  are  salts  of  carbonic  acid  (H2CO3)  and  are  secondary 
minerals,  formed  by  weathering  of  other  minerals  or  derived  from  deeper 
sources  within  the  earth.  They  may  be  deposited  either  in  place  or  else 
carried  in  solution  by  water  containing  carbon  dioxide  into  seas  and  lakes 
and  precipitated  by  means  of  organic  agencies  as  limestone,  etc.  Only 
two  species  of  the  calcite  group  (calcite  and  dolomite}  of  the  anhydrous 
carbonates  are  of  megascopic  importance  as  rock-forming  minerals. 


Calcite 
Calcite   is    calcium    carbonate, 


CaC03    in   which 


Composition. 
CaO  =  56.0  and  CO2  =  44.0  per  cent. 

Form.  —  Calcite  crystallizes  in  the  rhombohedral  division  of  the 
hexagonal  system.     Crystals  are  varied  in  habit,  are  often  perfect,  and 


FIG.  32. 


FIG.  33. 


sometimes  of  large  size.  The  rhombohedron  is  the  most  common 
crystal  form  (Figs.  32  to  34).  Other  forms  represented  by  Figs.  35  and 
36  sometimes  occur.  As  a  rock-forming  mineral  calcite  usually  occurs 
fine  to  coarse-crystalline  granular  in  marble,  compact  in  ordinary  lime- 
stones, loose  and  earthy  in  chalk,  spongy  in  tufa,  and  stalactitic  in  cave 
deposits. 


FIG.  34. 


FIG.  35. 


FIG.  36. 


FIG.  37. 


General  properties.  —  Perfect  rhombohedral  cleavage  in  three 
directions  intersecting  at  angles  of  75  and  105  degrees  (Fig.  37). 
Hardness  3.  Specific  gravity  2.72.  Color  usually  white  or  colorless, 
but  frequently  exhibits  a  variety  of  color  from  impurities.  Luster 


36  ENGINEERING  GEOLOGY 

vitreous  to  earthy.     Usually  transparent  to  translucent;    opaque  when 
impure.     Strong  double  refraction. 

Chemical  tests.  —  Infusible  before  the  blowpipe,  but  after  intense  ignition  the 
residue  reacts  alkaline  to  moistened  test  paper.  Readily  soluble  in  cold  dilute  acids 
with  brisk  effervescence. 

Occurrence.  —  Calcite  is  one  of  the  most  common  and  widely  dis- 
tributed of  minerals.  It  is  a  widespread  and  abundant  constituent  of 
calcareous  sedimentary  and  metamorphic  rocks,  in  which  it  is  the  pre- 
dominant, and  sometimes  the  only,  mineral  of  many  limestones,  chalk, 
calcareous  marls  and  tufas,  stalagrnitic  deposits,  marbles,  and  rocks 
composed  of  mixtures  of  calcite  and  silicate  minerals.  It  is  also  a 
common  mineral  of  many  veins.  Calcareous  shales  contain  a  variable 
quantity  of  it,  and  it  forms  the  cementing  material  of  some  sandstones. 
It  is  found  in  many  igneous  rocks  as  a  secondary  constituent  formed 
from  the  alteration  of  lime-bearing  silicates  by  waters  containing  carbon 
dioxide  in  solution,  but  in  such  cases  it  is  usually  present  in  only  small 
amounts.  It  also  occurs  as  a  lining  and  filling  of  amygdaloidal  cavities 
in  lavas. 

Uses.  —  Rocks  composed  chiefly  or  entirely  of  calcite  have  varied 
uses,  principal  ^among  which  may  be  mentioned  the  manufacture  of 
natural  and  Portland  cement,  the  manufacture  of  lime  for  mortars  and 
cements,  and  for  agricultural  purposes,  as  a  fluxing  material  in  blast 
furnaces,  as  ornamental  and  building  stone,  etc.  Iceland  spar,  the 
pure,  transparent  and  colorless  form  of  calcite,  is  valuable  for  optical 
instruments.  (See  Chapters  on  Building  Stones  and  Limes,  Cements 
and  Plasters.) 

Determination.  —  Calcite  is  distinguished  by  its  hardness  (3), 
perfect  rhombohedral  cleavage,  color,  and  luster.  It  is  readily  dis- 
tinguished from  dolomite  by  the  fact  that  it  effervesces  freely  in  cold 
dilute  acid,  while  dolomite  does  not. 

Aragonite 

Aragonite  has  the  same  chemical  composition  as  calcite,  but  differs  from  it  in 
crystalline  form  and  specific  gravity.  It  is  much  less  common  in  occurrence  than 
calcite,  and  has  no  special  importance  as  a  rock-making  mineral.  It  occurs  in  some 
onyx  marbles. 

Dolomite 

Composition. — A  carbonate  of  calcium  and  magnesium,  CaMg(C03)2. 
Carbon  dioxide  47.8,  lime  30.4,  magnesia  21.7. 

Form.  —  The  crystallization  of  dolomite  is  similar  to  that  of  calcite, 
hexagonal-rhombohedral.  Crystals  are  usually  simple  (unit)  rhombo- 


THE  ROCK-FORMING  MINERALS  37 

hedrons,  whose  faces  are  often  curved  (Fig.  38),  which  sometimes 
serve  to  distinguish  it  from  similar  crystals  of  calcite.  As  a  rock-form- 
ing mineral  it  seldom  shows  crystal  form,  but  usually  occurs 
massive,  frequently  fine  to  coarse  crystalline  granular  as  in 
some  marbles. 

General  properties.  —  Like  calcite,  dolomite  has  perfect 
rhombohedral  cleavage  in  three  directions,  which  intersect 
at  angles  of  nearly  74  and  106  degrees.     Hardness  3.5-4. 
Specific  gravity  2.85.     Color  frequently  some  shade  of  pink,  but  may 
be  white  or  colorless,  and  often  exhibits  a  variety  of  exotic  color  from 
the  presence  of  impurities.     Luster  vitreous;  pearly  in  some  varieties. 
Translucent  to  opaque. 

Chemical  tests.  —  Infusible  before  the  blowpipe,  but  after  intense  ignition  the 
residue  reacts  alkaline  to  moistened  test  paper.  Readily  soluble  with  effervescence 
in  hot  dilute  acid,  but  only  slowly  attacked  by  cold  dilute  acid,  which  serves  to  dis- 
tinguish it  from  calcite.  It  is  less  soluble  in  surface  or  rain  waters  than  calcite, 
but  on  exposure  to  weather  disintegrates  more  readily  than  the  latter. 

Occurrence.  —  As  a  rock-forming  mineral  dolomite  has  its  principal 
occurrence  in  sedimentary  and  metamorphic  rocks,  such  as  limestones 
and  marbles.  Its  occurrence  in  these  rocks  is  similar  to  that  of  calcite, 
and  the  two  are  often  intimately  mixed,  with  nearly  every  degree  of 
transition  between  them. 

Determination.  —  The  curved  faces  of  crystals  help  to  distinguish 
dolomite  from  calcite,  but  the  surest  test  is  the  difference  in  the  behavior 
of  the  two  minerals  to  cold  dilute  acid  (see  under  Calcite  above).  From 
other  minerals  which  it  may  resemble,  dolomite  is  distinguished  by  its 
rhombohedral  cleavage  and  inferior  hardness  (3.5-4). 

SULPHATES 

Of  the  large  number  of  sulphate  minerals,  only  two,  gypsum  and 
anhydrite,  are  of  importance  as  rock-forming  minerals.  Like  the  car- 
bonates the  rock-making  sulphates  are  secondary,  derived  from  pre- 
viously existent  minerals.  Most  of  the  sulphates  are  soluble  and  are 
carried  by  flowing  waters  to  the  sea  and  lakes  where  they  are  precipi- 
tated on  concentration  by  evaporation  under  proper  climatic  conditions 
(see  Chapter  on  Rocks).  The  sulphates  are  salts  of  sulphuric  acid 
(H2S04). 

Gypsum 

Composition.  —  A  hydrous:  calcium  sulphate  (CaS04.2  H2O)  con- 
taining sulphur  trioxide  46.6,  lime  32.5,  water  20.9. 


38 


ENGINEERING  GEOLOGY 


Form.  —  Gypsum  crystallizes  in  the  monoclinic  system.  The 
crystals  are  usually  simple  in  habit,  often  flattened  parallel  to  the  face 
b  as  shown  in  Figs.  39  and  40.  Twin  crystals  are  common,  and  are 
apt  to  be  of  arrow-head  form.  As  a  rock-forming  mineral  gypsum 
commonly  occurs  in  foliated  masses  with  sometimes  curved  faces, 
granular  to  compact,  and  fibrous. 


FIG.  40. 

The  common  varieties  of  gypsum  usually  recognized  are: 

(a)  Crystalline   sometimes    called    selenite,    in    crystals    or   foliated 
masses. 

(b)  Fibrous  (satin  spar),  coarse  to  fine  fibrous  in  appearance  with 
silky  luster. 

(c)  Alabaster,  a  fine-grained  white  variety. 

(d)  Rock  gypsum,  massive,  granular  or  earthy,  often  impure. 
General  properties.  —  Gypsum  has  one  perfect  cleavage  parallel 

to  the  face  b  (010)  by  which  it  may  be  parted  into  thin  folia,  and  a 
second  less  perfect  cleavage  —  the  two  intersecting  at  angles  of  66  and 
114  degrees,  so  that  a  cleavage  fragment  has  rhombic  form.  Hardness 
1.5-2.  Specific  gravity  2.32.  Colorless  or  white,  but  from  the  presence 
of  impurities  it  is  frequently  some  shade  of  red  or  yellow,  brown,  and 
black.  Luster  of  cleavage  surface  6  is  pearly  and  shining,  of  other  faces 
sub  vitreous;  fibrous  varieties  satin-like;  massive  varieties  frequently 
glistening,  sometimes  dull  earthy.  Transparent  to  translucent  and 
opaque.  Streak  white. 

Chemical  tests.  —  Gypsum  fuses  easily  before  the  blowpipe,  the  moistened  mass 
reacting  alkaline  to  test  paper.  When  fused  with  sodium  carbonate  on  charcoal  and 
the  melt  transferred  onto  silver  and  moistened,  it  gives  a  dark  stain.  Yields  water 
on  heating  in  a  closed  tube  and  becomes  opaque.  Soluble  in  hydrochloric  acid,  and 
in  400  to  500  parts  of  water.  Ignited  at  a  temperature  not  exceeding  200  degrees 
Cent.,  it  loses  a  part  of  its  water  and  becomes  plaster  of  Paris,  which  again  takes  up 
water  and  sets.  If  strongly  ignited  gypsum  loses  all  of  its  water,  and  is  known  as 
dead-burnt  plaster. 


THE  ROCK-FORMING  MINERALS  39 

Occurrence.  —  Gypsum  frequently  forms  more  or  less  extensive 
deposits  in  association  with  sedimentary  rocks,  especially  limestones, 
marls,  and  clays.  It  is  often  associated  with  anhydrite  and  sometimes 
with  rock  salt,  and  is  occasionally  found  in  crystalline  rocks,  or  more 
rarely  veins.  Its  chief  use  is  for  plaster  (see  Chapter  on  Limes,  Cements 
and  Plasters). 

Anhydrite 

Composition.  —  Anhydrous  calcium  sulphate,  CaSO4,  containing 
sulphur  trioxide  58.8,  lime  41.2. 

Form.  —  Anhydrite  crystallizes  in  the  orthorhombic  system,  but 
as  a  rock-making  mineral  crystal  form  is  rarely  developed.  Its  chief 
occurrence  in  rocks  is  in  granular  to  compact  masses,  less  often  in  foliated 
or  fibrous  forms. 

General  properties.  —  Anhydrite  has  three  directions  of  cleavage, 
but  of  different  degrees  of  perfection,  which  yield  rectangular  or  cube- 
like  forms.  Hardness  3-3.5.  Specific  gravity  2.95.  Fracture  uneven, 
sometimes  splintery.  Color  usually  white  but  variable  as  in  gypsum. 
Luster  varies  from  pearly  to  somewhat  greasy  and  vitreous  according 
to  direction;  in  massive  varieties  it  varies  to  dull. 

Chemical  tests.  —  Behavior  before  the  blowpipe  same  as  for  gypsum,  except  it 
does  not  yield  water  on  ignition  in  the  closed  tube,  which  serves  to  distinguish 
anhydrite  from  gypsum. 

Occurrence.  —  Anhydrite,  like  gypsum,  occurs  as  interstratified 
beds  in  sedimentary  rocks,  especially  limestones  and  shales,  and  is 
frequently  associated  with  gypsum  and  rock  salt.  Its  irregularity  is 
sometimes  puzzling  to  quarrymen,  and  its  slightly  greater  hardness 
than  gypsum  is  noticeable  to  the  driller. 

PHOSPHATES 

Of  the  large  number  of  known  phosphate  minerals  most  of  which 
are  rare,  only  one  (apatite)  is  of  any  importance  as  a  rock  constituent. 
As  a  megascopic  rock-mineral,  however,  apatite  is  not  of  wide  occurrence 
nor  of  general  importance. 

Apatite 

Composition.  —  Apatite  is  a  calcium  phosphate,  containing  F  or 
Cl  in  small  quantities;  fluor-apatite,  Ca4(CaF)  (P04)3;  less  often 
chlor-apatite,  Ca4(CaCl)  (PO4)3. 


40 


ENGINEERING  GEOLOGY 


Form.  —  Crystallizes  in  the  hexagonal  system ;  crystals  are  pris- 
matic in  habit,  usually  long,  sometimes  short,  and  may  have  rounded 
ends  or  be  terminated  by  pyramidal  faces  (Figs.  41  and  42).  It  some- 
times occurs  in  granular  massive  to  compact  form. 


FIG.  41. 


FIG.  42. 


General  properties.  —  Imperfect  basal  cleavage,  but  of  no  impor- 
tance megascopically.  Luster  vitreous.  Hardness  5,  just  scratched  by 
the  knife.  Specific  gravity  3.15.  Color  usually  some  shade  of  green 
or  brown,  sometimes  colorless  or  white  and  violet.  Brittle.  Transparent 
to  opaque. 

Chemical  tests.  —  Difficultly  fusible  before  the  blowpipe;  soluble  in  acids.  The 
addition  of  ammonium  molybdate  to  the  warm  nitric  acid  solution  yields  a  yellow 
precipitate  showing  the  presence  of  phosphorus. 

Occurrence.  —  Apatite  is  a  constant  accessory  constituent  of 
igneous  rocks,  and  as  such  is  usually  of  microscopic  importance  only. 
Its  principal  megascopic  occurrences  are  in  pegmatites,  and  metamor- 
phosed limestones.  In  many  of  its  occurrences  it  is  regarded  as  of 
pneumatolytic  origin  (see  Chapter  on  Ore-deposits). 

Crystalline  apatite  is  of  little  economic  value  at  present,  since  it  can 
not  compete  successfully  with  the  large  deposits  of  amorphous  (rock) 
phosphate  mined  for  the  manufacture  of  fertilizer. 

SULPHIDES 

The  sulphides  form  an  important  group  of  minerals.  They  include 
the  majority  of  the  ore  minerals  (see  Chapter  on  Ore-deposits),  but  on 
account  of  their  usual  sparing  occurrence  in  rocks,  only  one  of  them, 
pyrite,  has  any  special  importance  megascopically  as  a  rock-making 
mineral.  When  present  to  any  extent  in  rocks  used  for  building  and 
ornamental  purposes,  the  sulphides,  especially  those  of  iron,  are  injurious 
constituents,  because  of  their  ready  alteration  on  exposure  to  weather- 


THE  ROCK-FORMING  MINERALS 


41 


ing,  which  causes  disintegration  and  unsightly  discoloration  from  iron 
oxide  stain,  as  well  as  liberating  H2SO4  which  attacks  calcite. 

The  sulphides,  chalcopyrite,  galena  and  sphalerite  (zinc  blende)  while 
of  no  importance  as  rock-making  constituents  are  important  ore  minerals, 
and  since  they  are  frequently  referred  to  in  the  Chapter  on  Ore-Deposits, 
a  brief  general  description  of  each  one  is  given  below. 

Pyrite 

Composition.  —  Iron  disulphide,  FeS2,  containing  when  pure,  sulphur 
53.4,  iron  46.6. 

Form.  —  Pyrite  crystallizes  in  the  isometric  system,  the  most 
common  form  being  the  cube,  the  faces  of  which  are  usually  striated 
(Fig.  43);  also  as  the  octahedron  and  pentagonal  dodecahedron  (Fig. 


FIG.  43. 


FIG.  44. 


44),  known  as  the  pyritohedron.  Combinations  of  these  forms  are  also 
quite  common  (Figs.  45  and  46).  It  manifests  a  marked  tendency  to 
develop  as  crystals  in  rocks,  but  also  occurs  in  shapeless  grains  and 
masses. 


FIG.  45. 


FIG.  46. 


General  properties.  —  Pyrite  has  no  cleavage.  Fracture  conchoidal 
to  uneven.  Hardness  6-6.5.  Specific  gravity  4.95-5.10.  Color  brass- 
yellow,  becoming  darker  on  account  of  tarnishing.  Luster  metallic, 
splendent.  Streak  greenish-  to  brownish-black.  OpaqUe. 

Chemical  tests.  —  Easily  fusible  before  the  blowpipe  to  a  magnetic  globule,  giving 
off  sulphur  dioxide  gas.  Yields  sulphur  in  closed  glass  tube.  Insoluble  in  hydro- 
chloric acid,  but  soluble  in  boiling  nitric  acid  with  separation  of  sulphur. 


42  ENGINEERING  GEOLOGY 

Alteration.  —  Pyrite  alters  readily  on  exposure  to  weather  to  iron 
oxide,  especially  the  hydrated  oxide,  limonite.  Hence  rocks  containing 
much  of  it  are  not  suited  for  structural  or  ornamental  purposes  because 
of  its  ready  oxidation,  which  serves  both  to  disintegrate  the  rock  and 
stain  it  with  iron  oxide. 

Occurrence.  —  Pyrite  is  the  most  common  of  the  sulphide  minerals, 
and  occurs  in  all  kinds  of  rocks,  igneous,  metamorphic,  and  sedimentary. 
It  is  a  common  vein  mineral,  associated  with  many  different  minerals, 
frequently  chalcopyrite,  sphalerite,  galena,  etc.;  and  as  a  contact 
mineral  with  specularite,  magnetite,  etc. 

Determination.  —  The  crystal  form,  color,  and  hardness  are  usually 
sufficient  to  distinguish  pyrite  from  other  rock  minerals. 

Marcasite  and  Pyrrhotite 

Two  other  forms  of  iron  sulphide  are  marcasite  (FeS2)  and  pyrrhotite 
(Fe»S»+i,  chiefly  FenSi2).  These  occur  as  less  important  rock  constitu- 
ents than  pyrite,  but  decompose  more  readily  on  exposure  to  weather- 
ing processes,  and  hence  are  to  be  avoided  in  stones  used  for  building 
and  decoration.  Pyrrhotite  is  an  important  ore  mineral,  and  occurs  in 
magmatic  segregation  deposits,  contact  zones,  etc.  (See  Chapter  on 
Ore-Deposits.) 

Chalcopyrite 

Composition.  —  A  sulphide  of  copper  and  iron,  CuFe$2,  containing 
sulphur  35,  copper  34.5,  iron  30.5. 

Form.  —  Chalcopyrite  crystallizes  in  the  tetragonal  system.  Crystals 
are  sometimes  observed,  but  as  an  ore  mineral  its  usual  occurrence  is  in 
irregular  grains  and  masses. 

General  properties.  —  Color  brass-yellow  when  fresh,  but  often 
tarnished  from  exposure  to  weather.  Luster  metallic.  Streak  greenish- 
black.  Hardness  3.5.  Specific  gravity  4.25. 

Chemical  tests.  —  Easily  fusible  before  the  blowpipe  to  a  magnetic  globule.  Yields 
a  sublimate  of  sulphur  in  a  closed  tube.  Readily  soluble  in  nitric  acid  with  the  sep- 
aration of  sulphur. 

Occurrence.  —  Chalcopyrite  is  the  principal  ore  of  copper,  and 
occurs  widely  distributed  in  a  variety  of  types  of  ore-bodies.  It  occurs 
as  a  vein  mineral  associated  with  other  sulphides,  such  as  pyrite,  galena, 
sphalerite,  etc.;  as  a  magmatic  segregation  mineral  in  basic  igneous 
rocks  with  pyrrhotite,  as  at  Sudbury,  Canada;  as  a  contact  mineral 
with  magnetite  or  hematite,  etc.  Chalcopyrite  may  occur  either  as  a 
primary  or  a  secondary  mineral  (see  Chapter  on  Ore-Deposits). 


THE  ROCK-FORMING  MINERALS  43 

Determination.  —  Chalcopyrite  is  usually  identified  by  the  naked 
eye  by  its  brass-yellow  color,  softness,  and  greenish-black  streak.  It 
can  frequently  be  distinguished  from  pyrite  by  its  deeper  brass  color 
and  being  much  softer. 

Galena 

Composition.  —  Lead  sulphide,  PbS,  containing  sulphur  13.4,  lead 
86.6.  Frequently  contains  silver  in  sufficient  quantity  to  make  it  one 
of  the  most  important  silver  ore  minerals,  when  it  is  called  argentiferous 
galena. 

Form.  —  Galena  crystallizes  in  the  isometric  system,  the  cube  being 
the  most  common  form.  It  also  occurs  in  cleavable  and  coarse  or  fine 
granular  masses. 

General  properties.  —  It  has  perfect  cubic  cleavage.  Color  and 
streak  lead-gray.  Luster  metallic.  Hardness  2.5-2.75.  Specific 
gravity  7.5. 

Chemical  tests.  —  It  is  easily  fusible  before  the  blowpipe  yielding  a  malleable  lead 
globule  with  the  formation  of  a  yellow  to  white  coating  on  the  charcoal.  Soluble  in 
acids. 

Alteration.  —  Galena  may  be  converted  by  oxidation  into  the 
sulphate  (anglesite),  the  carbonate  (cerussite),  or  other  compounds. 

Occurrence.  —  As  an  ore  mineral  galena  may  have  a  variety  of 
occurrences,  namely,  (1)  in  veins  associated  with  other  sulphides,  such 
as  sphalerite,  pyrite,  chalcopyrite,  etc.;  (2)  as  irregular  masses  in 
metamorphic  rocks;  (3)  as  irregular  masses  or  disseminations  formed 
by  replacement  or  impregnation  in  limestones,  etc.;  (4)  as  a  contact 
metamorphic  mineral,  etc.  In  its  various  occurrences  galena  is  often 
associated  with  sphalerite,  and  both  are  persistent  minerals,  since  they 
are  formed  under  a  variety  of  physical  conditions. 

Determination.  —  Its  high  specific  gravity,  cubic  cleavage,  color, 
and  softness  usually  serve  to  distinguish  galena  from  other  minerals 
which  it  may  resemble. 

Sphalerite 

Composition.  —  Sphalerite,  known  also  as  blende,  black  jack,  etc., 
is  zinc  sulphide,  ZnS,  containing  sulphur  33,  and  zinc  67.  It  usually 
contains  some  iron  replacing  the  zinc,  and  frequently  a  small  amount  of 
cadmium. 

Form.  —  It  crystallizes  in  the  isometric  system,  the  tetrahedron, 
dodecahedron,  and  cube  being  the  common  forms.  As  an  ore  mineral  it 
usually  occurs  in  cleavable  masses,  coarse  to  fine  granular. 


44  ENGINEERING   GEOLOGY 

General  properties.  —  Sphalerite  has  perfect  dodecahedral  cleavage 
at  angles  of  60  and  90  degrees.  Color  varies  from  white  to  black  depend- 
ing upon  composition,  but  commonly  yellow,  brown  and  reddish-brown 
to  black.  Luster  resinous,  also  adamantine.  Streak  white  to  yellow 
and  brown.  Transparent  to  translucent.  Hardness  3.5-4.  Specific 
gravity  4.0. 

Chemical  tests.  —  Sphalerite  is  difficultly  fusible  before  the  blowpipe,  yielding  a 
white  coating  on  charcoal  when  cold,  yellow  when  hot.  Intensely  heated  on  coal 
with  cobalt  nitrate  solution  gives  a  green  color.  Soluble  in  hydrochloric  acid  with 
the  evolution  of  hydrogen  sulphide. 

Occurrence.  —  Sphalerite  is  a  very  common  mineral  and  is  the  chief 
ore  mineral  of  zinc,  the  Joplin  district,  Missouri,  being  the  most  important 
locality  in  the  United  States.  It  is  associated  usually  with  other  sul- 
phides, especially  galena,  pyrite,  marcasite  and  sometimes  chalcopyrite. 
It  occurs  under  a  variety  of  conditions,  the  principal  ones  of  which  are 
mentioned  under  galena  (p.  43).  It  may  be  either  a  primary  or  a 
secondary  mineral  (see  Chapter  on  Ore-Deposits). 

References  on  Rock-forming  Minerals 

1.  Dana,  E.  S.,  A  System  of  Mineralogy,  1900,  6th  edition,  1134  pp. 
Wiley  &  Sons,  New  York. 

2.  Dana,  E.  S.,  A  Text-book  of  Mineralogy,  1904,  593  pp.     Wiley  & 
Sons,  New  York. 

3.  Ford,   W.   E.,   Dana's  Manual   of  Mineralogy,    1912,   460  pp. 
Wiley  &  Sons,  New  York. 

4.  Iddings,  J.  P.,   Rock  Minerals,  Their  Chemical  and  Physical 
Characters,  and  their  Determination  in  Thin  Sections,  1906,  548  pp. 
Wiley  &  Sons,  New  York. 

5.  Kraus,  E.  H.,  Descriptive  Mineralogy  with  Especial  Reference 
to  the  Occurrences  and  Uses  of  Minerals,  1911,  334  pp.     Ann  Arbor, 
Michigan. 

6.  Moses,   A.   J.,   and   Parsons,   C.   L.,   Elements  of  Mineralogy, 
Crystallography  and  Blowpipe  Analysis,  1909,  4th  edition,  444  pp. 
D.  Van  Nostrand  Co.,  New  York. 

7.  Phillips,  A.  H.,  Mineralogy:   An  Introduction  to  the  Theoretical 
and  Practical  Study  of  Minerals,  1912,  699  pp.     The  Macmillan  Co., 
New  York. 

8.  Pirsson,  L.  V.,  Rocks  and  Rock  Minerals,  A   Manual  of  the 
Elements  of  Petrology  without  the  Use  of  the  Microscope,   1908, 
414  pp.     Wiley  &  Sons,  New  York. 


THE  ROCK-FORMING   MINERALS  45 

9.  Rogers,  A.  F.,  Introduction  to  the  Study  of  Minerals,   1912, 
522  pp.     McGraw-Hill  Book  Co.,  New  York. 

For  weathering  of  minerals  see: 

10.  Merrill,  G.  P.,  Rocks,  Rock-weathering  and  Soils,  1906,  400  pp. 
The  Macmillan  Co.,  New  York. 

For  use  of  minerals  in  deposits  see: 

11.  Merrill,  G.  P.,  The  Nonmetallic  Minerals,  1910,  2nd  edition, 
444  pp.     Wiley  &  Sons,  New  York. 

12.  Ries,  H.,  Economic  Geology,  1910,  3rd  edition,  589  pp.     The 
Macmillan  Co.,  New  York. 


CHAPTER  II 

ROCKS,    THEIR    GENERAL    CHARACTERS,    MODE    OF 
OCCURRENCE,   AND   ORIGIN 

Introduction.  —  Knowledge  of  rocks  —  kinds,  their  mineral  com- 
position and  general  properties,  structures  and  textures,  mode  of 
occurrence,  etc.,  —  especially  the  important  and  more  commonly  occur- 
ring varieties  of  igneous,  sedimentary,  and  metamorphic  ones,  is  of 
fundamental  importance  to  the  engineer.  Among  the  more  important 
reasons  why  the  engineer  should  possess  a  good  knowledge  of  the 
different  kinds  of  rocks  may  be  mentioned  the  following:  (1)  Rocks 
differ  greatly  in  their  value  for  building  purposes;  (2)  they  vary 
markedly  in  their  weathering  qualities  —  resistance  to  atmospheric 
agents;  (3)  they  vary  in  hardness,  which  materially  affects  the  rate  of 
drilling  them  and  necessarily  the  cost;  (4)  they  differ  widely  in  struc- 
ture, a  factor  which  has  to  be  considered  in  connection  with  tunneling, 
quarrying  operations,  stability  of  rock  cuts,  dam  foundations,  reser- 
voir sites,  value  for  the  various  uses  to  which  they  are  put,  etc. 

Definition  of  a  rock.  —  Broadly  speaking,  a  rock  in  the  geological 
sense  is  the  material  that  forms  an  essential  part  of  the  earth's  solid 
crust,  and  includes  loose  incoherent  masses,  such  as  a  bed  of  sand, 
gravel,  clay,  or  volcanic  ash,  as  well  as  the  very  firm,  hard,  and  solid 
masses  of  granite,  sandstone,  limestone,  etc.  Most  rocks  are  aggre- 
gates of  one  or  more  minerals,  but  some  are  composed  entirely  of  glassy 
matter,  or  of  a  mixture  of  glass  and  minerals.  When  consisting  en- 
tirely of  mineral  aggregates,  a  rock  may  be  simple  if  composed  of  a 
single  mineral,  such  as  pure  marble  made  up  of  calcite,  or  pure  quartzite 
of  quartz;  or  compound  if  composed  of  several  minerals,  such  as  com- 
mon granite  which  is  made  up  of  a  mixture  of  grains  of  feldspar,  quartz, 
and  mica. 

Many  common  rock  names  are  loosely  used,  and  this  often  leads  to 
trouble.  In  letting  contracts  for  quarrying,  tunneling,  etc.,  the  con- 
tractor may  often  base  his  estimates  on  the  nature  of  the  rock  to  be 
removed,  and  neglect  on  the  part  of  either  party  to  properly  identify 
or  designate  the  kind  of  material  to  be  taken  out  has  not  infrequently 
led  to  serious  misunderstanding  and  disagreement,  inconvenient  as 
well  as  expensive  to  one  party  or  the  other. 

46 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       47 

The  common  minerals  which  enter  into  the  composition  of  rocks 
have  been  treated  at  length  in  Chapter  I. 

In  the  study  of  rocks  the  following  essential  features  should  be  con- 
sidered before  describing  the  individual  types  under  each  of  the  three 
main  divisions  named  below:  (1)  Mode  of  occurrence  or  geological 
relations;  (2)  composition  or  character  of  the  component  minerals; 

(3)  texture  or  manner  of  aggregation  of  the  component  minerals;   and 

(4)  structure  or  mode  of  arrangement.     These  subjects  are  treated  in 
the  following  pages  of  this  chapter,  and  in  every  case  the  practical 
bearing  is  pointed  out  so  far  as  is  possible. 

Varieties  of  rocks.  —  Many  principles  have  been  made  the  bases 
of  various  schemes  for  grouping  or  classifying  rocks,  among  the  more 
important  of  which  may  be  mentioned:  (a)  texture  and  structure; 
(6)  mineralogical  composition;  (c)  chemical  composition;  (d)  geologi- 
cal age;  (e)  origin  or  genesis;  or  a  combination  of  several  of  these. 
A  discussion  of  these  is  not  only  unnecessary  but  beyond  the  scope  of 
this  book. 

Based  on  the  principle  of  genesis  or  mode  of  origin  rocks  may  be 
grouped  into  three  large  classes,  now  recognized  quite  generally  by  all 
geologists.  These  are: 

(I)  Igneous  rocks,  those  which  have  solidified  from  molten  material. 

(II)  Sedimentary  rocks  (also  called  stratified  rocks),  those  which  have 
been  laid  down  chiefly  under  water  (aqueous)  by  mechanical,  chemical, 
or  organic  agents,     Under  this  division  is  included  also  a  smaller  group 
of  wind-formed  rocks  (ceoliari). 

(III)  Metamorphic  rocks,  those  which  have  been  formed  from  original 
igneous   or  sedimentary  rocks  by  alteration,   through  the  action  of 
subsequent  processes  (the  work  chiefly  of  pressure,  heat,  and  water), 
which  have  resulted  in  wholly  or  partly  obscuring  their  original  char- 
acters. ^^^ 

These  three  divisions  will  be  adopted  in  the  following  p^es,  each 
division  being  separately  treated  in  the  order  named. 

IGNEOUS  ROCKS 
OCCURRENCE  AND  ORIGIN 

When  fresh  and  unaltered  the  igneous  rocks  frequently  possess 
certain  characters  by  which  they  may  be  distinguished  from  the  sedi- 
mentary and  metamorphic  ones.1 

1  The  igneous  rocks  forming  the  walls  of  some  ore  deposits  are  sometimes  so 
altered  by  hot  ascending  solutions,  that  it  is  difficult  to  identify  them,  except  by 
careful  microscopic  study.  (See  Chapter  on  Ore-Deposits.) 


48  ENGINEERING  GEOLOGY 

The  evidence  gained  by  careful  study  in  the  field  as  to  the  mode 
of  occurrence  or  geologic  relations  of  the  rocks  to  surrounding  ones 
whether  formed  as  dikes,  lava  sheets,  etc.,  will  frequently  determine 
the  igneous  origin  of  a  rock.  Again,  mineral  composition  serves  as  an 
important  distinguishing  characteristic.  If  composed  wholly  or  partly 
of  glass,  the  rock  is  certainly  of  igneous  origin;  or,  if  made  up  entirely 
of  mineral  aggregates,  the  presence  of  certain  minerals  is  usually 
regarded  as  strong  evidence  of  igneous  origin.  Finally,  structure  and 
texture  oftentimes  furnish  an  important  means  of  identification.  An 
igneous  rock  usually  appears  homogeneous  and  massive,  without  evi- 
dence of  stratification1  and  foliation  or  banding,  structures  that  are 
common  to  sedimentary  and  metamorphic  rocks,  although  occasionally 
observed  in  some  igneous  masses  (for  example  volcanic  tuffs).  Amyg- 
daloidal  texture  (p.  69)  is  characteristic  of  many  surface  lava  flows. 
At  times  the  igneous  rock  may,  by  its  temperature  or  in  other  ways, 
have  altered  the  surrounding  rock  near  the  contact  in  a  characteristic 
manner.  Fossils  are  not  found  in  igneous  rocks,  except  rarely  in  tuffs. 

Mode  of  Occurrence 

As  previously  stated,  igneous  rocks  have  been  formed  by  the  con- 
solidation of  molten  material,  the  source  of  which  was  within  the  earth 
at  some  unknown  depth  beneath  the  surface.  At  times  and  in  various 
localities,  this  molten  material  under  proper  conditions  is  forced  up- 
ward for  one  cause  or  another  towards  the  surface  of  the  earth,  cutting 
through  or  intruding  any  other  kind  of  rock.  It  may  be  arrested  at 
some  depth  below  the  surface  where  it  is  cooled  and  solidified  under 
the  influence  of  the  surrounding  rocks,  or  it  may  reach  the  surface  and 
be  poured  out  upon  it,  solidifying  to  form  hard  rock. 

This  conception  leads  to  a  two-fold  division  of  igneous  rocks.  (1) 
Those  that  have  solidified  at  considerable  depths  beneath  the  surface, 
designated  intrusive  or  plutonic;  and  (2)  those  that  have  solidified  at 
or  on  the  surface,  designated  extrusive  or  volcanic.  Each  of  these  may 
be  further  subdivided. 

Intrusive  or  Plutonic  Rocks 

Forms  of  intrusive  rocks.  —  The  principal  modes  of  occurrence  of 
intrusive  igneous  rocks  recognized  by  geologists  are  as  follows:  Dikes, 
sheets,  laccoliths,  necks,  stocks,  and  batholiths. 

1  Occasionally  regular  horizontal  jointing  is  mistaken  for  stratification  by  per- 
sons having  but  slight  geological  knowledge. 


PLATE  I,  FIG.  1.  —  Parallel  dikes  of  diabase  cutting  pegmatite  dike,  near  Pourpour, 
Quebec.     (H.  de  Schmid,  photo.) 


FIG.   2.  —  Irregular   granite   dikes   cutting   gneiss,   Moose   Mountain,   Ont. 

(H.  Hies,  photo.) 

(49) 


50 


ENGINEERING  GEOLOGY 


Dikes.  —  A  dike  results  from  the  filling  of  a  fissure  in  other  rocks 
(Plate  I)  by  molten  material  from  below,  and  there  solidified.  It  is 
the  simplest  form  of  intrusion,  and  has  great  length  as  compared  with 
thickness;  hence,  it  is  an  elongated  and  relatively  narrow  body,  which 
may  range  from  a  fraction  of  an  inch  in  width  and  a  few  yards  in  length 
to  a  hundred  feet  and  more  across  and  miles  in  length.  In  inclination 
dikes  may  vary  from  vertical  to  horizontal,  the  most  frequent  attitude 
being  that  of  vertical  or  nearly  so. 

Frequently  they  may  be  observed  extending  outward  from  larger 
masses  of  intruded  rock  (Fig.  52),  but  in  many  cases  such  a  relation- 
ship is  not  visible.  They  may  continue  along  remarkably  straight 
lines  or  follow  irregular  or  sinuous  courses  (Plate  I,  Fig.  2).  A  large 
dike  may  divide  into  two  or  more  smaller  ones  which  continue  usually 
in  the  same  general  direction,  and  apophyses  or  stringers  are  common. 
The  igneous  rock  of  the  dike  may  be  acid  or  basic  in  character,  and 


FIG.  47.  —  Section  through  dike  more 
resistant  to  weathering  than  the  in- 
closing rock,  marking  the  position  of 
a  ridge,  (a)  dike;  (6)  inclosing  rock. 


FIG.  48.  —  Section  through  dike  less 
resistant  to  weathering  than  the 
inclosing  rock,  marking  the  position 
of  a  valley,  (a)  dike;  (6)  wallrock. 


dikes  of  each  are  common  over  many  parts  of  the  eastern  or  Atlantic 
province  of  crystalline  rocks  (see  plate  showing  granite  areas  in  Chap- 
ter XI).  The  large  dikes  almost  invariably  show  finer-grained  texture 
along  the  margins  than  in  the  centers,  whereas  the  narrow  dikes  are 
apt  to  be  fine-grained  throughout.  Also  some  of  the  large  dikes  show 
alteration  of  the  inclosing  rocks  along  the  contacts. 

Subsequent  erosion  and  weathering  of  a  dike  may  or  may  not  result 
in  topographic  expression  (Figs.  47  to  49).  Usually  if  the  dike  rock  is 
more  resistant  to  weathering  and  erosion  than  the  inclosing  rocks,  the 
position  of  the  dike  will  be  marked  by  a  ridge  (Fig.  47).  Sometimes 
the  opposite  effect  is  shown  and  a  valley-like  depression  results  (Fig. 
48).  Again,  it  frequently  happens  that  no  topographic  expression  is 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       51 

shown  (Fig.  49),  and  as  in  the  crystalline  province  of  the  eastern  United 
States,  the  only  surface  indication  remaining  to  mark  the  position  of 
the  dike  is  a  line  of  large  and  small  boulders  of  the  original  dike  scat- 
tered loose   over   the   surface  and 
partly  buried  hi  the  resulting  resid- 
ual rock  decay  (clay)  (Plate  XXXI). 
Dikes  are  so  abundant  that  the 
engineer  frequently  encounters  them 
in  the  field.     They  are  often  not  of 
any  value  as  road  or  building  ma- 
terial, because  of  their  narrow  width, 
and    their    occurrence    hi    quarries 
FIG.  49. -Section  through  dike  and    (Plate  VI,  Fig.  1)  is  objectionable  be- 
inclosing    rock,    showing    no    topo-  , ,  M  , ,  , 

graphic    expression   from   weather-    cause  they  spoil  the  stone,  and  some- 
ing,      (a)  dike;    (6)  inclosing  rock.    times  crack  jt  UP  badly.    Abundant 

dikes    therefore    may   mean   much 

waste,  unless  the  defective  stone  can  be  broken  up  for  road  material. 
In  some  localities  the  dike  rock  may  be  weathered  (but  not  eroded) 
to  such  an  extent  that  it  permits  access  of  surface  water.  If  then 
these  decayed  dikes  are  encountered  in  underground  operations,  the 
water  seeping  downward  along  them  may  give  trouble.1 

Ore  bodies  sometimes  but  not  always  are  associated  with  dikes, 
while  at  other  times  a  dike  of  later  age  may  cut  across  the  ore  deposit, 
a  condition  which  has  sometimes  been  misinterpreted,  and  led  to  the 
belief  that  the  ore  had  given  out. 

Another  case  of  error  has  been  caused  by  the  occurrence  of  somewhat 
broad  parallel  dikes,  whose  adjoining  boundaries  were  hidden  by  sur- 
face material,  leading  the  engineer  to  suspect  that  the  two  were  one 
large  dike. 

Intrusive  sheets.  —  Intrusive  sheets,  known  also  as  sitts,  are  the 
solidified  bodies  of  molten  material  intruded  between  the  stratification 
or  foliation  planes  of  sedimentary  and  metamorphic  rocks,  and  hence 
they  assume  a  somewhat  bedded  aspect  (Fig.  50).  They  are  character- 
ized by  relatively  great  lateral  extent  as  compared  with  their  thickness. 
Probably  the  basic  and  intermediate  igneous  rocks,  such  as  andesites 
and  basalts,  assume  the  form  of  intrusive  sheets  more  frequently  than 
the  acid  rocks. 

Sheets  may  range  from  a  foot  to  several  hundred  feet  or  more  in 

1  A  band  of  clayey  rock  encountered  underground  does  not  always  represent 
decayed  dike  rock,  but  is  sometimes  rock  which  has  been  first  crushed  by  movement 
along  a  fracture  (faulting).,  and  subsequently  weathered  by  percolating  water. 


52 


ENGINEERING   GEOLOGY 


thickness,  and  may  cover  an  area  many  miles  in  extent.     "The  Pali- 
sades of  the  Hudson  are  formed  by  a  sheet  of  unusual  thickness;  its  out- 


Surfa( 


FIG.  50.  —  Section  through  (a),  extrusive  and  (6)  intrusive  sheets,  and  (c)  conduit. 

crop  is  70  miles  long  from  north  to  south,  and  its  thickness  varies  from 
300  to  850  feet"  (Scott).  Sheets  sometimes  break  across  the  strata 
and  are  continued  at  a  new  horizon  (Fig.  51).  Frequently  thick  sheets 


Surface 


FIG.  51.  —  Section  of  intrusive  sheet,  breaking  across  the  strata  and  continuing  in 
the  same  general  direction  at  a  higher  horizon;  sheet  shows  apophyses  and 
inclusions  of  country  rock. 

or  sills  divide  into  several  subordinate  ones,  each  following  more  or 
less  closely  a  plane  of  bedding. 

Intrusive  sheets  may  sometimes  be  mistaken  for  surface  lava  flows 
that  have  subsequently  been  buried.  They  may  often  be  distinguished 
from  contemporaneous  sheets  or  flows  by  (a)  alteration  by  heat  of  the 
beds  immediately  above  and  below;  (6)  breaking  across  the  beds  at 
any  point  and  continued  along  another  horizon;  (c)  giving  off  of 
tongues  or  apophyses  into  the  overlying  as  well  as  underlying  beds; 
(d)  the  general  absence  from  the  upper  surface  of  scoriaceous  or  tuffaceous 
material  and  of  vesicular  and  amygdaloidal  textures  (which  see) ;  (e)  in- 
corporation of  rock  fragments  in  the  sheet  torn  from  the  overlying  bed, 
etc. 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC. 


53 


Sheets  or  sills  do  not  always  show  the  same  mineral  composition  from 
top  to  bottom  (see  magmatic  differentiation,  p.  70).  Where  such 
variation  exists  the  rock  may  be  dark-colored  or  basic  at  the  bottom 
and  lighter-colored  and  siliceous  at  the  top,  affording  two  different 
types  of  building  stone.  Such  a  difference  exists  in  the  sill  of  Sudbury, 
Ontario,  where  the  nickel-copper  ores  are  found  only  in  the  basic  or 
lower  portion  of  the  sill.  Sheets  or  sills  are  not  of  much  importance  as 
a  source  of  building  stone. 

Laccoliths.  —  A  laccolith  is  a  lenticular  or  dome-shaped  mass 
of  igneous  rock  intruded  between  strata.  It  may  be  considered  as  a 
special  case  of  an  intrusive  sheet  in  which  the  supply  of  molten  material 
from  below  exceeds  the  rate  of  lateral  spreading,  and  is  accompanied  by 
arching  of  the  overlying  beds  at  the  surface.  A  section  through  the 
igneous  mass  usually  shows  a  flat  base  and  a  convex  upper  surface 
(Fig.  52),  resembling  a  half  lens.  Figs.  52  and  53  show  variations  in 


FIG.  52.  —  Section  through  laccolith  showing  associated  sheets  and  dikes. 
Compare  outline  of  laccolith  with  that  of  Fig.  53. 


FIG.  53.  —  Section  through  partly  eroded  laccolith  showing  different  outline 

from  Fig.  52. 

the  general  structure  of  laccoliths  due  probably,  as  has  been  suggested 
by  some,  to  progressive  increase  of  viscosity  of  the  magma  during  its 
intrusion.  In  plan  the  mass  approximates  a  circle,  but  may  be  some- 
what elongated  and  oval-shaped,  and  in  size  (thickness  and  lateral 
extent)  is  subject  to  great  variation.  In  some  cases  the  laccolith  is 
accompanied  by  intrusive  sheets  and  dikes  (Fig.  52),  and  like  the 


54 


ENGINEERING  GEOLOGY 


latter  they  may  and  do  frequently  alter  by  metamorphism  the  overly- 
ing and  underlying  beds.  The  pressure  of  the  intruded  magma  form- 
ing the  laccolith  usually  causes  a  lifting  of  the  overlying  strata  and 
produces  a  dome-like  elevation  at  the  surface  (Fig.  52).  Laccoliths 
may  occur  singly,  though  they  often  occur  in  groups,  a  dozen  or  more 
being  clustered  together  in  some  instances. 

The  Henry  Mountains  of  Utah,  first  described  by  G.  K.  Gilbert,  form 
a  typical  representative  of  the  laccolithic  method  of  intrusion.  Here, 
many  stages  of  erosion  are  represented  and  may  be  observed.  Many 
other  examples  of  laccoliths  are  known  in  the  western  United  States 
and  in  Europe. 

Laccoliths,  like  sills,  may  sometimes  show  a  zonal  structure,  and 
hence  the  center  and  margins  might  supply  different  kinds  of  rock. 


CP-T?\V,  \  /  4>>^£faee      \ 


FIG.  54.  —  Section  through  volcanic  neck  or  plug  (a),  volcanic  cone  shown 
by  dotted  lines,  removed  by  erosion. 


FIG.  55.  —  Plan  of  volcanic  neck  or  plug  (a). 

Necks.  —  These  are  roughly  cylindrical  masses  of  igneous  rock 
having  probably  great  but  unknown  depth,  which  fill  the  vents  or  con- 
duits of  volcanoes.  Erosion  may  remove  practically  all  trace  of  the 
surrounding  beds  of  more  porous  and  softer  volcanic  ejectments,  leav- 
ing the  plug  of  resistant,  consolidated  igneous  rock  as  a  more  or  less 
conspicuous  topographic  form  (Fig.  54).  Volcanic  necks  may  range 
up  to  a  mile  or  more  across,  and  are  usually  more  or  less  circular  in 
plan  (Fig.  55).  Good  examples  of  necks  are  noted  in  places  over  the 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC. 


55 


western  half  of  the  United  States,  especially  those  of  western  New 
Mexico. 

Stocks.  —  Stocks,   known   also   as  bosses,   are   irregular,   rounded 
masses  of  igneous  rock  intruded  and  solidified  at  some  depth  beneath 


FIG.  56.  —  Section  through  stock  or  boss,     (a)  granite  boss;   (6)  inclosing  rock. 

the  surface,  and  now  exposed  from  stripping  by  erosion  of  the  thickness 
of  overlying  rocks  (Fig.  56  and  Plate  XXXIV,  Fig.  2). 

Stocks  may  range  in  size  from  a  few  hundred  feet  to  several  miles; 
and  in  plan  they  may  vary  from  more  or  less  circular  to  elliptical  in 
outline  (Fig.  57).  They  may  cut  across  the  inclosing  (country)  rock 


FIG.  57.  —  Plan  of  stock  or  boss,     (a)  granite;   (6)  inclosing  rock. 

with  frequently  steeply-inclined   contacts,  along  which  characteristic 
metamorphism  is  often  observed. 

Because  the  rock,  especially  granite,  composing  stocks  or  bosses 
is  frequently  of  more  resistant  character  than  the  surrounding  or 
country  rock,  they  become  dome-like  masses  of  steep  or  gentle  slopes, 
and  oftentimes  on  account  of  size  are  conspicuous  topographic  forms 
(Plate  XXXIV,  Figure  2).  Many  of  them  show  an  elevation  of  several 
hundred  feet,  and  in  extreme  cases  700  or  800  feet  and  more  above  the 
surface  of  the  surrounding  rocks,  such  as  Stone  Mountain,  Georgia, 
and  the  splendid  granite  domes  of  the  Yosemite  in  California.  On  the 
other  hand  in  regions  of  old  land  surfaces  which  have  been  continuously 


56 


ENGINEERING  GEOLOGY 


exposed  to  weathering  and  erosion  for  very 
long  periods  of  time,  the  surface  of  the  boss 
shows  no  topographic  expression,  but  is  more 
or  less  flat  and  coincident  with  that  of  the  in- 
closing rocks. 

Batholiths.  —  These  are  huge  masses  of 
plutonic  rock  hundreds  of  miles  in  extent 
which  are  now  exposed  at  the  surface  by  ero- 
sion (Fig.  58).  They  are  similar  to  stocks, 
but  differ  from  them  mainly  in  their  much 
larger  size,  the  small  batholith  and  the  large 
stock  grading  into  each  other.  If  they  could 
be  followed  down,  probably  many  stocks 
would  prove  to  be  protrusions  from  batho- 
liths  (Fig.  58).  Batholiths  are  shown  in  the 
oldest  regions  of  the  earth,  such  as  eastern 
Canada,  etc.,  or  forming  the  core  of  many 
mountain  ranges,  like  the  Sierra  Nevada  and 
Rocky  Mountains.  They  usually  consist  of 
some  granitoid  rock,  such  as  granite,  syenite, 
diorite,  etc.,  but  probably  granite  is  the  com- 
monest rock  forming  them.  The  country  rock 
surrounding  them  is  also  variable. 

Both  batholiths  and  stocks  are  important 
sources  of  granitic  rock  for  .use  in  structural 
work.  The  massive  character  of  the  rock,  and 
the  arrangement  and  spacing  of  the  joints 
make  the  material  well  adapted  for  the  extrac- 
tion of  dimension  blocks. 

In  the  West  important  ore  bodies  are  some- 
times found  along  the  borders  of  such  batho- 
liths. 

Extrusive  or  Volcanic  Rocks 

These  may  be  (a)  molten  material  poured 
out  onto  the  surface  from  a  volcanic  vent  or 
along  a  fissure  and  solidified,  or  (6)  fragmental 
material  (pyroclastic)  of  all  sizes  erupted  from 
volcanic  vents.  The  first  forms  surface  lava 
flows  and  sheets,  the  second  ash-beds  (Plate 
VI,  Fig.  2),  and  coarser  fragmental  material, 
which  on  consolidation  yield  beds  of  tuffs  and 


\ 


\    I 


\ 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.        57 

volcanic  breccias.  The  crystalline  (lava  flows)  and  fragmental  mate- 
rials frequently  occur  interstratified  as  shown  in  Fig.  59.  The  frag- 
mental materials  show  all  varieties  of  texture  and  structure,  some 
being  very  fine-grained  while  others  are  very  coarse,  but  bedding  is 
usually  pronounced. 

Lava  flows  and  sheets.  —  These  are  formed  on  the  surface  from 
quiet  outwellings  of  highly  molten  material  through  (a)  a  localized 
opening  or  volcanic  vent  and  hence  connected  with  volcanic  eruptions, 
or  (6)  from  fissures  not  connected  with  volcanic  eruptions.  The  lava 
flow  may  be  either  subaerial  (on  land)  or  submarine,  according  to 
whether  the  eruption  takes  place  on  land  or  on  the  sea  bottom.  The 
flows  vary  much  in  thickness,  some  being  only  a  few  feet  while  others 
are  measured  in  yards. 

Subaerial  flows  from  volcanic  vents  may  build  cones  having  very  low 
angles  of  slope  and  of  great  lateral  extent,  according  to  the  fluidity  of 
the  lava  erupted,  such  as  the  volcanic  cones  of  Hawaii  and  Iceland. 
Thus  the  more  basic  lavas  are  the  more  fluid.  These  may  alternate 


FIG.  59.  —  Section  through  a  series  of  interbedded  lava  flows,  and  fragmental 
materials,     (a)  lava  flows;   (6)  fragmental  materials. 

with  extrusions  of  fragmental  material  (Fig.  59),  when  a  cone  of  com- 
posite character  and  steeper  slopes  is  formed  (Plate  II,  Fig.  1). 

In  many  places  over  the  earth's  surface  lava  flows  have  resulted  from 
the  quiet  outpouring  onto  the  surface  through  fissures,  spreading  in  some 
cases  hundreds  of  miles  in  extent  and  several  thousand  feet  in  thickness. 
Such  fissure-eruptions  have  occurred  on  a  gigantic  scale  in  the  Columbia 
River  region  of  the  northwestern  United  States,  in  eastern  India,  in 
the  north  of  the  British  Isles,  and  in  historic  times  in  Iceland. 

In  some  cases  surface  lava  sheets  have  later  become  buried  by  de- 
position of  other  rocks  on  them  through  depression  below  sea-level.  In 
such  cases  the  buried  sheet  resembles  one  of  intrusion,  but  can  usually 
be  distinguished  from  the  latter  by  absence  of  metamorphism  of  the 
overlying  beds,  and  the  structures  characteristic  of  the  surface  of  lavas, 
such  as  scoriaceous,  amygdaloidal,  vesicular,  etc. 


PLATE  II,  FIG.  1.  —  Volcanic  cone  of  Colima,  Mexico.  Built  up  of  ash  and  lava 
flows.  Parasitic  cone  of  1865  on  left.  Ridge  in  foreground  part  of  base  of 
original  cone  destroyed  by  an  explosive  eruption.  (H.  Ries,  photo.) 


FIG.  2.  —  Table  Mountain,  Golden,  Colo.     Capped  by  several  flows  of  resistant 
basalt.     Under  these  upturned  beds  of  sedimentary  rocks.     (H.  Ries,  photo.) 

(58) 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       59 

The  fragmental  (pyroclastic)  materials  are  those  which  have  been 
thrown  out  with  great  force  and  in  enormous  volume,  during  violent 
volcanic  eruptions.  They  have  settled  down  over  the  surrounding 
country,  either  on  land  (Plate  VI,  Fig.  2)  or  hi  water,  and  hence  often 
show  a  stratified  structure. 

In  the  western  states  and  Mexico,  where  these  volcanic  rocks  are 
abundant  the  engineer  has  to  deal  with  them. 

Lava  flows,  though  often  thick,  are  sometimes  shallow,  and  overlie 
stream  gravel  or  other  deposits  (Plate  XLIII,  Fig.  1).  When  testing  a 
rock  foundation  for  dams,  reservoirs  or  other  structures,  which  are  to 
be  placed  on  lava  flows,  care  should  be  taken  to  see  that  the  lava  cap 
is  sufficiently  thick  to  give  a  solid  and  impermeable  base.1 

Lava  flows  are  not  as  a  rule  adapted  to  the  production  of  large  blocks. 
Many  show  a  columnar  jointing  (Plate  III,  Fig.  2).  The  stone  at  the 
surface  of  the  flow  may  be  broken  up  (Plate  III,  Fig.  1),  or  if  massive 
is  often  full  of  gas  cavities,  which  may  be  absent  deeper  down  (Plate 
HI,  Fig.  2). 

The  more  porous  and  softer  volcanic  rocks,  like  tuffs  and  agglomer- 
ates, can  often  be  cut  into  larger  blocks  than  the  consolidated  lavas. 
They  are  however  usually  very  porous,  and  should  not  if  possible  be 
used  in  moist  situations.  Curiously  enough  however  many  of  these 
very  porous  volcanic  rocks  are  not  injured  by  frost,  probably  because 
they  do  not  absorb  enough  water  to  completely  fill  their  pores.  (See 
absorption  under  Building  Stones.) 

The  high  porosity  of  tuffs  and  breccias  may  also  cause  trouble  in  dam 
and  reservoir  construction,  because  they  permit  seepage  under  the 
walls,  so  that  the  bed  rock  may  have  to  be  filled  with  grout,  or  sealed 
up  in  other  ways.  In  the  case  of  one  dam  foundation  on  the  Clagamas 
River  in  Oregon,  grout  forced  down  a  50-foot  pipe  under  a  200  pounds 
pressure,  crossed  a  six-foot  interval  in  the  volcanic  breccia,  rushed  up 
another  pipe  to  the  surface  and  spurted  30  feet  into  the  air.  For  similar 
reasons  a  tunnel  driven  through  them  should  be  lined. 

The  use  of  volcanic  ash  for  hydraulic  cement  is  referred  to  in  Chapter 
XII. 

Composition  of  Igneous  Rocks 

Under  this  heading  is  discussed  (a)  chemical  and  (6)  mineralogical 
composition  of  igneous  rocks.  As  previously  stated,  most  igneous  rocks 
are  made  up  of  mineral  aggregates.  For  such  rocks  mineral  compo- 
sition is  dependent  hi  large  measure  on  chemical  composition  of  the 

1  For  example  see  case  of  Zuni  Dam,  Eng.  News,  LXIV,  p.  203,  1909. 


60  ENGINEERING  GEOLOGY 

rock  magmas.1  When  solidified  under  different  physical  conditions, 
rock  magmas  having  similar  chemical  composition  may  yield  different 
minerals;  and  differences  in  chemical  composition  usually  result  in 
variations  in  mineral  composition.  Chemical  composition  plays  a  fun- 
damental role  in  the  classification  of  igneous  rocks,  as  discussed  later. 

Chemical  composition.  —  It  is  obvious  that  rock  magmas  as  such 
cannot  be  subjected  to  chemical  analysis,  but  their  solidified  products 
(rocks)  can;  and  from  the  very  large  number  of  analyses  made  of 
igneous  rocks  from  all  parts  of  the  world,  they  are  shown  to  be,  with- 
out exception,  silicate  magmas.  The  many  hundreds  of  analyses  that 
have  been  made  of  igneous  rocks  invariably  show  that  they  contain 
the  following  principal  oxides:  Silica  (Si02);  alumina  (A1203);  iron 
oxides,  ferric  (Fe203)  and  ferrous  (FeO) ;  magnesia  (MgO) ;  lime  (CaO) ; 
soda  (Na2O);  and  potash  (K20).  Other  lesser  oxides,  including  water, 
are  present,  but  no  account  is  taken  of  them  here,  since  they  usually 
occur  in  such  small  amounts  that  they  do  not  exert  any  important 
influence  on  the  rock. 

Igneous  rocks  show  varying  chemical  composition,  which  is  used  by 
the  geologist  to  study  their  relationships,  but  to  the  engineer  chemical 
analysis  is  not  of  much  practical  value.  Igneous  rocks  form  a  series 
ranging  from  acid  ones  (high  in  silica),  with  dominant  alkali  feldspar 
and  quartz,  to  basic  ones  (low  in  silica)  with  ferromagnesian  silicate 
minerals  predominating. 

Since  the  acid  magmas  contain  silica  in  excess  of  the  bases,  these  will  develop 
free  quartz  in  the  rocks  crystallized  from  them.  The  total  percentage  of  silica 
in  them  may  reach  80  per  cent.  On  the  other  hand,  many  magmas  are  low  in 
silica,  as  shown  in  the  analyses  of  the  rocks  formed  from  them.  In  the  basic 
rocks  the  percentage  of  silica  may  be  as  low  as  40  per  cent,  and  in  some  ultra- 
basic  ones  it  may  be  even  lower,  not  exceeding"  30  per  cent.  The  amount  of  silica 
present  exercises  an  important  influence  on  the  crystallization  of  the  magma,  as 
discussed  later. 

The  eight  principal  oxides  enumerated  above  as  composing  igneous  rocks  do  not 
exist  as  free  oxides  except  in  a  few  cases  and  with  but  few  exceptions  only  in  small 
amounts.  Of  these  the  iron  oxides  are  the  most  frequently  occurring  ones,  al- 
though alumina  as  the  mineral  corundum  is  sometimes  present.  With  these  ex- 
ceptions, the  oxides  of  aluminum,  iron,  magnesium,  calcium,  sodium,  and  potassium 
are  combined  in  the  form  of  silicate  minerals,  which,  with  rare  exceptions,  com- 
pose the  igneous  rocks. 

Alumina  may  range  from  nothing  in  some  of  the  nonfeldspathic  rocks,  such  as 
the  peridotites,  to  20  per  cent  and  more  in  some  syenites.  It  is  present  chiefly  in 

1  Magma  is  now  generally  employed  for  the  molten  masses  of  igneous  rock  be- 
fore they  have  crystallized.  An  original  parent  magma  may  break  up  into  several 
derived  ones.  J.  F.  Kemp,  Handbook  of  Rocks,  1906,  p.  202. 


62 


ENGINEERING  GEOLOGY 


rocks  in  combination  with  silica  and  the  alkalies,  and  in  some  cases  lime,  as  feld- 
spars and  feldspathoids.  It  also  enters  into  the  composition  of  some  of  the  so-called 
ferromagnesian  minerals,  such  as  mica,  hornblende,  augite,  etc.  As  noted  above, 
alumina  is  sometimes  present  in  rocks  as  the  mineral  corundum. 

The  oxides  of  iron  and  magnesium  combine  with  silica  to  form  the  so-called 
ferromagnesian  minerals,  which  comprise  the  pyroxene,  hornblende,  biotite,  and 
olivine  groups  as  the  principal  rock-forming  ones  (see  Chapter  I) .  Lime  enters  into 
combination  with  the  same  bases  and  silica  in  the  monoclinic  pyroxenes  and  amphi- 
boles,  and  is  an  important  constituent  in  the  more  calcic  (basic)  plagioclase  feld- 
spars. It  is  essentially  absent  from  the  orthorhombic  pyroxenes  and  biotite. 

The  ferromagnesian  minerals  are  usually  present  in  only  subordinate  amounts  in 
the  acid  rocks,  but  increase  in  quantity  and  are  the  predominant  minerals  in  the 
basic  rocks. 

The  alkalies,  potash  and  soda,  in  combination  with  alumina,  silica,  and  in  some 
cases  lime,  are  of  fundamental  importance  in  the  feldspars  (orthoclase  and  plagio- 
clase groups),  and  the  feldspathoids.  They  are,  especially  soda,  important  con- 
stituents in  the  alkali-rich  pyroxenes  and  amphiboles;  and  potash  enters  into  the 
composition  of  biotite. 

Phosphoric  anhydride  (P2O5)  and  titania  (Ti02)  among  the  lesser  oxides  are  quite 
generally  present  in  igneous  rocks;  the  former  in  combination  with  lime  as  the 
mineral  apatite  is  of  most  importance  in  the  basic  rocks;  while  the  latter  occurs 
as  free  oxides  in  the  minerals  ilmenite  and  sometimes  rutile,  as  the  lime  titano- 
silicate  sphene,  the  lime  titanate  perovskite,  and  in  variable  but  small  quantities  in 
the  ferromagnesian  silicates. 

Boron,  fluorine,  and  chlorine  frequently  occur  in  minute  quantities  in  igneous 
rocks;  as  do  also  sulphur  and  carbon,  the  former  as  sulphides,  especially  as  the 
mineral  pyrite,  and  the  latter  in  elementary  form  as  graphite. 

The  annexed  table  will  serve  in  some  measure  to  give  a  general  idea  of  the 
composition  of  the  principal  types  of  plutonic  igneous  rocks.  Analyses  of  the 
corresponding  volcanic  rocks  are  omitted  from  the  table,  since  they  have  similar 
composition  to  their  equivalent  plutonic  types. 

TABLE  OF  ANALYSES  OF  PLUTONIC  IGNEOUS  ROCKS 


I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

SiO2         

72.27 

65.43 

60.39 

70.36 

62.71 

46.85 

45.05 

33  84 

A12O3      

14.30 

16.11 

22.57 

15.47 

17.06 

19.72 

6.50 

5  88 

FesO^ 

1.16 

1.15 

0.42 

0.98 

3.79 

3  22 

3  83 

7  04 

FeO 

0.97 

2.85 

2.26 

1.17 

2.74 

7  99 

7  69 

5  16 

MgO 

0.70 

0.40 

0.13 

0.87 

1.78 

7.75 

12  07 

22  96 

CaO                       .     .     . 

1.56 

1.49 

0.32 

3.18 

5.51 

13.10 

18  82 

9  46 

Na^jO                         .    .    . 

3.46 

5.00 

8.44 

4.91 

3.54 

0.09 

0  94 

0  33 

K2O             

5.00 

5.49 

4.77 

1.71 

2.96 

1.56 

0  78 

2  04 

Rest         

0.83 

2.26 

0.65 

1.43 

0.14 

0.56 

5  20 

13  83 

100.25 

100.18 

99.95 

100.08 

100.33 

100.84 

100.88 

100.54 

T.  Biotite  granite,  near  Richmond,  Virginia;  II.  Syenite,  Mount  Ascutney,  Vermont;  III.  Nepheline  sy- 
enite (Litchfieldite),  Litchfield  County,  Maine;  IV.  Quartz  diorite,  near  Enterprise,  Butte  County, 
California;  V.  Diorite,  Bush  Creek,  Elk  Mountains,  Colorado;  VI.  Gabbro,  Baltimore,  Maryland; 
Average  of  23  samples:  VII.  Pyroxenite,  Brandberget,  Norway;  VIII.  Peridotite,  Crittenden  County, 
Kentucky. 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.        63 

Study  of  this  table  of  analyses  of  the  principal  types  of  plutonic  igneous  rocks 
discloses  wide  variations  in  the  eight  chief  component  oxides.  Silica,  alumina,  and 
the  alkalies  (soda  and  potash)  are  the  principal  components  in  the  most  acid  rock 
granite,  which  indicates  that  feldspar  and  quartz  are  the  dominant  minerals.  As 
the  basic  and  ultrabasic  types  are  approached,  these  oxides  decrease  in  quantity 
and  the  oxides  of  iron,  magnesium,  and  calcium  increase,  which,  when  expressed 
mineralogically,  emphasizes  the  increase  of  ferromagnesian  minerals  with  decrease 
of  quartz  and  feldspar;  the  former  being  quite  generally  absent  and  the  latter 
(feldspar)  failing  entirely  in  the  ultrabasic  rocks. 

F.  W.  Clarke  has  calculated  the  average  composition  of  igneous  rocks,  based  on 
the  most  reliable  data  available,  to  be  as  follows: 

TABLE  SHOWING  AVERAGE  COMPOSITION  OF  IGNEOUS  ROCKS 
(Reduced  to  100  per  cent) 

SiO2 59.93 

A12O3 14.97 

FeaOa 2.58 

FeO 3.42 

MgO 3.85 

CaO 4.78 

NaaO 3.40 

K2O 2 . 99 

H20 1.94 

Rest 2.14 

ioo.oo 

Under  "rest"  in  the  table  above  is  included  TiO2,  ZrO2,  C02,  P205,  S,  Cl,  F, 
BaO,  SrO,  MnO,  NiO,  Cr2O3,  V2O3,  and  Li2O. 

Mineral  composition.  —  Most  igneous  rocks  are  aggregates  of 
minerals;  a  few  are  composed  wholly  of  glass,  and  still  others  are  made 
up  of  a  mixture  of  minerals  and  glass.  Given  magmas  of  similar 
chemical  composition  and  vary  the  physical  conditions  of  cooling  on 
solidifying,  and  development  of  different  minerals  will  result. 

The  mineral  composition  affects  the  hardness,  durability,  beauty,  and 
ability  of  the  rock  to  take  a  polish. 

From  the  discussion  under  "chemical  composition"  it  has  been  shown 
that  the  principal  oxides  found  on  analysis  are  combined  with  each 
other  to  form  silicate  minerals,  the  chief  components  of  igneous  rocks. 
The  important  groups  of  these  include  feldspars,  quartz,  and  the  ferro- 
magnesian minerals.  For  convenience  of  classification  the  more  im- 
portant minerals  of  igneous  rocks  may  be  tabulated  under  two  groups 
as  follows: 


Siliceous-aluminous  Group 
(Salic). 

Ferromagnesian  Group 
(Femic). 

Alkalic  feldspar 
Plagioclase  feldspar 
Quartz 
Nephelite 
Sodalite 
Corundum 

Pyroxenes 
Amphiboles 
Biotite 
Olivine 
Iron  ores 

64  ENGINEERING  GEOLOGY 

Considered  mineralogically,  the  acid  rocks  are  characterized  by  the  presence  of 
dominant  alkali  feldspar  and  more  or  less  quartz,  with  subordinate  ferromagnesian 
minerals.  They  are  rich  in  silica,  alumina,  and  alkalies,  but  contain  only  small 
amounts  of  iron,  lime,  and  magnesia,  hence  these  rocks  are  usually  light  in  color, 
have  a  low  density  or  specific  gravity  (average  about  2.6),  and  comparatively  high 
fusion  point. 

Intermediate  rocks  contain  little  or  no  quartz,  but  consist  chiefly  of  alkalic  and 
soda-lime  feldspars,  with  in  some  cases  the  feldspathoids  (nephelite,  sodalite,  etc.), 
with  or  without  ferromagnesian  minerals. 

In  the  basic  igneous  rocks  ferromagnesian  minerals  predominate;  the  dominant 
feldspar  is  a  member  of  the  lime-soda  series,  quartz  is  absent,  and  olivine  is  fre- 
quently present.  They  contain  less  silica  and  alkalies  than  the  acid  rocks,  but  are 
higher  in  iron,  lime,  and  magnesia.  The  rocks  are,  therefore,  much  more  fusible, 
are  dark  in  color,  and  have  a  relatively  high  density  or  specific  gravity,  being  about 
3.0  to  3.2,  reaching  in  the  ultra-basic  rocks  as  much  as  3.6. 

In  the  ultrabasic  rocks,  both  feldspar  and  quartz  are  essentially  absent,  and  one 
or  more  of  the  ferromagnesian  minerals  is  the  dominant  component,  either  horn- 
blende, a  pyroxene,  olivine,  or  a  mixture  of  these. 

According  to  F.  W.  Clarke,1  "a  statistical  examination  of  about  700  igneous  rocks, 
which  have  been  described  petrographically,  leads  to  the  following  rough  estimate 
of  their  mean  mineralogical  composition:" 

Quartz . 12. 0 

Feldspars 59.5 

Hornblende  and  pyroxene 16.8 

Mica 3.8 

Accessory  minerals 7.9 

100.0 

Grouping  of  minerals.  —  A  convenient  and  useful  division  of  the  rock- 
forming  minerals  which  enter  into  the  composition  of  igneous  rocks  is 
into  (a)  essential  and  (b)  accessory.  Essential  minerals  influence  greatly 
the  character  of  a  rock  and  their  presence  is  therefore  necessary  for  the 
naming  of  it.  For  example,  quartz  with  certain  other  minerals  is  essen- 
tial to  the  naming  of  a  rock  granite,  but  if  quartz  be  practically  absent 
or  present  in  only  very  small  amount  the  rock  composed  of  the  same 
mineral  aggregates  would  be  designated  a  quartzless  granite  or  syenite. 

On  the  other  hand,  accessory  minerals  occur  only  sparingly  or  in 
small  quantity  and  their  presence  or  absence  does  not  materially  affect 
the  nature  of  the  rock.  Thus,  quartz  and  feldspar  are  essential  miner- 
als in  granite,  while  zircon  and  apatite  are  accessory. 

Another  important  distinction  that  is  frequently  made  between 
minerals  of  igneous  rocks  is  whether  they  are  original  or  secondary. 
Original  minerals,  known  also  as  pyrogenetic  or  primary,  have  formed 

1  The  Data  of  Geochemistry,  1911,  Bull.  491,  U.  S.  Geol.  Survey,  p.  30. 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.        65 

from  the  solidification  of  the  magma,  while  secondary  minerals  have 
formed  subsequent  to  the  crystallization  of  the  magma,  and  from  the 
original  ones  by  alteration  (weathering,  contact  or  dynamic  metamor- 
phism,  etc.)-  Thus  kaolinite,  sericite,  talc,  calcite,  and  epidote  are 
secondary  minerals  hi  igneous  rocks. 

Essential  minerals  are  original,  but  not  all  original  minerals  are 
essential.  For  example,  quartz  and  feldspar  in  granite  are  both  essen- 
tial and  original  minerals,  while  zircon  and  apatite  in  the  same  rock  are 
original,  but  they  are  not  essential  minerals.  An  essential  mineral 
may  sometimes  be  replaced  by  a  secondary  one,  such  as  hornblende 
(uralite)  which  replaces  pyroxene  in  gabbros  that  have  been  subjected 
to  metamorphism. 

Order  of  crystallization.  —  The  order  in  which  minerals  crystallize  from  a  magma 
is  indicated  by  the  mutual  relations  of  the  components  as  viewed  in  thin  sections 
under  the  microscope,  or,  as  in  the  case  of  coarse-grained  rocks,  from  polished  sur- 
faces. Thus  far  experience  shows  that  minerals  crystallizing  from  magmas  do  so 
not  simultaneously  but  successively,  with  in  some  cases  overlapping  of  their  periods 
of  crystallization,  as  shown  in  quartz  and  feldspar,  from  the  study  of  thin  sections 
of  granite. 

Rosenbusch  states  that  in  general  the  order  of  crystallization  of  minerals  from 
magmas  is  in  four  groups  as  follows: 

I.  Iron  ores  and  accessory  constituents  (magnetite,  hematite,  ilmenite,  apatite, 
zircon,  spinel,  sphene,  etc.). 

II.  Ferromagnesian  silicates  (olivine,  pyroxene,  amphibole,  mica,  etc.). 

III.  Feldspathic   constituents    (feldspars  and  feldspathoids,   including  leucit 
nephelite,  sodalite,  etc.). 

IV.  Free  silica  (quartz). 

This  order  of  crystallization  applies  especially  to  nonporphyritic  rocks.  "More 
explicitly,"  as  Harker1  states,  "what  is  regarded  as  the  normal  sequence  is  laid 
down  in  the  following  rules:" 

I.  "The  separation  of  crystals  in  a  silicate-magma  follows  an  order  of  decreasing 
basicity,  so  that  at  every  stage  the  residual  magma  is  more  acid  than  the  aggregate 
of  the  compound  already  crystallized  out." 

II.  "The  relative  amounts  of  the  several  constituents  present  in  the  magma 
affect  the  order  of  crystallization  in  such  a  manner  that,  in  general,  those  present 
in  smaller  amount  crystallize  out  first." 

III.  "Having  regard  to  the  several  bases  represented  in  the  various  constituents 
crystallization  begins  with  the  separation  of  iron  oxides  and  spinellids,  proceeds 
with  the  formation  of  magnesium  and  iron  silicates,  then  silicates  of  calcium,  then 
those  of  the  alkali  metals,  and  ends  with  the  crystallization  of  the  remaining  free 
silica." 

Mineralizers.  —  Study  of  extrusive  lavas  at  the  time  of  expulsion 
shows  the  presence  of  considerable  quantities  of  volatile  substances, 
chief  among  which  is  water  vapor.  Besides  water  vapor  there  are 

1  The  Natural  History  of  Igneous  Rocks,  1909,  pp.  180-181. 


66  ENGINEERING  GEOLOGY 

carbon  dioxide,  fluorine,  chlorine,  boric  acid,  sulphur,  etc.  These  dis- 
solved vapors,  known  as  mineralizers,  for  the  reason  that  they  exercise 
an  important  influence  on  mineral  composition  and  to  some  extent 
texture,  are  regarded  as  being  more  generally  present  in  acid  than  in 
basic  magmas,  although  known  to  occur  in  both.  These  substances 
play  an  important  role  in  the  crystallization  of  igneous  rocks,  and  their 
action  in  the  production  of  minerals  from  solidifying  magmas  may  be 
either  chemical  or  physical. 

For  the  formation  of  certain  minerals,  such  as  hornblende,  biotite,  tourmaline, 
etc.,  which  contain  small  quantities  of  water  as  hydroxyl  (OH),  fluorine,  and  boric 
acid,  the  presence  of  mineralizers  in  the  magma  is  essential,  and  their  function  is 
a  chemical  one.  On  the  other  hand,  many  minerals  cannot  be  produced  by  dry 
fusion,  but  require  for  their  production  the  presence  of  certain  mineralizers,  es- 
pecially water  vapor,  which  acts  physically  in  lowering  the  melting  point  of  the 
fusion  and  increasing  its  fluidity,  as  in  the  formation  of  orthoclase,  albite,  and 
quartz. 

Texture  of  Igneous  Rocks 

By  texture  of  an  igneous  rock  is  meant  size,  shape,  and  manner  of 
aggregation jrf  its  component  minerals.  It  serves  an  important  means 
of  determining  the  physical  condition  under  which  the  rock  was  formed, 
whether  at  or  near  the  surface,  or  at  some  depth  below,  and  hence  is 
recognized  as  one  of  the  important  factors  in  the  classification  of  igneous 
rocks. 

Some  rocks  are  sufficiently  coarse-grained  in  texture  for  the  principal 
minerals  to  be  readily  distinguished  by  the  unaided  eye;  in  others  the 
minerals  are  so  small  in  size  as  to  defy  identification  by  the  naked  eye 
or  even  with  the  aid  of  a  pocket  lens;  and  in  still  others  no  minerals 
have  crystallized,  but,  instead,  the  magma  has  solidified  as  a  glass. 
These  express  the  physical  (rate  of  cooling)  and  not  the  chemical  con- 
ditions under  which  magmas  have  solidified,  and  in  turn  serve  in  a 
general  way  to  express  the  position  in  the  earth's  crust  in  which  this 
solidification  took  place.  The  rate  of  cooling,  therefore,  is  one  of  the 
most  prominent  factors  in  determining  rock  texture.  Other  impor- 
tant factors  that  influence  the  development  of  rock  texture  are  chemical 
composition,  temperature,  pressure,  and  the  presence  of  mineralizers. 

Kinds  of  texture.  —  Since  texture  expresses  so  closely  the  con- 
ditions under  which  rock  magmas  solidify,  it  is  recognized  as  an  im- 
portant property  of  rocks,  and  is  made  one  of  the  principal  factors  in 
their  classification  (see  page  70).  In  the  megascopic  description  of 
igneous  rocks,  including  their  pyroclastic  (volcanic)  equivalents,  five 
principal  textures  are  recognized.  These  are  glassy,  dense  or  felsitic 
(aphanitic),  porphyritic,  granitoid,  and  fragmental. 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.        67 

Glassy  texture.  —  Under  conditions  of  quick  chilling,  magmas,  espe- 
cially the  more  siliceous  ones,  freeze  or  solidify  into  a  glass,  without  dis- 
tinct crystallization  and  the  formation  of  visible  minerals.  Such  rocks 
do  not  show  definite  minerals  and  are  composed  of  glass,  examples  of  this 
being  obsidian,  pitchstone,  etc.  Some  glasses,  such  as  pumice,  are  highly 
vesicular  due  to  the  escape  of  water  vapor  at  high  temperature  through 
relief  of  pressure. 

Dense  or  felsitic  (aphanitic)  texture.  —  This  texture  is  characteristic 
of  crystalline  rocks,  but  the  individual  minerals  are  too  small  in  size  to 
be  distinguished  by  the  eye.  The  general  appearance  of  the  rock  is 
homogeneous  and  stony  but  not  glassy.  Examples,  many  felsites  and 
basalts. 

Porphyritic  texture.  —  Porphyritic  texture  is  characteristic  of  those 
rocks  composed  of  mineral  grams  or  crystals  of  larger  size  set  in  a 
groundmass  (Plate  IV,  Fig.  2)  that  is  more  finely  crystalline  or  even 
glassy,  or  both.  The  larger  crystals  or  grains  are  termed  phenocrysts 
and  may  show  distinct  crystal  outline  (idiomorphic) ,  or  may  have 
irregular  and  corroded  surfaces  (allotriomorphic) .  They  may  be  very 
abundant  in  some  rocks,  exceeding  occasionally  the  groundmass  in 
amount,  or  they  may  be  very  scantily  developed.  Great  variation  in 
size  is  also  shown,  from  an  inch  and  more  in  diameter  down  to  those 
so  small  that  they  are  scarcely  discernible.  They  may  consist  of  the 
light-colored  minerals  (quartz  and  feldspar)  or  of  the  dark-colored 
ferromagnesian  ones  (hornblende,  pyroxene,  olivine,  etc.),  or  of  a  mix- 
ture of  light-  and  dark-colored  minerals. 

Porphyritic  texture  is  frequently  developed  in  lavas,  dikes,  sheets, 
and  laccoliths,  and  is  less  often  observed  in  the  deeper-seated  rocks, 
but  by  no  means  uncommon  in  some,  as  in  granites. 

In  porphyritic  rocks  the  groundmass  often  weathers  more  rap- 
idly  than  the  phenocrysts,  leaving  the  latter  in  more  or  less  strong 
relief. 

Granitoid  texture.  —  Those  igneous  rocks  which  are  composed  en- 
tirely of  recognizable  minerals  of  approximately  the  same  size  possess 
granitoid  or  even-granular  texture.  The  individual  minerals  seldom 
exhibit  definite  crystal  boundaries.  Example,  normal  granite. 

According  to  the  size  of  mineral  grains,  we  may  recognize:  (1)  Fine- 
grained rocks,  average  size  of  particles  less  than  'one  millimeter;  (2) 
medium-grained,  between  1  and  5  millimeters;  and  (3)  coarse-grained, 
greater  than  5  millimeters. 

Other  things  equal,  fine-grained  granitoid  rocks  are  more  durable 
than  coarse-grained  ones. 


PLATE  IV,  FIG.  1.  —  Banded  felsite,  showing  flow  structure. 


FIG.  2.  —  Trachyte,  showing  porphyritic  texture. 


(68) 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       69 

Fragmental  texture.  —  Fragmental  is  a  textural  term  used  hi  describ- 
ing volcanic  tuffs  and  breccias,  which  represent  the  consolidation  of 
pyroclastic  materials  of  all  sizes  erupted  by  volcanoes. 

Porous  texture.  —  The  effusive  igneous  rocks,  showing  glassy  and 
felsitic  textures,  may  vary  texturally  from'very  compact  and  dense  to 
very  porous,  with  nearly  all  gradations  between  these  extremes  observed. 
According  to  the  abundance  of  spacings  or  cavities,  caused  by  escaping 
vapors  from  the  magma  during  cooling,  the  rock  may  be  termed  vesic- 
ular (Plate  V,  Fig.  1),  scoriaceous,  or  pumiceous. 

When  these  cavities  have  been  filled  with  mineral  matter  deposited 
from  solution,  the  rock  is  described  as  having  amygdaloidal  texture. 
The  fillings,  which  may  be  any  one  or  more  of  a  variety  of  minerals, 
usually  zeolites,  calcite,  epidote,  quartz,  or  feldspar,  are  termed  amyg- 
dules,  because  of  their  resemblance  to  almond-shaped  forms.  Amygda- 
loidal texture  is  especially  common  in  the  surface  lava  flows  (basalts) 
of  all  ages  occurring  in  the  United  States. 

During  the  cooling  of  granitoid  (plutonic)  rocks,  irregular  small 
cavities  are  sometimes  developed,  especially  in  some  granites,  into 
which  the  minerals  project  as  well-formed  crystals.  These  cavities  are 
called  miarolitic. 

Differentiation  of  Igneous  Rocks 

It  is  a  matter  of  common  observation  that  magmas  of  different  composition  have 
been  erupted  not  only  from  different  vents,  but  from  the  same  vent  at  different 
periods  of  time.  This  was  formerly  explained  by  some  that  at  an  unknown  depth 
beneath  the  surface  of  the  earth,  there  existed  two  layers  of  unlike  magma,  one 
lighter  and  more  acid,  the  other  heavier  and  more  basic,  and  that  the  eruptions 
came  from  one  or  the  other  of  these  or  a  mixture  of  both.  From  the  observed  facts 
in  the  field  it  is  now  recognized  that  this  assumption  is  inadequate  as  an  explanation. 

Plutonic  igneous  masses,  such  as  granite  stocks,  etc.,  exposed  now  at  the  sur- 
face through  erosion,  frequently  show  a  somewhat  zoned  arrangement;  an  outer 
margin  of  irregular  width  and  extent  whose  mineral  composition  is  essentially  differ- 
ent from  that  of  the  larger  central  mass.  That  is  to  say,  a  border  zone  consisting  of 
a  greater  concentration  of  the  more  basic,  and  sometimes  the  more  acid,  minerals 
than  in  the  central  mass.  The  two  parts  of  the  igneous  mass  usually  contain  the 
same  minerals,  but  in  different  concentrations,  and  the  passage  from  one  to  the  other 
is  frequently  gradual. 

A  similar  zonal  arrangement  has  been  observed  in  some  laccoliths.  Also  similar 
evidence  is  afforded  from  the  study  of  complementary  dikes.  Dikes  composed  of 
unlike  mineral  composition,  one  set  light  in  color  and  density,  and  therefore  acid  in 
character;  the  other  dark  in  color,  heavier,  and  of  basic  character,  have  been  observed 
cutting  the  rocks  of  a  given  area  and  closely  associated.  If  this  series  of  unlike 
dike  material  were  sampled  in  proportion  to  their  volumes  and  carefully  analyzed, 
the  bulk  sample  would  reproduce  the  composition  of  the  original  parent  magma. 
Such  a  system  of  dikes  is  termed  complementary. 


70  ENGINEERING  GEOLOGY 

These  geological  facts  are  now  generally  agreed  to  by  petrographers  as  being 
most  satisfactorily  explained  on  the  assumption  that  magmas  have  the  capacity, 
under  certain  conditions,  of  separating  into  submagmas  of  unlike  composition  as 
well  as  differing  from  that  of  the  original  magma,  but  if  mixed  in  proper  proportions 
they  would  reproduce  the  parent  magma.  "Regarding  the  division  there  seems  to 
be  in  general  two  opposite  poles  towards  which  the  submagmas  tend;  to  one  con- 
centrate the  iron,  magnesia,  and  to  a  large  extent  the  lime,  to  the  other  the  alkalies, 
alumina,  and  to  a  great  extent  the  silica.  The  one  gives  us  ferromagnesian  rocks 
such  as  gabbro,  the  other  feldspathic  rocks  such  as  granite"  (Pirsson). 

The  process  of  a  magma  separating  into  two  submagmas  is  known  as  magmatic 
differentiation,  and  it  may  take  place  prior  to  intrusion  or  extrusion,  or  it  may  go 
forward  in  place.  The  process  has  been  an  important  one  in  the  genesis  of  some 
ore  bodies  (see  Chapter  on  Ore  Deposits). 

It  has  been  shown  that  the  variety  of  igneous  rock  types  occurring  within  a  given 
area  exhibit  certain  distinctive  features  which  indicate  their  kinship,  and  therefore 
their  derivation  from  a  common  parent  magma.  These  kinship  characters  may  be 
shown:  (1)  by  the  presence  of  certain  minerals;  (2)  in  the  peculiarity  of  chemical 
composition;  (3)  in  some  cases  by  peculiar  textures;  or  (4)  in  a  combination  of  these. 
To  express  this  kinship  of  associated  igneous  rock  types,  Iddings  has  proposed  the 
convenient  term  consanguinity;  and  the  area  within  which  such  related  types  occur 
is  called  a  petrographic  province,  or  a  comagmatic  area  or  region. 

Classification  of  Igneous  Rocks       >( 

Igneous  rocks  possess  certain  features  by  which  the  many  different 
varieties  recognized  may  be  distinguished  from  each  other,  such  as 
mode  of  occurrence,  texture,  mineral  composition,  chemical  compo- 
sition, etc.  One  or  more  of  these  features  has  been  employed  in  classify- 
ing igneous  rocks,  but  thus  far  not  one  of  the  many  classifications 
proposed  has  been  universally  adopted.  The  difficulty  lies  chiefly 
in  the  fact  that  hard  and  fast  lines  cannot  be  drawn,  since  each  of 
the  several  features  enumerated  above  shows  gradations,  hence  equal 
emphasis  has  not  been  placed  on  the  same  feature  by  all. 

The  scheme  of  classification  of  igneous  rocks  most  generally  em- 
ployed by  petrographers  is  based  on  three  fundamental  principles, 
namely,  (1)  texture,  (2)  mineral  composition,  and  (3)  chemical  com- 
position. It  very  often  happens  that  the  identification  of  the  exact 
variety  or  kind  of  igneous  rock  is  not  possible  by  megascopic  methods, 
such  as  involves  a  naked  eye  examination  or  the  use  of  a  pocket  lens, 
but  must  be  determined  by  microscopic  and  chemical  study.  The 
engineer,  however,  must  rely  on  megascopic  characters  of  igneous 
rocks  in  classifying  them,  using  a  scheme  that  is  both  useful  and  prac- 
tical, and  one  that  is  based  on  the  principal  rock  characters,  such  as 
texture  and  mineral  composition. 

Volcanic  rocks  may  be  glassy,  stony,  cellular,  or  porphyritic,  while 


PLATE  V,  FIG.  1.  —  Basalt,  showing  vesicular  texture. 


FIG.  2.  —  Graphic  granite,  showing  characteristic  intergrowth  of  quartz  (dark) 

and  feldspar  (light). 

(71) 


72 


ENGINEERING  GEOLOGY 


the  plutonic  rocks  are  generally  massive  and  holocrystalline,  with  por- 
phyritic  texture  by  no  means  uncommon.  A  rock,  therefore,  may  have 
a  uniform  mineral  composition,  but  vary  in  texture,  depending  upon 
the  conditions  under  which  it  solidified.  On  the  other  hand,  plutonic 
rocks  may  possess  similar  texture,  but  differ  in  mineral  composition. 
These  differences,  either  mineralogical  or  textural,  lead  to  the  develop- 
ment of  different  varieties  of  igneous  rocks. 

The  following  table,  taken  from  Pirsson,  expresses  simply  the  min- 
eralogical and  textural  characters  of  the  more  common  kinds  of  igneous 
rocks,  and  is  admirably  adapted  to  the  needs  of  the  engineer.  There 
are  many  more  varieties  of  igneous  rocks,  as  shown  in  the  table  on 
page  73,  but  these  can  hardly  be  distinguished  megascopically. 

MEGASCOPIC  CLASSIFICATION  OF  IGNEOUS  ROCKS 

(A)  Grained,  constituent  grains  recognizable.    Mostly  intrusive. 


(a)  Feldspathic  rocks,  usually  light  in  color. 

(6)  Ferromagnesian  rocks,  generally 
dark  to  black. 

With  quartz. 

Without  quartz. 

With  subordinate 
feldspar. 

Without 
feldspar. 

Nonporphy  ritic  . 
Porphyritic  

GRANITE. 
(a)  Aplite. 

GRANITE-PORPHYRY. 

SYENITE. 
(a)  Syenite. 
(6)  Nephelite  syenite, 
(c)  Anorthosite. 
SYENITE-PORPHYRY. 

DIORITE. 
GABBRO. 
DOLERITE. 

DlORITE-PORPHYRY. 

PERIDOTITE. 
Pyroxenite. 
Hornblendite. 

(B)  Dense,  constituents  nearly  or  wholly  unrecognizable.    Intrusive  and  extrusive. 


(a)  Light  colored,  usually 
feldspathic. 

(6)  Dark  colored  to  black, 
usually  ferromagnesian. 

Nonporphyritic  

FELSITE. 
FELSITE-PORPHYRY. 

BASALT. 
BASALT-PORPHYRY. 

Porohvritic  .  .  . 

(C)  Rocks  composed  wholly  or  in  part  of  glass.    Extrusive. 


Nonporphyritic. 
Porphyritic 


OBSIDIAN,  pitchstqne,  pearlite,  pumice,  etc. 
Vitrophyre  (obsidian-  and  pitchstone-porphyry). 


(£>)  Fragmental  igneous  material.    Extrusive. 


TUFFS,  BRECCIAS  (Volcanic  ashes,  etc.). 


In  the  next  table,  taken  from  Kemp,  the  arrangement  vertically 
from  top  to  bottom  is  based  on  texture,  and  from  left  to  right  on 
mineral  composition,  chiefly  in  accordance  with  the  predominant  feld- 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC. 


73 


ll 


i 


3 


1! 

Is 


il 

fl 

3 


14 


I 


il 


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— ^  ar;   rv 

®     I      c  £7 

fe     I    73  5 

—      c  a 


. 
1111 b II 


ill    « 

CX!   P. 


331 

£oa 


lit 


£ 


a* 

1 


1 1 


r||  i- 

l1!1^  I 


.20 


ill 

pi 


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.1.1 


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I 

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si 


•^uau 
-Traaid 


•ajeut 
-mopafd 


-ouaqj 


74 


ENGINEERING   GEOLOGY 


spar  present.  This  cannot  always  be  determined  by  megascopic  means, 
but  requires  the  use  of  the  polarizing  microscope  in  the  study  of  thin 
rock  sections.  The  arrangement  transversely  also  emphasizes  in  a 
general  way  the  acid  character  of  the  rocks  on  the  left  side  of  the 
table  and  the  basic  nature  of  those  on  the  right  side.  The  percentages 
of  silica  given  at  the  bottom  of  the  table  serve  to  indicate  this  general 
relationship  of  the  rocks  chemically. 

DESCRIPTION   OF  IGNEOUS   ROCKS 
INTRUSIVE  ROCKS 

Granite 

Mineral  composition.  —  Granites  are  granular  rocks  composed  of  feld- 
spar (microcline,  orthoclase,  albite,  or  their  mixtures)  and  quartz,  with 
usually  mica  (biotite  or  muscovite)  or  hornblende,  rarely  pyroxene. 
Some  granites  consist  of  feldspar  and  quartz  alone.  Soda-lime  feldspar 


FIG.  60.  —  Granite  cut  by  pegmatite  dikes.     (After  Watson,  U.  S.  Geol.  Surv., 

Bull.  426.) 

is  generally  present  and  frequently  in  large  amount.  Accessory  min- 
erals, such  as  apatite,  zircon,  magnetite,  etc.,  in  small  amounts  and 
usually  of  microscopic  size  are .  always  present.  The  light-colored 
minerals  are  in  marked  excess,  and  feldspar  is  the  predominant  one. 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.        75 

Chemical  composition.  —  The  chemical  composition  of  granite,  though  con- 
ditioned by  mineral  composition,  is^now  regarded  to  be  of  less  economic  importance 
than  the  latter.  The  range  in  chemical  composition  is  shown  in  ten  analyses  of 
United  States  granites  given  below: 

SiO2 66.28-77.68 

A12O3 11.63-16.38 

Fe2O3 0.00-  2.73 

FeO 0.09-  1.88 

MgO 0.04-  1.63 

CaO 0.12-  3.75 

NaoO 2.85-  5.16 

K2O 1.87-  6.50 

TiO2 Trace-  0.54 

P2O6 Trace-  0.30 

Varieties.  —  Mineralogically,  on  the  basis  of  essential  minerals 
accompanying  quartz  and  feldspar  present,  we  may  have  (a)  muscovite 
granite,  containing  muscovite;  (6)  biotite  granite,  containing  biotite; 
(c)  muscovite-biotite  granite,  containing  both  muscovite  and  biotite; 
or  (d)  hornblende  granite,  containing  hornblende;  etc.  Aplite,  a  name 
formerly  applied  to  those  granites  poor  or  lacking  in  mica,  is  now  used 
for  the  fine-grained,  muscovite  granites,  occurring  in  dikes.  Pegmatite 
is  a  variety  of  granite,  of  usually  very  coarse  crystallization  of  quartz, 
feldspar,  and  mica,  with  frequently  rarer  minerals,  occurring  in  dikes 
or  veins.  Each  of  the  three  principal  minerals  may  be  utilized;  the 
quartz  and  feldspar  for  pottery  manufacture,  etc.;  and  the  mica, 
when  in  large  colorless  and  transparent  sheets,  for  lamp  chimneys, 
stove  doors,  electrical  purposes,  etc.  Pegmatite  frequently  shows  a 
curious  intergrowth  of  feldspar  and  quartz  crystallized  simultaneously, 
which  on  a  cross  fracture  suggest  cuneiform  characters,  and  called 
graphic  granite  (Plate  V,  Fig.  2).  Pegmatites  are  igneous  in  origin  and 
have  resulted  by  crystallization  of  the  residual  magma,  unusually  rich  in 
mineralizers,  especially  water.  The  name  unakite  is  given  to  a  granite 
with  pink  feldspars  and  rich  in  epidote. 

Physical  properties.  —  The  usual  color  of  granite  is  some  shade 
of  gray,  though  pink  or  red  varieties  are  not  uncommon,  dependent 
chiefly  upon  that  of  the  feldspar,  and  the  proportion  of  the  feldspar  to 
the  dark  minerals.  Specific  gravity  ordinarily  ranges  from  2.63  to  2.75, 
according  to  the  kinds  and  relative  amounts  of  the  principal  minerals. 
As  shown  in  Chapter  XI,  the  percentage  of  absorption  is  very  small, 
less  than  a  fraction  of  one  per  cent;  and  the  crushing  strength  is  high, 
ranging  from  15,000  to  20,000  pounds  per  square  inch,  properties  which 
render  the  rock  especially  desirable  for  building  purposes. 


PLATE  VI,  FIG.  1.  —  Dikes  of  pegmatite  in  granite,  Richmond,  Va.     (H.  Hies, 
photo.)     Much  of  the  rock  in  quarry  rejected  because  of  these  dikes. 


FIG.  2.  —  Volcanic  ash  deposits,  on  lower  slopes  of  extinct  volcano  of  Toluca  in 

Mexico.     (H.  Ries,  photo.)     Note  how  the  ash  has  been  gullied  by  rain. 
(76) 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC. 


77 


Texture  and  structure.  —  Texturally,  granites  are  holocrystalline, 
even-granular  to  porphyritic  rooks.  Feldspars  form  the  phenocrysts  in 
porphyritic  granites.  Granites,  therefore,  possess  a  minimum  of  pore 
space  and  a  maximum  degree  of  strength.  Structurally,  normal 
granite  is  a  massive  rock  without  foliation  or  bands.  When  it  takes  on 
a  foliated  or  banded  structure,  subsequent  to  its  crystallization,  it  is 
no  longer  a  true  granite,  but  a  granite-gneiss.  For  other  structural 
features  of  granite,  such  as  jointing,  rift  and  grain,  segregations  (knots), 
and  inclusions  of  foreign  rocks,  see  under  granites  as  a  building  stone 
in  Chapter  XI. 

Mode  of  occurrence.  —  Granites  are  plutonic  rocks  that  have 
cooled  at  depth  beneath  the  surface.  They  form  large  irregular  masses 
known  as  batholiths,  also  rounded  exposures  in  other  rocks  (stocks  or 
bosses),  and  dikes. 

Weathering,  distribution,  and  uses  of  granite  are  discussed  under 
granites  as  a  building  stone  in  Chapter  XI,  to  which  the  reader  is  re- 
ferred, so  that  their  economic  features  need  not  be  repeated  here. 


Syenite 

Mineral  composition. —  Syenites  are  granular  rocks  composed  chiefly 
of  feldspars  of  the  same  varieties  as  granite,  or  of  the  feldspathoids 
(nephelite,  sodalite,  etc.),  with  usually  hornblende,  mica,  or  pyroxene. 
They  differ  from  granite  hi  containing  little  or  no  quartz,  and  are 
therefore  lower  in  silica.  More  or  less  soda-lime  feldspar  is  always 
present,  and  when  this  approximately  equals  the  potash  feldspar  in 
amount,  the  rock  is  called  a  monzomte,  which  marks  a  transition  to 
diorite.  Magnetite,  ilmenite,  apatite,  and  zircon  are  common  ac- 
cessory minerals.  All  gradations  exist  between  syenite  and  granite  on 
the  one  hand,  and  between  syenite  and  diorite  on  the  other.  Likewise 
syenite  and  nepheline  syenite  may  grade  into  each  other. 

Chemical  composition.  —  Syenites  are  lower  in  silica  than  granites,  but  generally 
show  an  increase  in  all  the  bases,  especially  alumina  and  the  alkalies,  more  par- 
ticularly soda,  which  indicates  the  passage  to  the  nephelite  syenite  variety.  The 
chemical  composition  of  syenite  is  shown  in  the  two  following  analyses: 


SiOj    I   A12O3 

FejOa 

FeO 

MgO 

CaO 

NajO 

K20 

H20 

Rest. 

Total. 

I. 
II. 

60.2  '  20.4 

58.8     22.5 

) 

1.7 
1.5 

1.9 
1.0 

1.0 
0.2 

20 
0.7 

6.3 

9.6 

6.1 
4.9 

0.3 
1.0 

0.4 
0.3 

100.3 
100.5 

I.  Fourche  Mountain,  Arkansas;  II.  Salem  Neck,  Massachusetts. 


78  ENGINEERING  GEOLOGY 

Varieties.  —  Like  granites,  syenites,  according  to  the  predominant  ferromagne- 
sian  mineral,  may  be  grouped  into  (a)  mica  syenites  which,  when  occurring  in  dikes 
and  of  dark  color,  have  been  called  minette;  (6)  hornblende  syenite;  and  (c)  augite 
syenite.  Of  more  importance,  however,  and  the  one  recognized  in  the  two-fold 
division  of  syenites  in  rock  classification  is  that  based  on  the  presence  or  absence 
of  the  feldspathoids,  the  most  frequent  one  of  which  is  nephelite,  which  serves  to 
divide  the  syenites  into: 

(a)  Syenite  (common  syenite)  composed  chiefly  of  feldspars,  with  or  without 
dark  minerals;  and 

(6)  Nephelite  syenite  composed  chiefly  of  feldspars  and  nephelite,  with  or  with- 
out dark  minerals. 

Physical  properties.  —  Syenites  are  light-colored  rocks  and  show 
a  range  in  color  similar  to  granites,  from  nearly  white  through  shades 
of  gray  to  pink  being  the  most  common.  Specific  gravity  ordinarily 
varies  between  2.6  and  2.8,  dependent  on  the  kinds  and  proportions  of 
minerals  present. 

Texture  and  structure.  —  Syenites'are  massive  even-granular  rocks, 
but  porphyritic  texture  is  sometimes  developed.  Like  granites  they 
may  be  characterized  by  joints,  segregations  (knots),  and  inclusions. 
They  may  show  foliation  or  banding  from  metamorphism,  when  they 
are  more  properly  called  syenite-gneiss. 

Mode  of  occurrence.  —  Syenites  are  not  common  rocks,  and  are  of 
little  importance  as  building  stone,  although  they  have  equal  value 
as  granite  for  constructional  purposes.  Like  granite,  they  form  in- 
dependent irregular  masses  and  dikes,  and  are  frequently  associated 
with  large  bodies  of  granite,  into  which  they  grade  by  increase  of 
quartz. 

Weathering  and  uses  of  syenites  are  similar  to  granites  (page  77). 
They  are  very  much  more  restricted  in  distribution  than  granite. 
(See  Chapter  XI  on  Building  Stone  for  distribution.) 

Diorite 

Mineral  composition.  —  The  diorites  are  granular  rocks  composed 
of  plagioclase  as  the  chief  feldspar  and  hornblende  or  biotite,  or  both. 
Augite  in  subordinate  amount  is  often  present,  and  some  orthoclase 
occurs  in  all  diorites.  Quartz  enters  into  the  composition  of  some 
diorites  as  an  important  constituent  and  the  rock  is  then  distinguished 
as  quartz  diorite.  Iron  ores,  apatite,  zircon,  and  titanite  are  common 
microscopic  accessory  minerals. 

As  now  used  the  name  diorite  is  applied  to  those  granular  rocks  in 
which  hornblende  equals  or  exceeds  feldspar  in  amount.  Because  of 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC. 


79 


fine-grained  texture,  it  is  not  possible  in  many  cases  to  determine  by 
megascopic  examination  the  dominant  feldspar. 

By  increase  of  quartz  and  alkalic  feldspar  the  diorites  proper  pass 
into  the  granites  on  the  one  hand,  and  by  increase  of  pyroxene  into  the 
gabbros  on  the  other.  Monzonite  is  the  intermediate  type  between 
syenite  and  diorite;  and  quartz  monzonite,  known  also  as  grano-diorite, 
is  intermediate  between  granite  and  quartz  diorite. 

Chemical  composition.  —  The  most  important  points  to  be  observed  in  the 
chemical  composition  of  normal  diorites  are  the  lower  silica  but  notably  increased 
percentages  of  the  bases,  iron,  lime,  and  magnesia,  over  the  granites  and  syenites, 
resulting  in  the  increase  in  quantity  of  hornblende.  Also  soda  is  in  excess  of  potash, 
which  follows  from  the  chief  feldspar  being  plagioclase.  Quartz  diorites,  on  the 
other  hand,  show  a  higher  silica  percentage  than  diorites,  but  averaging  lower  than 
for  granites,  while  lime  and  soda  may  be  higher  on  account  of  the  chief  feldspar 
being  plagioclase.  These  differences  become  apparent  on  examination  of  the  analyses 
of  quartz  diorite  and  diorite,  tabulated  below: 


Si02 

A1203 

FezO;, 

FeO 

MgO 

CaO 

NajO 

K20 

H20 

Rest. 

Total. 

I. 

70.36 

15.47 

0.98 

1.17 

0.87 

3.18 

4.91 

1.71 

1.06 

0.37 

100.08 

II. 

67.54 

17.02 

2.97 

0.34 

1.51 

2.94 

4.62 

2.28 

0.55 

1.20 

101.01 

III. 

58.05 

18.00 

2.49 

4.56 

3.55 

6.17 

3.64 

2.18 

0.86 

1.29 

100.79 

IV. 

57.87 

16.30 

1.71 

3.86 

5.50 

5.53 

5.01 

0.75 

2.66 

0.93 

100.12 

I.  Quartz  diorite,  near  Enterprise,  Butte  Co.,  California;  II.  Quartz  diorite,  Electric  Peak,  Yellowstone 
Park;  III.  Diorite,  Electric  Peak,  Yellowstone  Park;  IV.  Diorite,  South  Husent  Creek,  Butte 
County,  Calif. 

Varieties.  —  Mineralogically,  we  may  effect  a  two-fold  division  of 
the  diorite  family  into  common  diorite  and  quartz  diorite  (or  tonalite)  on 
the  basis  of  the  absence  or  presence  of  appreciable  quartz. 

Also,  according  to  the  ferromagnesian  mineral  present,  one  may  distinguish 
hornblende  diorite,  which  is  diorite  in  its  restricted  sense,  mica  (biotite)  diorite,  and 
augite  diorite.  Camptonite  is  a  variety  of  hornblende  diorite;  kersantite  is  a  dioritic 
rock  containing  both  biotite  and  plagioclase,  and  occurring  in  dikes.  Diorites  con- 
taining notable  amounts  of  pyroxene,  marking  their  passage  into  gabbros,  have 
been  called  gabbro-diorite. 

Physical  properties.  —  Diorites  are  usually  of  a  dark  gray  or  green- 
ish color,  sometimes  almost  black,  dependent  upon  the  color  of  horn- 
blende and  its  proportion  to- feldspar.  Because  of  the  increased  amounts 
of  ferromagnesian  minerals,  hornblende  or  biotite,  or  both,  diorites  have 
a  higher  specific  gravity  than  granites,  ranging  usually  between  2.85  and 
3.0.  They  show  a  high  compressive  strength  and  a  low  percentage  of 
absorption. 

Texture  and  structure.  —  Typical  diorites  have  granitoid  texture, 
ranging  from  fine  to  coarse  even-granular.  Porphyritic  texture  is 


80 


ENGINEERING  GEOLOGY 


sometimes  developed  but  is  probably  less  common  than  in  granites. 
Structurally,  diorites  are  massive  rocks,  but  may  be  rendered  foliated 
or  schistose  through  dynamic  metamorphism,  and  pass  into  gneisses 
and  hornblende  schists.  Orbicular  or  spheroidal  structure  is  well  de- 
veloped in  some  diorites  (Chap.  XI),  and  the  rock  has  had  a  very 
limited  use  as  an  ornamental  stone. 

Mode  of  occurrence.  —  Diorites  are  common  and  widely  distributed 
rocks.  They  frequently  occur  as  independent  intruded  masses  in  the 
form  of  stocks  and  dikes,  less  often  ^as  batholiths,  and  are  found  con- 
nected with  granite  and  gabbro  masses  into  which  they  may  grade. 

Weathering,  distribution,  and  uses  of  diorite  are  stated  under 
building  stone,  in  Chapter  XI,  but  it  may  be  added  here  that  they  are 
not  as  important  commercially  as  granites. 

Gabbro 

Mineral  composition.  —  The  gabbros  are  granitoid  intrusive  rocks 
which,  when  typically  developed,  consist  of  pyroxene  and  plagioclase 
feldspar  (labradorite  or  more  calcic  varieties).  In  typical  gabbros  the 
dark  silicate  minerals  predominate  over  the  light-colored  ones,  but 
rocks  are  included  in  the  gabbro  group  which  are  composed  practically 
of  all  plagioclase  (chiefly  labradorite),  to  which  the  name  anorthosite 
has  been  given.  Olivine  is  notably  present  in  some  gabbros  which  are 
known  as  olivine  gabbro.  Common  accessory  minerals  include  iron  ores 
(magnetite  and  ilmenite)  and  apatite. 

Extensive  areas  of  anorthosite  are  known  in  Canada,  the  Adiron- 
dack Mountains,  and  elsewhere.  Pyroxene  occurs  at  times  in  sub- 
ordinate amount,  and  by  its  increase  the  rock  passes  into  gabbro  proper. 
Iron  ore  minerals  (magnetite  or  ilmenite)  and  more  or  less  biotite  and 
hornblende  may  also  be  present. 

Chemical  Composition.  —  The  gabbros  are  characterized  chemically  by  low 
silica  (55  to  45  per  cent),  and  high  iron,  magnesia,  and  lime.  The  alkalies  are  low 
but  alumina  is  generally  quite  high.  These  general  characters  of  the  gabbro  family 
are  brought  out  in  the  table  of  analyses  below. 


Si02 

AI203 

Fe.03 

FeO 

MgO 

CaO 

NaaO 

K2O 

H20 

Rest. 

Total. 

I. 

48.23 

18.26 

1.26 

6.10 

10.84 

9.39 

1.34 

0.73 

2.26 

1.50 

99.91 

II. 

47.16 

14.45 

1.61 

13.81 

5.24 

8.13 

3.09 

1.20 

0.60 

4.69 

99.98 

III. 

46.24 

29.85 

1.30 

2.12 

2.41 

16.24 

1.98 

0.18 

1.03 

101.35 

IV. 

45.66 

16.44 

0.66 

13.90 

11.57 

7.23 

2.13 

0.41 

0.90 

1.13 

100.03 

V. 

44.76 

18.82 

2.19 

4.73 

11.32 

14.58 

0.89 

0.11 

2.53 

0.36 

100.29 

L  Gabbro  (bronzite  norite),  Crystal  Falls,  Michigan;  II.  Gabbro  (norite),  Elizabethtown,  Essex  Co., 
New  York;  III.  Anorthosite,  mouth  of  Seine  River,  Rainy  Lake  Region,  Ontario;  IV.  Olivine  gab- 
bro, Birch  Lake,  Minnesota;  V,  Hypersthene  gabbro,  Wetheredville,  Maryland. 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.        81 

Varieties.  —  The  gabbro  group  is  a  large  one,  and  many  varieties 
of  rock  are  represented,  most  of  them  being  based  on  microscopic  dis- 
tinction in  mineral  composition  and  texture.  The  following  may  be 
enumerated,  the  first  being  the  most  important. 

Diabase,  when  typically  developed,  is  intermediate  between  gabbro 
proper  and  basalt,  differing  from  the  former  in  having  lathe-shaped 
feldspars,  resulting  in  the  characteristic  texture  called  ophitic  or  diabasic 
(Plate  VII,  Fig.  2).  It  occurs  commonly  as  dikes,  but  also  as  sills  in 
the  eastern  Atlantic  states  (Palisades  of  Hudson  River),  and  is  used 
chiefly  for  road  material  and  paving  blocks. 

Olivine  gabbro  is  a  variety  of  the  rock  rich  in  olivine.  If  the  pyroxene  is  an 
orthorhombic  species,  usually  hypersthene,  the  rock  is  called  norite.  As  stated 
under  mineral  composition,  when  the  rock  consists  almost  wholly  of  lime-soda 
feldspars  with  negligible  amounts  of  other  minerals,  it  is  known  as  anorthosite  (some- 
tunes  called  plagioclasite  or  plagioclase  rock,  common  in  Adirondack  Mountains  and 
eastern  Canada.  When  the  rock  is  composed  of  plagioclase  and  olivine  without 
pyroxene  it  is  called  troctolite,  a  rare  variety  of  gabbro. 

Physical  properties.  —  Gabbros  proper  are  dark  gray  or  greenish  to 
black  in  color.  Anorthosites  are  normally  white  or  light-colored,  but 
the  rock  is  often  grayish  and  sometimes  almost  black.  The  specific 
gravity  will  average  slightly  higher  in  typical  gabbros  than  for  diorites, 
the  usual  range  being  between  2.9  and  3.2.  They  possess  a  high  degree 
of  compressive  strength  and  low  absorptiveness,  and  are  well  suited  for 
constructional  purposes,  in  which  they  have  had  a  limited  use.  They 
are  susceptible  of  a  high  degree  of  polish,  and  have  been  used  to  some 
extent  as  monumental  stock,  but  their  very  dark  color  has  militated  in 
part  against  their  very  extended  use  in  this  direction. 

Texture  and  structure.  —  The  gabbros,  both  texturally  and  struc- 
turally, are  similar  to  the  diorites.  They  are  massive  even-granular 
rocks,  with  porphyritic  texture  rarely  developed.  Orbicular  texture, 
similar  to  that  of  some  granites  and  diorites,  though  known,  is  but 
seldom  observed. 

Original  banded  structure  may  be  noted  in  some  gabbros,  and  dynamic 
metamorphism  may  mash  them  into  their  foliated  equivalents,  gneisses 
or  schists.  Another  change  which  is  a  molecular  one  usually  results 
from  the  action  of  pressure  metamorphism.  This  is  the  transformation 
of  the  pyroxene  to  hornblende  (uralite),  the  process  being  known  as 
uralitization.  This  change  may  or  may  not  be  accompanied  by  the  pro- 
duction of  schistosity,  and  the  rock  may  retain  its  original  massive 
structure. 

Segregations  both  large  and  small  of  iron  ores  (magnetite,  but  usually 


82 


ENGINEERING  GEOLOGY 


ilmenite,  or  a  mixture  of  the  two,  and  of  the  sulphides,  especially 
pyrrhotite)  are  common  in  gabbros  of  many  localities,  especially  those 
of  Wyoming,  Minnesota,  New  York,  and  Canada,  and  of  Norway  and 
Sweden  in  Europe.  (See  Chapter  on  Ore-Deposits.) 

Alteration.  —  The  change  of  pyroxene  to  hornblende  (uralite)  in 
gabbros  under  the  action  of  dynamic  metamorphism  has  already  been 
stated.  Under  the  action  of  metamorphism  garnet  is  frequently  de- 
veloped as  a  new  mineral. 

A  second  mode  of  alteration  frequently  observed  in  gabbros  subjected  to  meta- 
morphic  action  is  that  which  changes  the  feldspar  to  saussurite,  a  mixture  chiefly 
of  albite  and  zoisite  with  other  minerals.  The  process  is  known  as  saussuritization 
and  the  rocks  showing  it  have  been  called  saussurite-gabbros. 

Through  the  action  of  atmospheric  agencies  (weathering)  gabbros 
ultimately  alter  to  deep  red  ferruginous  clay  soils. 

Mode  of  occurrence.  —  Gabbros  are  fairly  common  rocks  and  have 
rather  wide  distribution.  Their  geological  occurrence  is  similar  to  gran- 
ite, and  they  may  form  batholiths,  stocks,  or  bosses,  and  dikes. 

For  the  distribution  and  uses  of  gabbros  the  reader  is  referred  to 
Chapter  XI  on  Building  Stone. 

Peridotite 

Mineral  composition.  —  Peridotites  are  ultrabasic  intrusive  rocks 
consisting  chiefly  of  olivine,  with  usually  more  or  less  pyroxene,  some- 
times hornblende,  and  without  feldspar,  or  if  present  in  such  small 
amount  as  to  be  negligible. 

Chemical  composition.  —  Chemically  the  peridotites  are  characterized  by  very 
low  silica,  little  or  no  alumina  and  alkalies,  and  very  large  amounts  of  magnesia 
and  iron  oxides,  and  to  a  less  extent  lime.  The  following  analyses  will  make  clear 
these  features  in  chemical  composition. 


SiO2 

A12O3 

Fe203 

FeO 

MgO 

CaO 

Na20 

K20 

H20 

Rest. 

Total. 

I. 
II. 
III. 

43.87 
40.11 
39.99 

1.64 

0.88 
3.55 

8.94 
1.20 

2.60 
6.09 
8.56 

27.32 

48.58 
41.26 

6.29 
4.19 

.... 

0.50 

8.72 
2.74 

0.75 
0.74 
2.07 

100.63 
100.34 
99.62 

IV. 
V. 

39.37 
38.40 

4.47 
0.29 

4.96 
3.42 

9.13 
6.69 

26.53 
45.23 

3.70 
0.35 

6.50 

0.26 
0.008 

7.95 
4.35 

3.07 
1.34 

99.94 
100.38 

I.  Peridotite,  Baltimore  County,  Maryland;  II.  Dunite,  Corundum  Hill,  North  Carolina;  III.  Perido- 
tite, Olivine  Range,  New  Zealand;  IV.  Peridotite,  near  Open  Lake,  Michigan;  V.  Dunite,  Tula- 
meen  River,  British  Columbia. 

Varieties.  —  The  more  important  varieties  of  peridotites  usually  recognized  are : 
Dunite,  composed  chiefly  of  all  olivine;  cortlandite,  composed  chiefly  of  olivine 
and  hornblende,  with  sometimes  pyroxene  (hypersthene) ;  saxonite  (harzburgite), 


PLATE  VII,  FIG.  1.  —  Photomicrograph  of  a  section  of  granite. 


FIG.  2.  —  Photomicrograph  of  a  section  of  diabase.     (Both  photos  by 
A.  B.  Cushman,  from  Hies'  Economic  Geology.) 


(83) 


84  ENGINEERING  GEOLOGY 

composed  of  olivine  and  orthorhombic  pyroxene;  wehrlite,  composed  of  olivine  and 
pyroxene  (diallage) ;  Iherzolite,  composed  of  ob'vine  and  monoclinic  and  orthorhombic 
pyroxenes  (diallage  and  hypersthene).  Mica  (biotite)  occurs  in  some  peridotites, 
designated  mica  peridotite. 

These  varieties  may  grade  into  each  other  and,  while  they  are  holocrystaUine 
rocks,  it  is  hardly  possible  to  distinguish  between  them  by  megascopical  means 
alone.  In  addition  to  the  principal  minerals  mentioned  as  entering  into  the  com- 
position of  peridotites,  accessory  ones  usually  are  present,  such  as  ilmenite,  chro- 
mite,  and  sometimes  garnet. 

For  megascopic  purposes,  a  simplified  classification  of  peridotites  as  given  by 
Hatch,  based  on  the  ferromagnesian  minerals  present,  is  in  some  respects  preferable 
to  the  one  given  above.  It  is:  Dunite,  olivine  rock;  hornblende  peridotite,  pyroxene 
peridotite,  hornblende-pyroxene  peridotite,  and  hornblende-biotite  peridotite. 

Physical  properties.  —  The  peridotites  are  usually  very  dark  in  color, 
varying  ordinarily  from  some  shade  of  green  to  black.  The  variety 
dunite  is  frequently  some  shade  of  light  green.  The  specific  gravity 
ordinarily  ranges  between  3.0  and  3.3. 

Texture  and  structure.  —  Peridotites  are  granitoid  rocks,  with 
porphyritic  texture  essentially  wanting.  Those  varieties  containing 
pyroxene  or  hornblende,  or  both,  frequently  exhibit  a  mottling  of  the 
individuals  of  these  minerals  from  inclosures  of  smaller  grains  of  olivine; 
such  texture  has  been  called  poikilitic.  They  are  massive  rocks  but 
may  be  rendered  schistose  by  pressure  metamorphism. 

Alteration.  —  Under  atmospheric  conditions  peridotites  are  very 
susceptible  to  rapid  alteration,  the  chief  product  being  serpentine  al- 
though talc  is  not  uncommon.  A  certain  amount  of  serpentization  is 
nearly  always  noted,  as  indicated  in  the  analyses  by  the  large  percent- 
ages of  water.  They  finally  break  down  into  ferruginous  soils.  The 
change  to"  serpentine,  as  indicated  by  Pirsson,  may  be  represented  by 
the  following  reaction: 

Olivine  Enstatite  Water  Serpentine 

Mg2SiO4     -h     MgSi03     +     2H2O       =      H4Mg3Si209. 

Mode  of  occurrence.  —  The  peridotites,  as  independent  masses,  occur 
chiefly  as  dikes,  although  other  forms,  such  as  sheets,  stocks,  etc.,  are 
known.  They  are  also  associated  at  times  with  large  intrusive  masses 
of  gabbro,  into  which  they  may  grade. 

For  distribution  and  uses  of  peridotites  see  Chapter  on  Building 
Stone. 

Pyroxenite  and  Hornblendite 

These  are  rocks  related  to  peridotite  and  are  ordinarily  treated  as  members  of 
the  peridotite  group,  but  by  some  are  included  separately  under  the  group  name 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC. 


85 


perknite.  They  are  not  very  common  rocks  and  are  not  of  great  geologic  impor- 
tance. According  to  whether  the  dominant  mineral  is  pyroxene  or  hornblende, 
we  have  either  pyroxenite  or  hornblendite.  Biotite,  olivine,  and  iron  ores  may  occur 
as  accessory  minerals. 

Typical  pyroxenite  and  hornblendite  contain  neither  feldspar  nor  olivine.  By 
the  addition  of  feldspar  they  mark  the  passage  into  gabbros  on  the  one  hand,  and 
of  olivine  into  the  peridotites  on  the  other.  They  are  very  dark-colored  rocks  of 
high  specific  gravity,  and  occur  both  as  dikes  and  deep-seated  masses.  Like  the 
peridotites  they  alter  readily  on  exposure  to  weather,  and  ultimately  yield  heavy 
ferruginous  soils. 

As  indicated  in  the  analyses  below,  they  are  characterized  chemically  by  low 
silica,  alumina,  and  alkalies,  and  by  large  percentages  of  magnesia,  lime,  and  iron. 


Si02 

A1203 

FejOa 

FeO 

MgO 

CaO 

NajO 

K20 

H20 

Rest. 

Total. 

I. 
II. 
III. 

50.80 
45.05 
46.4 

3.40 
6.50 
10.8 

1.39 
3.83 
5.9 

8.11 

7.69 
5.6 

22.77 

12.07 
22.2 

12.31 
18.66 
3.7 

Trace 
0.94 
0.3 

Trace 
0.78 
1.2 

0.52 
2.40 
3.8 

0.73 
2.96 

100.03 
100.88 
100.1 

I.  Pyroxenite,  Baltimore  County,  Maryland;  IT.  Pyroxenite,  Brandberget,  Gran,  Norway;  III.  Horn- 
blendite, Valbonne,  Pyrenees. 


J 


VOLCANIC  OR  DENSE  IGNEOUS  ROCKS 


Introduction.  —  In  this  group  are  included  those  igneous  rocks 
that  have  formed  on  or  near  the  surface,  and  because  of  the  conditions 
of  rapid  cooling  the  resulting  component  minerals  are  so  small  in  size 
that  they  cannot  be  distinguished  by  the  naked  eye,  but  require  the 
microscope  to  identify  them.  Because  of  this  fact  they  are  usually 
referred  to  as  dense  igneous  rocks  hi  contradistinction  to  the  grained  or 
plutonic  rocks  that  have  formed  at  great  depth  beneath  the  surface, 
and  whose  principal  minerals  are  usually  large  enough  to  be  identified 
megascopically,  chiefly  because  of  the  slower  rate  of  cooling.  The  two 
groups  of  rocks,  however,  grade  into  each  other,  and  no  sharp  line  of 
demarcation  can  be  drawn  between  them. 

The  volcanic  rocks  may  be  classified  in  the  same  manner  as  the 
plutonic  igneous  rocks,  and  for  every  type  of  the  latter  a  volcanic 
equivalent  is  recognized.  For  such  division  of  the  volcanic  rocks  we 
must  rely  on  the  methods  of  microscopic  study  of  thin  rock  sections, 
since  it  is  not  possible  to  distinguish  between  them  by  megascopic 
study  for  the  reasons  previously  stated.  On  this  basis  we  may  make 
the  following  divisions  of  the  volcanic  rocks  corresponding  to  the 
plutonic  equivalents  described  in  the  following  pages  and  presented  in 
tabular  form  on  page  86. 


86 


ENGINEERING  GEOLOGY 


Volcanic 
Plutonic 

Rhyolite  " 
Granite 

Trachyte 

Syenite 

Phonolite 
Nephelite-syenite 

Dacite 

(Quartz  andesite) 

Quartz  diorite 

Volcanic 

Andesite 

Augite 

(Andesite) 

Basalt 

Augitite 

Limburgite 

Plutonic 

Diorite 

Gabbro 

Olivine  gabbro 

Pyroxenite 

Peridotite 

For  megascopic  purposes  this  grouping  of  volcanic  rocks  cannot 
be  followed,  since  the  principal  minerals  are  indistinguishable  by  the 
naked  eye.  By  adopting  color  as  the  basis  of  classification,  which  ex- 
presses in  a  general  way  the  mineral  composition  of  the  rocks  as  to 
whether  light-  or  dark-colored  minerals  predominate,  we  may  group 
the  volcanic  rocks  into  two  principal  divisions,  namely,  (a)  felsites  and 
(6)  basalts. 

On  the  color  basis,  felsites  comprise  the  light-colored  volcanic  rocks, 
which  are  dominantly  feldspathic,  with  or  without  quartz  and  the 
feldspathoids,  and  would  include  rhyolite,  trachyte,  and  phonolite  of 
the  table  on  page  73.  The  remaining  types  comprising  andesite, 
basalt,  augitite,  and  limburgite,  consisting  of  nearly  equal  or  larger 
amounts  of  ferromagnesian  minerals,  with  or  without  lime-soda  feld- 
spar, would  be  included  under  the  single  term  basalt. 

The  felsites  and  basalts  as  thus  defined  are  described  below. 

Felsite 

The  felsites  include  the  dominantly  feldspathic  varieties  ~  of  fine- 
grained volcanic  rocks,  with  or  without  quartz  and  the  feldspathoids, 
which  are  light  in  color,  white,  light  to  medium  gray,  red,  yellow, 
brown,  or  green,  and  comprising  the  microscopic  types  rhyolite,  trachyte, 
and  phonolite.  They  sometimes  show  porphyritic  texture,  and  may 
be  designated  felsite  porphyry.  Vesicular  or  cellular  structure  is  less 
common  in  the  felsites  than  in  the  basalts.  Frequently,  flow  structure 
is  visible  to  the  naked  eye.  The  usual  range  in  specific  gravity  is 
from  2.4  to  2.7.  Since  the  rocks  grouped  as  felsites  are  very  fine- 
granular,  it  is  not  possible  to  affect  a  classification  of  them  megascopi- 
cally  on  the  basis  of  mineral  composition.  Megascopically,  then,  such 
division  as  may  be  made  of  them  must  be  based  on  color  and  texture. 

Chemical  composition.  —  The  chemical  composition  of  felsites  is  quite  variable, 
dependent  upon  the  mineral  composition.  The  rhyolites  correspond  to  granites, 
trachytes  to  syenites,  phonolites  to  nepheline  syenites,  etc.  The  table  below  will 
serve  to  indicate  the  composition  of  some  of  the  more  important  varieties  of  felsite. 
These  should  be  compared  with  analyses  of  their  plutonic  equivalents  on  pages  62, 
75,  77. 


ROCKS,   THEIR  GENERAL  CHARACTERS,  ETC. 
ANALYSES  OF  FELSITES 


87 


SiO, 

A1203 

Fe^O, 

FeO 

MgO 

CaO 

NaaO 

K20 

H20 

Rest. 

Total. 

I. 

75.98 

12.34 

0.85 

0.93 

0.15 

0.13 

4.02 

4.44 

0.88 

0.30 

100.02 

II. 

75.34 

12.51 

0.42 

1.55 

0.32 

1.07 

3.31 

4.17 

0.86 

0.49 

100.04 

III. 

63.24 

17.98 

2.67 

0.85 

0.63 

0.93 

6.27 

5.47 

1.17 

0.73 

100.14 

IV. 

62.17 

18.58 

2.15 

1.05 

0.73 

1.57 

7.56 

3.88 

1.70 

99.22 

V. 

57.86 

20.26 

2.35 

0.39 

0.04 

0.89 

9.47 

5.19 

2.61 

0.91 

99.97 

VI. 

56.24 

21.43   2.01 

0.55 

0.15 

1.38 

10.53 

5.74 

0.98 

0.85 

99.86 

VII. 

68.10 

15.50   3.20 

None 

0.10 

3.02 

4.20 

3.13 

2.72 

0.24 

100.21 

I.  Rhyolite  from  Haystack  Mountain,  Aroostook  County,  Maine;  II.  Rhyolite  from  "Elephants'  Back," 
Yellowstone  National  Park;  III.  Trachyte,  Dike  Mountain,  Yellowstone  National  Park;  IV.  Tra- 
chyte, Crazy  Mountains,  Montana;  V.  Phonolite,  Black  Hills,  South  Dakota;  VI.  Phonolite,  Pleas- 
ant Valley,  Colfax  County,  New  Mexico;  VII.  Dacite,  Bear  Creek  Falls,  Shasta  County,  California. 

Mode  of  occurrence.  —  The  chief  occurrences  of  felsites  are  as  dikes 
and  lava  flows  or  sheets,  the  latter  being  the  more  common.  They  are 
found  in  many  localities  in  the  eastern  United  States,  but  are  especially 
abundant  as  lava  flows  and  sheets  in  the  West. 

Uses.  —  The  felsites  have  only  been  utilized  to  a  small  extent, 
since  their  textures  usually  render  them  unsuited  for  any  save  the 
rougher  classes  of  constructional  work.  As  a  rule  they  will  not  polish, 
and  their  rough  appearance  makes  them  unfit  for  interior  decorative 
purposes.  In  the  western  states  and  Mexico,  for  example,  they  give 
satisfaction  for  dimension  blocks. 


Basalt 

The  basalts  include  the  very  dark-colored  igneous  rocks  which  cor- 
respond to  the  felsites  in  texture.  Mineralogically  they  agree  with  the 
diorites  and  gabbros,  and  are  gray  black  to  black  in  color,  but  are  less 
lustrous  in  appearance  than  many  of  the  felsites.  Cellular  and  amyg- 
daloidal  structures  are  common  in  the  basalts,  and  while  porphyritic 
texture  is  sometimes  observed  it  is  less  frequent  than  in  the  felsites. 
Pyroxene,  olivine,  and  feldspar  may  occur  as  phenocrysts,  when  the 
rock  exhibiting  such  porphyritic  texture  is  conveniently  called  basalt 
porphyry,  which  bears  the  same  relation  to  basalt  that  felsite  porphyry 
does  to  felsite. 

The  range  in  specific  gravity  is  high,  usually  from  2.9  to  3.1.  The 
basalts  megascopically  are  recognized  by  their  dark  color  and  high 
specific  gravity.  Columnar  jointing  is  common  (Plate  VIII),  one  of  the 
best  examples  being  that  of  the  Giants'  Causeway  on  the  north  coast  of 
Ireland. 


PLATE  VIII.  —  Columnar  jointing  in  basalt,  Le  Puy,  France. 


(88) 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC. 


89 


Chemical  composition.  —  Like  felsite,  the  chemical  composition  of  basalt  is 
variable  and  depends  on  mineral  composition.  The  range  in  composition  is  shown 
in  the  following  chemical  analyses: 

ANALYSES  OF  BASALTS 


SiO, 

AUO, 

FejO, 

FeO 

MgO 

CaO 

NajO 

KjO 

H,0 

Rest.      Total. 

I. 

56.88 

18.25 

2.35 

4.45 

4.07 

7.53 

3.29 

1.42 

0.74 

1.08    100.06 

II. 

56.63 

16.85 

3.62 

3.44 

4.23 

7.53 

3.08 

2.24 

1.31 

1.25    100.18 

III. 

52.40 

13.55 

2.73 

9.79 

5.53 

10.01 

2.32 

0.40 

1.67 

1.59     99.99 

IV. 

48.76 

15.89 

6.04 

4.56 

5.98 

8.15 

3.43 

2.93 

1.88 

2.61    100.23 

V. 

45.11 

12.44 

2.67 

9.36 

11.56 

10.61 

3.05 

1.01 

0.94 

3.27    100.02 

VI. 

43.35 

11.46 

11.98 

2.26 

11.69 

7.76 

3.88 

0.99 

3.00 

3.97    100.34 

VII. 

38.62 

13.90 

5.97 

8.65 

11.21 

15.54 

2.01 

0.57 

1.46 

2.76    100.69 

I.  Andesite,  Franklin  Hill,  Plumas  County,  California;  II.  Andesite,  Unga  Island,  Alaska;  III.  Basalt, 
Pine  Hill,  South  Britain,  Connecticut;  IV.  Basalt,  Saddle  Mountain,  Pike's  Peak,  Colorado;  V.  Ba- 
salt, Pinto  Mountain,  Uvalde  County,  Texas;  VI:  Augitite,  Hutberg,  Tetschen,  Bohemia;  VII.  Lim- 
burgite,  Dakar  Peak,  Cape  Verde  Islands. 

Mode  of  occurrence.  —  Basalts  are  widespread  in  occurrence,  chiefly 
as  lava  flows  or  sheets,  and  dikes.  They  are  abundantly  developed 
both  in  the  eastern  and  western  United  States,  the  most  extensive  area 
being  that  of  the  Snake  River  region  covering  parts  of  Idaho,  Oregon, 
and  Washington,  the  dark  lava  beds  having  an  areal  extent  of  many 
thousand  square  miles  and  hundreds  of  feet  in  thickness. 

Uses.  —  The  porous,  cellular  varieties  of  basalt  should  be  excluded 
from  use  as  a  constructional  material,  but  there  seems  no  reason  why 
the  dense  compact  varieties  should  not  be  used  hi  those  regions  where 
it  occurs,  although  its  toughness  and  abundant  jointing  make  the  ex- 
traction of  dimension  blocks  difficult.  Color  and  lack  of  susceptibility 
to  good  polish  preclude  it  from  use  as  an  interior  decorative  stone. 
Its  principal  uses  at  present  are  for  macadamizing  and  paving  roads 
and  streets. 

GLASSY  IGNEOUS  ROCKS 

Under  glassy  rocks  are  included  those  igneous  rocks  which  are  com- 
posed essentially  or  entirely  of  glass.  They  represent,  with  only  rare 
and  unimportant  exceptions,  molten  lavas  poured  out  onto  the  surface 
which  have  undergone  extremely  quick  solidification,  aided  probably 
to  some  extent  by  the  rapid  escape  of  mineralizers.  Any  magma 
under  proper  conditions  of  rapid  chilling  may  solidify  as  glass,  but  the 
most  common  ones  show  a  high  percentage  of  silica  (acid)  and  cor- 
respond to  granite  in  composition. 


90 


ENGINEERING  GEOLOGY 


Some  glassy  rocks  may  contain  distinct  crystals  or  phenocrysts,  and 
are  known  as  glass  porphyry  or  vitrophyre.  More  often,  however, 
porphyritic  texture  is  not  developed,  and  we  may  recognize  the  follow- 
ing principal  varieties  of  glassy  rocks,  based  on  luster  and  structure: 
Obsidian,  a  homogeneous  glass,  of  bright  vitreous  luster,  jet  black  to 
red  in  color,  and  having  conchoidal  fracture;  pitchstone,  a  homogeneous 
glass  of  dull  or  resinous  luster,  black  to  red,  brown,  and  green  in  color, 
and  containing  from  5  to  10  per  cent  of  water;  perlite,  a  glass  broken 
by  concentric  cracks  on  cooling,  and  made  up  of  small  spheroidal 
masses,  usually  of  gray  color,  rarely  red;  pumice,  an  excessively  porous 
or  cellular  glass,  due  to  the  escape  of  water  vapor  at  high  temperature 
through  relief  of  pressure,  and  usually  white  or  gray  in  color,  though 
darker  shades  sometimes  occur. 

The  glassy  rocks  vary  from  dense  and  compact  homogeneous  rocks, 
having  conchoidal  fracture,  to  those  that  are  highly  vesicular  or  cellular, 
and  may  show  characteristic  flow  structure  (Plate  IV,  Fig.  1).  The 
usual  range  in  specific  gravity  is  from  2.34  to  2.7.  Under  conditions  of 
metamorphism  (pressure,  heat,  water,  etc.)  the  glassy  rocks,  especially 
the  older  ones,  alter  into  rocks  composed  of  definite  minerals  (feldspar 
and  quartz  chiefly)  and  of  stony  texture,  the  process  being  described  as 
devitrification. 

Chemical  composition.  —  The  range  in  composition  of  some  of  the  more  important 
varieties  of  glassy  rocks  is  shown  in  the  analyses  tabulated  below.  These  should  be 
compared  with  analyses  of  the  granular  rocks  on  pages  80,  97. 

ANALYSES  OF  VOLCANIC  GLASS 


Si02 

A1203 

Fe203 

FeO 

MgO 

CaO 

NasO 

K20 

H20 

Rest. 

Total. 

I. 

75.52 

14.11 

1.74 

0.08 

0.10 

0.78 

3.92 

3.63 

0.39 

0.11 

100.38 

II. 

73.11 

13.16 

0.62 

0.23 

0.19 

0.54 

2.85 

5.10 

4.05 

0.14 

99.99 

III. 

78.84 

12.47 

0.32 

0.90 

0.25 

1.08 

2.88 

5.38 

2.76 

Trace 

99.88 

IV. 

79.49 

11.60 

0.32 

0.49 

0.09 

1.64 

4.04 

1.52 

0.68 

None 

99.88 

V. 

53.52 

13.56 

4.93 

6.61 

7.37 

7.39 

3.22 

0.68 

1.03 

1.84 

100.05 

VI. 

42.25 

16.87 

5.24 

10.72 

6.91 

3  33 

3.96 

0.77 

6.01 

3.67 

99.84 

* 

I.  Obsidian,  Obsidian  Cliff,  Yellowstone  National  Park;  II.  Pitchstone,  Rosita,  Colorado;  III.  Perlite, 
Midway  Geyser  Basin,  Yellowstone  National  Park;  IV.  Pumice,  Cinder  Cone,  California;  V.  Basalt 
obsidian,  Londorf,  Vogelsberg,  Hesse;  VI.  Diabase  glass,  Mars  Hill,  Aroostook,  Maine. 

Mode  of  occurrence.  —  The  glassy  rocks  sometimes  occur  as  in- 
dependent sheets  and  dikes,  but  usually  they  form  the  surface  of  lava 
flows  and  at  times  the  marginal  portions  of  dikes.  They  are  found, 
therefore,  in  volcanic  regions,  and  are  especially  abundant  in  the  West, 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       91 

but  are  also  found  in  the  eastern  United  States.     Obsidian  Cliff  in  the 
Yellowstone  National  Park  is  a  noted  locality. 

Uses.  —  Volcanic  glasses  have  not  been  quarried  for  commercial 
purposes,  but  some  of  them  could  be  used  to  advantage  as  interior 
decorative  stone,  since  some  are  quite  ornamental  and  are  susceptible 
of  a  high  polish. 

PORPHYRITIC  IGNEOUS  ROCKS  (PORPHYRIES) 

The  term  porphyry,  when  applied  hi  the  broad  sense,  includes  all 
igneous  rocks,  regardless  of  mineral  composition  and  therefore  of  rock- 
type,  that  show  porphyritic  texture.  It  is  in  this  sense  that  the  word 
porphyry  is  used  in  this  book.  As  previously  stated  hi  the  description 
of  the  different  kinds  of  igneous  rocks,  porphyritic  texture  may  be  de- 
veloped in  either  the  granitoid,  felsitic,  or  glassy  rocks.  Megascopically 
then  the  porphyries  may  be  subdivided  into  (1)  those  porphyritic 
rocks  whose  groundmass  is  sufficiently 'coarse-granular  that  its  prin- 
cipal minerals  may  be  distinguished  by  the  naked  eye;  and  (2)  those 
porphyritic  rocks  whose  groundmass  is  either  felsitic  or  glassy  in 
texture,  and  in  which  only  the  phenocrysts  may  be  identified  by  the 
unaided  eye. 

On  this  basis  of  classifying  the  porphyritic  rocks,  the  first  group  will 
include  the  granitoid  rocks  having  porphyritic  texture,  such  as  porphy- 
ritic granite,  porphyritic  syenite,  etc.  Such  texture  is  developed  chiefly 
in  the  feldspathic  members  of  this  group,  with  feldspar  probably  the 
most  frequent  mineral  formed  as  phenocrysts,  which  may  or  may  not 
show  outward  crystal  form. 

The  second  group  includes  all  felsitic  and  glassy  igneous  rocks  hav- 
ing porphyritic  texture,  such  as  felsite  porphyry,  basalt  porphyry,  and 
glass  porphyry  or  vitrophyre.  The  phenocrysts  may  consist  of  either 
light-colored  (quartz  and  feldspar)  or  dark-colored  (hornblende,  pyrox- 
ene, biotite,  or  olivine)  minerals.  Porphyries  are  common  to  both  the 
light  (acid)  and  dark  (basic)  igneous  rocks  belonging  to  the  group  of 
dense-textured  ones. 

In  chemical  and  mineral  composition,  specific  gravity,  alteration, 
etc.,  the  porphyries  are  similar  to  their  corresponding  granular  types 
of  rock,  and  from  the  standpoint  of  durability  they  may  be  utilized  for 
the  same  purposes.  They  have  wide  distribution,  and  show  a  variety 
of  colors.  Many  of  our  important  ore  deposits  of  the  West  are  asso- 
ciated with  porphyries,  and  in  the  West  the  word  porphyry  is  used  for 
almost  every  igneous  rock  occurring  in  sheets  or  dikes  in  connection 
with  ore  deposits  (Kemp).  (See  Chapter  on  Ore  Deposits.) 


92  ENGINEERING  GEOLOGY 

In  many  of  the  porphyries,  the  phenocrysts  contrast  strongly  in 
color  with  that  of  the  groundmass,  and  exhibit  a  beautiful  effect  on 
polished  surfaces.  They  are  hard  and  durable,  usually  without  rift  or 
grain,  and  often  of  beautiful  color,  but  have  been  used  to  a  very  limited 
extent  as  decorative  stone  in  the  United  States. 

PYROCLASTIC  OR  VOLCANIC  FRAGMENTAL  ROCKS 

Under  pyroclastic  rocks  are  included  all  fragmental  materials  erupted 
by  volcanoes,  regardless  of  size  and  shape.  Masses  of  rock  weighing 
tons  are  sometimes  thrown  out,  and  from  this  size  the  material  grades 
down  to  that  of  dust-like  particles. 

The  different  kinds  of  volcanic  fragmental  material  recognized  are: 
(1)  Volcanic  blocks,  the  large  irregular-shaped  masses,  angular  to  some- 
what rounded,  and  measuring  several  feet  and  more  in  size;  (2)  bombs, 
round  or  elliptical-shaped  masses  of  lava,  ranging  from  a  few  inches 
up  to  a  foot  and  more  in  diameter;  (3)  lapilli,  fragments  of  lava  of  in- 
definite shape,  ranging  in  size  from  a  pea  to  that  of  a  walnut;  and 
(4)  volcanic  ash  (Plate  VI,  Fig.  2)  or  dust,  the  finer  particles  of  lava 
ejected,  including  all  sizes  below  that  of  a  pea. 

The  larger  fragments  accumulate  near  the  vent  or  opening,  while 
the  finer  material  may  travel  some  distance  before  falling  to  the  sur- 
face. They  may  cover  extensive  areas  and  accumulate  to  consider- 
able depths,  and  are  sometimes  interbedded  with  lava  flows  as  shown 
in  Fig.  59.  Consolidation  of  the  fragmental  material  into  more  or 
less  firm  rock  may  take  place  either  on  land  or  under  water;  in 
either  case  the  rock  usually  shows  stratification.  The  finer  volcanic 
materials  after  consolidation  yield  volcanic  tuffs;  the  larger  and 
coarser  materials  give  volcanic  breccias.  Other  names,  such  as  vol- 
canic agglomerate  and  volcanic  conglomerate,  have  been  applied  to 
the  consolidated  coarse  material,  according  to  size  and  shape  of  the 
fragments. 

The  volcanic  tuffs  and  breccias  may  receive  different  names,  ac- 
cording to  the  nature  of  fragments  composing  them;  such  as,  rhyo- 
lite-tuffs,  trachyte-tuffs,  andesite-tuffs,  basalt-tuffs,  etc.  Those  magmas 
of  acid  composition  (high  silica),  corresponding  to  felsite,  are  more  apt 
to  yield  fragmental  material  than  the  more  basic  or  low  silica  ones  of 
the  composition  of  basalt,  because  chiefly  of  their  greater  viscosity  and 
greater  difficulty  of  escape  of  the  vapors. 

Chemical  composition.  —  The  following  analyses  show  the  chemical  composition 
of  some  of  the  types  of  volcanic  fragmental  rocks: 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC. 
ANALYSES  OP  VOLCANIC  FRAGMENTAL  ROCKS. 


93 


Si02 

AIA 

Fe,03 

FeO 

MgO 

CaO 

Na,O 

K,0 

H,0 

Rest. 

Total. 

I. 

70.01 

12.61 

1.47 

0.50 

0.72 

1.06 

1.94 

5.12 

4.68 

O.Q4 

100.52 

II. 

61.15 

15.70 

4.31 

1.12 

3.04 

2.84 

1.54 

2.22 

7.05 

1.44 

100.59 

III. 

52.24 

21.08 

4.41 

n.d. 

0.60 

2.68 

4.58 

6.43 

8.33 

0.08 

100.43 

IV. 

57.16 

20.06 

2.84 

1.95 

1.55 

4.41 

5.84 

4.52 

1.09 

2.67 

102.09 

V. 

47.44 

16.51 

15.33 

3.19 

8.80 

6.02 

1.60 

0.30 

1.12 

100.48 

I.  Rhyolite  tuff,  Willard's  Creek,  Lassen  County,  California;  II.  Trachyte  tuff,  Two  Ocean  Pass, 
Yellowstone  National  Park;  III.  Phonolite  tuff,  Schorenberg,  Eifel  Rh.,  Prussia;  IV.  Andesito  tuff, 
Nightingale  Island,  Tristan  d'  Acunha,  South  Atlantic;  V.  Basalt  tuff  (not  fresh,  14.12  per  cent 
HjO),  Salt  Lake,  Oahu,  Hawaiian  Islands. 

The  volcanic  fragmental  rocks,  especially  the  tuffs,  show  a  variety 
of  color.  The  more  recent  ones  are  usually  only  partially  consolidated, 
are  soft  and  porous,  and  are  capable  of  absorbing  large  quantities  of 
water.  On  the  other  hand,  the  older  ones  are  often  compact  and  hard 
and  their  fragmental  character  may  not  be  evident  to  the  naked  eye. 
They  may  be  moderately  strong,  but  are  usually  light  in  weight. 

Volcanic  tuffs  have  wide  distribution  in  the  West,  and  have  more 
restricted  occurrence  in  the  East.  They  have  been  employed  only  to 
a  limited  extent  for  building  purposes  in  this  country,  but  have  a  more 
extended  use  in  Mexico  and  locally  in  several  of  the  European  countries. 
They  are  usually  soft  and  easy  to  work,  but  owing  to  their  porous 
nature  they  may  be  used  to  best  advantage  only  in  dry  climates.  As 
a  rule,  they  will  not  polish  because  of  their  textures. 


SEDIMENTARY  ROCKS 

Introduction.  —  The  rocks  included  under  this  head,  known  also  as 
stratified  rocks,  are  of  a  secondary  or  derivative  origin,  since  they  have 
been  formed  chiefly  from  pre-existent  ones.  A  few  have  been  formed 
from  the  remains  of  plants  and  animals.  The  source  of  the  material 
entering  into  the  composition  of  most  sedimentary  rocks  may  have 
been  derived  from  pre-existing  igneous,  metamorphic,  or  stratified 
rocks.  Indeed,  the  earliest  sediments  are  regarded  by  most  geolo- 
gists as  having  been  derived  from  already  existing  igneous  rocks. 

The  materials  composing  the  sedimentary  rocks  have  been  laid 
down  under  water  or  on  land,  and  have  been  derived  from  the  dis- 
integration (physical)  and  decomposition  (chemical)  of  pre-existing 
minerals  and  rocks,  and  of  plants  and  animals,  as  discussed  under 
Weathering  in  Chapter  IV.  As  a  rule  this  material  has  been  moved 
from  its  original  position  by  various  agents:  (1)  Partly  as  mechanical 


94  ENGINEERING  GEOLOGY 

sediments  in  the  form  of  solid  particles  of  different  sizes  and  shapes; 
and  (2)  partly  as  dissolved  salts  in  solution.  The  principal  agents 
involved  in  shifting  the  position  of  this  material  are:  (1)  Moving 
water,  the  most  important  one,  forming  aqueous  sediments,  which 
comprise  the  vast  majority  of  sedimentary  rocks;  (2)  mechanical 
action  of  wind  forming  ceolian  sediments,  which  are  of  less  impor- 
tance; and  (3)  ice,  chiefly  glacial,  forming  in  this  case  glacial  sedi- 
ments. 

According  to  the  agents  involved  in  the  deposition  of  sedimentary 
rocks  we  may  have  (a)  mechanically-formed  sediments;  (6)  chem- 
ically-formed sediments;  and  (c)  organically-formed  sediments. 

GENERAL  PROPERTIES  OF  SEDIMENTARY  ROCKS 

Variation  in  size  of  material.  —  The  products  of  rock  decay  vary 
greatly  in  size,  but  when  subjected  to  the  action  of  running  water 
they  are  sorted  and  graded  into  particles  of  approximately  equal 
size,  in  accordance  with  the  strength  of  current,  as  explained  in 
Chapter  V.  Grouped  then  according  to  size,  beginning  with  the 
coarsest,  the  following  names  for  this  material  may  be  employed: 
(1)  Boulders  and  cobbles,  the  coarsest  material,  ranging  down  to  3 
or  4  inches  in  diameter;  (2)  gravel,  including  all  material  below  cobble 
size  down  to  1  millimeter  (^  inch)  in  diameter;  (3)  sand,  ranging 
from  1  to  0.05  millimeter  (^V  to  ^^  in.)  in  diameter;  and  (4)  clay 
and  silt,  ranging  from  .05  to  .0001  millimeter  (3-^  in.  to  ^51100  m-) 
in  diameter.  Gradation  of  these  into  each  other  is  very  common. 

Texture  of  sedimentary  rocks.  —  Texture  as  here  defined  relates 
to  size  and  shape  of  the  individual  grains  or  particles  composing  the 
rocks.  The  size  of  individual  grains  varies  within  very  wide  limits, 
from  coarse  material  like  boulders  and  gravel  which  form  when  con- 
solidated conglomerates  (Plate  X,  Fig.  2),  through  sand  cemented 
into  sandstone  (Plate  X,  Fig.  1),  to  fine  material  like  mud,  silt  or 
clay,  forming  shales.  The  shape  of  the  component  grains  depends 
chiefly  upon  whether  they  have  been  transported  or  not,  and  if  moved 
by  running  water  the  amount  of  wear  they  have  suffered  in  transit. 
Thus,  the  range  in  shape  is  from  smooth  and  well  rounded,  through 
sub  angular,  to  entirely  angular  material.  The  rounded  water-worn 
coarse  material  when  consolidated  yields  conglomerates  (Fig.  62), 
but  when  angular  and  consolidated  produces  breccias  (Fig.  61).  The 
texture  of  a  sedimentary  rock  affects  its  value  as  a  building  stone 
to  some  extent.  Other  things  being  equal,  fine-grained  ones  carve 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       95 

and  split  better,  as  well  as  being  often  more  durable,  although  there 
are  occasional  exceptions  to  this  rule. 

Cementation  of  sedimentary  material  into  solid  rock.  —  After 
the  loose  materials  of  different  sizes  described  above  have  been  de- 
posited, they  may  become  cemented  into  solid  rock  through  the  de- 
position of  mineral  matter  held  in  solution  by  the  percolating  waters. 


FIG.  61.  —  Sketch  showing  structure  of        FIG.  62.  —  Sketch  showing  structure  of 
a  breccia.  a  conglomerate. 


This  cement  binds  the  pebbles,  grains,  and  small  particles  together, 
converting  them  from  loose  masses  into  solid  firm  rock.  The  com- 
mon cementing  substances  deposited  from  solution,  which  serve  to 
bind  the  loose  materials,  are  silica,  calcium  carbonate,  and  iron 
oxide.  Sometimes  the  finer  clay-like  substances  mechanically  de- 
posited with  the  coarser  material  act  as  the  cement  or  binder.  The 
finer  sediments  like  clay,  mud,  etc.,  may  be  converted  into  solid 
rocks  by  pressure,  without  the  deposition  of  a  cementing  material. 
In  some  sandstones,  for  example,  the  cement  is  composed  to  a 
large  extent  of  secondary  minerals.  Thus  in  the  case  of  certain 
feldspathic  sandstones  which  were  being  examined  with  a  view  to 
using  them  in  the  construction  of  the  Ashokan  dam  in  the  Catskill 
Mountains,  it  was  found  that  their  exceptional  strength  was  due  to 
"  modifications  of  texture  that  have  resulted  from  the  alteration  and 
reconstruction  of  the  mineral  constituents.  The  breaking  up  of 
the  orthoclase  feldspar,  and  the  accompanying  changes  in  the  ferro- 
magnesian  minerals,  have  furnished  considerable  secondary  quartz, 


96  ENGINEERING   GEOLOGY 

which  has  in  part  attached  to  the  original  quartz  grains  making 
them  more  angular  and  developing  an  interlocking  tendency.  At 
the  same  time  the  fibrous  sericitic  and  chloritic  aggregates  have  de- 
veloped to  such  extent  as  to  fill  most  of  the  remaining  pores,  and  in 
many  cases  the  fibrous  extensions  have  actually  grown  partly  around 
the  adjacent  quartz  grains.  The  effect  has  been  to  develop  a  siliceous 
binding  of  unusual  toughness.  This  combination  of  changes  has 
made  a  rock  that  is  now  remarkably  well  bound  and  interlocked  for  a 
sedimentary  type."  l 

Quantity  of  cement.  —  All  gradations  may  exist  between  hard,  firm, 
and  compact  rocks  to  more  or  less  loose  and  friable  ones.  A  rock  may 
be  composed  entirely  of  hard  grains,  such  as  quartz,  and  yet  be  bound 
together  by  so  little  cement  that  the  rock  as  a  whole  is  soft  and  porous. 
On  the  other  hand,  a  rock  although  composed  of  soft  mineral  grains 
like  calcite,  may  be  so  well  fastened  by  the  cement,  as  to  form  a 
hard,  dense  mass.  We  can  see  from  this  that  the  strength  of  a  sedi- 
mentary rock  must  depend  mainly  on  the  tightness  with  which  the 
grains  are  bound  together,  for  the  particles  do  not  interlock  as  they 
do  in  igneous  rocks.  The  quantity  as  well  as  kind  of  cement  may 
therefore  influence  the  stone's  porosity,  hardness,  and  resistance  to 
abrasion  and  frost. 

Color  of  cement.  —  A  wide  range  of  color  may  be  shown  according 
to  the  composition  of  the  cement.  Iron  oxide  cement  is  some  shade 
of  yellow,  red,  or  brown;  silica  and  calcium  carbonate  if  free  from  im- 
purities would  be  white;  and  clay,  if  present  in  appreciable  amount, 
may  impart  a  grayish  color.  Silicates,  sometimes  of  secondary  char- 
acter, may  give  the  stone  a  bluish  or  greenish  tint.  Two  kinds  of 
cement  may  be  present  in  the  same  rock. 

Durability  of  Cement.  —  Other  things  being  equal,  silica  forms  the 
most  durable  kind  of  cement  in  rocks  exposed  to  the  chemical  action 
of  the  atmosphere;  iron  oxide  is  next,  and  calcium  carbonate  the  least 
durable.  Clay  if  present  in  small  amounts  and  evenly  distributed, 
probably  does  no  harm  and  facilitates  the  working  qualities  of  the 
stone,  but  if  very  abundant  it  tends  to  attract  moisture  to  the  rock 
and  lower  its  frost  resistance. 

Structure  of  sedimentary  rocks.  —  Most  sedimentary  rocks  are 
characterized  by  a  more  or  less  pronounced  original  bedded  struc- 
ture (Plate  XI),  known  as  bedding  or  stratification  (called  lamination 
in  the  finer  sediments),  which  represent  the  lines  of  parting  between 
individual  beds  or  strata,  resulting  from  the  sorting  action  of  water, 
1  Berkey,  Sch.  of  M.  Quart.,  XXIX,  p.  140,  1908. 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       97 

and  consequently  disposed  in  sheet-like  form.  The  sediment  is  de- 
posited in  layers,  usually  horizontally  or  nearly  so,  and  in  super- 
position, and  the  process  of  sedimentation  may  be  more  or  less  rapid, 
or  gradual  and  protracted.  The  layers  may  vary  as  to  kind  of  ma- 
terial, color,  texture,  and  thickness.  Variation  in  thickness  of  indi- 
vidual layers  may  range  from  a  very  small  fraction  of  an  inch  up  to 
100  feet  and  more,  hence  we  distinguish  beds  or  layers,  and  lamince 
(Fig.  65).  The  terms  bed  and  layer  as  generally  used  are  synony- 
mous and  refer  to  the  thicker  divisions,  while  lamince  are  applied  to 
the  thinner  ones.  Stratum  is  generally  applied  to  a  single  bed  or 
layer  of  rock,  while  a  group  of  beds  deposited  in  sequence  one  above 
another  and  during  the  same  period  of  geologic  time  is  known  as  a 
formation.  The  thickness  of  the  individual  layers  affects  the  value 
of  the  rock  for  building  stone,  as  well  as  its  stability  and  strength 
in  tunnel  construction,  etc. 

Composition  of  the  sedimentary  rocks.  —  Mineralogically,  the  sedi- 
mentary rocks  are  in  general  more  simple  than  most  of  the  igneous 
ones.  Fewer  minerals,  of  less  complex  composition  chemically,  and 
as  a  rule  of  more  stable  character,  enter  as  the  principal  components 
of  the  sedimentary  rocks.  This  follows  naturally  for  the  reason  that 
the  sediments  are  composed  principally  of  those  minerals  derived  by 
the  processes  of  sedimentation  from  igneous  rocks  able  to  resist  the 
various  changes  to  which  they  have  been  subjected,  together  with 
recombinations  to  form  new  minerals  of  a  less  complex  and  more 
stable  character,  under  surface  conditions.  The  most  common  min- 
erals entering  into  the  composition  of  sedimentary  rocks  are  quartz, 
kaolinite,  feldspar,  mica,  and  the  iron  oxides,  both  hydrous  and 
anhydrous,  together  with  those  precipitated  from  solution,  such  as 
the  carbonates  (calcite,  dolomite,  and  siderite),  and  the  sulphates, 
gypsum  and,  to  a  less  extent,  anhydrite,  as  well  as  a  few  less  com- 
monly-occurring ones. 

Chemically  the  sedimentary  rocks  are  subject  to  greater  variations  in  compo- 
sition than  the  igneous  masses,  owing  to  the  nature  of  the  processes  involved  in 
their  genesis.  Composite  analyses  of  sedimentary  rocks  as  averaged  and  tabulated 
by  Clarke  are  as  in  table  on  following  page. 

CLASSIFICATION  OF  SEDIMENTARY  ROCKS 

The  classification  of  sedimentary  rocks  best  suited  to  the  needs  of 
the  engineer,  and  the  one  that  is  adopted  in  this  book,  is  based  (a) 
on  mode  of  formation  or  genesis,  and  (6)  on  composition  and  physical 


98 


ENGINEERING  GEOLOGY 
COMPOSITE  ANALYSES  OF  SEDIMENTARY  ROCKS 


I. 

II. 

III. 

SiO2 

58  38 

78.66 

5  19 

A12O3 

15  47 

4.78 

0  81 

Fe2O3 

4.03 

1.08  ) 

FeO  

2.46 

0.30  \ 

0.54 

MgO.  . 

2.45 

1.17 

7.90 

CaO... 

3.12 

5.52 

42.61 

Na2O 

1  31 

0.45 

0  05 

K2O 

3.25 

1.32 

0  33 

H2O  at  110°  

1.34 

0.31 

0.21 

H2O  above  110°.  . 

3.68 

1.33 

0.56 

TiO2 

0.65 

0  25 

0  06 

CO2  .  . 

2.64 

5  04 

41  58 

P2O5  

0.17 

0.08 

0  04 

S... 

0.09 

8O2.. 

0.65 

0.07 

0.05 

Cl 

Trace 

0  02 

BaO 

0  05 

0  05 

None 

SrO  .  .  . 

None 

None 

None 

MnO.  . 

Trace 

Trace 

0.05 

Li20...  
C,  organic.  

Trace 
0.81 

Trace 

Trace 

100.46 

100.41 

100.09 

'_!.  Composite  analysis  of  78  shales;  or,  more  strictly,  the  average  of  two  smaller  composites,  properly 
weighted.  II.  Composite  analysis  of  253  sandstones.  III.  Composite  analysis  of  345  limestones. 

When  sedimentary  rocks  are  used  for  building  purposes  the  chemical  analysis  is  as  a  rule  of  little  value, 
but  for  some  other  uses  it  is  important. 

characters.     The  one  tabulated  below  is,  in  all  essentials,  that  given 
by  Pirsson.1     It  follows: 

I.  Sediments  of  mechanical  origin. 

1.  Water  deposits. 

a.  Conglomerates  and  breccias. 

b.  Sandstones. 

c.  Shales. 

2.  Wind  deposits. 

a.  Loess. 

b.  Sand  dunes. 

II.  Sediments  of  chemical  origin  formed  from  solution. 

1.  Concentration. 

a.  Sulphates:  Gypsum  and  anhydrite. 

b.  Chlorides:  Halite  (rock  salt) . 

c.  Silica:  Flint,  geyserite,  etc. 

d.  Carbonates:  Limestone,  travertine,  etc. 

e.  Oxides:  Iron  ores. 

1  Rocks  and  Rock  Minerals. 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       99 

2.  Organic,  formed  through  the  agency  of  animals  and  plants. 

a.  Carbonates:  Limestone  of  several  kinds. 

b.  Silica:  Flint,  diatomaceous  earth,  etc. 

c.  Phosphate:  Phosphate  rock. 

d.  Carbon:  Coal,  etc. 

From  the  very  nature  of  sedimentary  processes  the  principal  kinds 
of  sediments  tabulated  above  grade  into  each  other,  and  frequently 
it  is  difficult,  if  indeed  not  impossible,  to  determine  whether  a  partic- 
ular rock  should  be  classified  in  one  division  or  in  another. 

I.  SEDIMENTARY  ROCKS  OF  MECHANICAL  ORIGIN 

The  rocks  included  under  this  head  have  resulted  mainly  from  the 
mechanical  action  of  water  or  sometimes  wind  action,  and  are  there- 
fore stratified,  that  is,  arranged  in  layers  or  beds.  With  few  excep- 
tions, which  will  be  specifically  mentioned,  they  represent  the  land 
waste  derived  by  weathering  of  pre-existing  rocks,  transported  and 
deposited  by  moving  waters  and  subsequently  consolidated.  Be- 
cause they  are  composed  of  fragments  of  pre-existing  rocks  they  are 
sometimes  referred  to  as  fragmental  or  clastic  sediments.  In  com- 
position they  are  chiefly  siliceous  and  argillaceous,  sometimes  cal- 
careous. In  texture  they  vary  greatly  from  very  coarse  to  very 
fine-grained  rocks,  and  may  frequently  contain  fossils  —  remains 
of  animals  and  plants.  They  may  be  described  below  under  (a) 
breccias,  (b)  conglomerates,  (c)  sandstones,  and  (d)  shales. 

Breccias 

Breccias  are  composed  of  angular  instead  of  rounded  fragments, 
cemented  together  into  solid  masses  (Fig.  61).  They  are  not,  strictly 
speaking,  water-laid  rocks,  as  is  shown  by  the  angular  character 
of  the  fragments,  and  usually  in  a  general  absence  of  stratification. 
When  deposited  in  water,  as  they  sometimes  are,  the  character  of 
the  fragments  clearly  indicates  that  they  have  not  been  moved  by 
running  water  any  distance  from  their  source.  They  have  not  all 
been  formed  in  the  same  way,  hence  we  usually  distinguish,  on  basis 
of  origin,  several  different  kinds  of  breccias,  which  for  convenience 
and  not  for  genetic  reasons  are  given  below: 

(1)  Talus  breccias,1  composed  of  the  angular  material  (Fig.  63), 
derived  by  physical  weathering,  which  accumulates  at  the  base  of 

1  See  further  regarding  these  under  Weathering,  Chapter  IV,  and  Landslides, 
Chapter  VII. 


100 


ENGINEERING  GEOLOGY 


cliffs  (Plate  IX,  Fig.  2),  and  sometimes  becomes  cemented  from  the 
action  of  circulating  waters.  (2)  Friction  or  fault  (Fig.  64)  breccias, 
formed  of  angular  material  derived  from  earth-movements  which 


FIG.  63.  —  Section  of  cliff,  illustrating  talus  slope  at  base.     By  cementation 
the  talus  is  converted  into  breccia. 

crush  and  break  up  the  rock  on  the  two  sides  of  a  fault  by  rubbing 
of  the  walls  against  each  other,  or  by  intense  crushing  (Plate  IX, 
Fig.  1)  incident  to  folding.  The  coarse  and  fine  angular  fragments  so 


FIG.  64.  —  Section  showing  a  fault  breccia. 

derived  are  often  cemented  together  by  deposition  from  circulating 
waters.  Of  the  substances  deposited  in  the  interstices  of  the  rock 
fragments  and  which  serve  to  bind  them  together,  calcite  or  dolomite, 
and  quartz,  are  probably  the  commonest.  Sometimes  ore-minerals 


PLATE  IX,  FIG.  1.  —  Breccia  formed  by  crushing  of  marble  by  rock  movements. 


FIG.  2.  —  Talus  breccia  formed  by  disintegration  of  limestone  seen  in  cliffs 
on  right,  Lake  Louise,  Alberta.     (J.  S.  Hook,  photo.) 


(101) 


102  ENGINEERING  GEOLOGY 

are  deposited  by  the  circulating  waters  along  with  the  common  non- 
metallic  ones,  which  give  rise  to  breccia-ore  deposits,  such  as  the 
zinc  deposits  of  southwest  Virginia  and  east  Tennessee  (see  Chapter 
XVII).  (3)  Volcanic  or  eruptive  breccias,  formed  from  the  coarse  and 
fine  angular  material  ejected  by  volcanic  action,  and  afterwards  con- 
solidated into  solid  rock.  This  last  type  of  breccia  if  of  recent  for- 
mation is  usually  very  porous. 

The  angular  fragments  composing  breccias  may  vary  greatly  in 
size,  ranging  from  large  irregular-shaped  blocks  down  to  rock  parti- 
cles just  large  enough  to  be  readily  distinguished  by  the  naked  eye; 
and  these  different  sized  materials  may  be  and  usually  are  heteroge- 
neously  mixed.  The  fragments  may  all  be  derived  from  a  single  rock 
type  —  igneous,  sedimentary,  or  metamorphic  —  or  from  several  dis- 
similar types.  When  derived  from  a  single  kind  of  rock,  the  breccia 
may  be  designated  by  the  name  of  the  original  material,  as  lime- 
stone or  marble  breccia,  sandstone,  or  quartzite  breccia,  gneiss  breccia, 
etc. 

Breccias  may  show  a  wide  range  of  color,  due  partly  to  kind  and 
color  of  the  rock  fragments  and  partly  to  the  character  and  amount 
of  the  cement.  They  have  not  been  used  to  any  extent  as  a  stone  for 
building  purposes,  chiefly  because  of  their  heterogeneous  character 
and  appearance,  but  some  of  the  more  compact  varieties  which  are 
susceptible  of  a  polish  are  of  great  ornamental  value,  such  as.  some 
of  the  brecciated  marbles  (Plate  IX,  Fig.  1).  These,  however,  are 
often  lacking  in  durability  and  may  be  of  very  irregular  hardness. 
(For  properties  and  uses  of  breccias  as  decorative  or  ornamental  stone, 
see  Chapter  on  Building  Stone.) 

Conglomerate 

Conglomerates  are  composed  of  rounded  and  water-worn  material 
of  different  sizes  (Plate  X,  Fig.  2),  ranging  up  to  large  boulders, 
cemented  together  into  solid  rock  (Fig.  62).  The  compact  pebbles 
are  rounded  and  water-worn  from  water  action.  They  are  usually 
made  up  of  the  more  resistant  varieties  of  minerals  and  rocks  that 
may  have  traveled  some  distance  from  their  original  source,  and  the 
interstices  are  commonly  filled  with  fine  sediment,  such  as  sand,  etc. 
Among  the  commonest  cements  binding  the  pebbles  together  are  silica, 
calcite,  and  iron  oxide.  The  component  pebbles  may  be  essentially  of 
a  single  kind  of  mineral  or  rock,  or  several  kinds  may  be  mingled  to- 
gether. Thus,  we  may  have  quartz  conglomerate,  limestone  conglomerate, 
quartzite  conglomerate,  gneiss  conglomerate,  etc. 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       103 

The  rock  pebbles  entering  into  the  composition  of  conglomerates 
may  be  derived  from  original  igneous  or  sedimentary  rocks,  or  their 
equivalent  metamorphic  ones.  Volcanic  conglomerate  is  usually  ap- 
plied to  material  ejected  during  igneous  activity  that  has  fallen  into 
water  and  become  rounded  and  cemented  into  solid  rock. 

Like  breccias,  conglomerates  are  subject  to  a  wide  range  of  color, 
and  texturally  present  a  heterogeneous  appearance.  The  ratio  of 
cement  to  pebbles  is  very  variable.  Those  showing  much  cement 
and  with  sharp  contrast  between  it  and  the  pebbles  have  received  the 
name  pudding  stone.  Conglomerates  grade  into  sandstones  and  nearly 
all  gradations  between  the  two  may  be  observed. 

Conglomerates  are  entirely  aqueous  in  origin  and  usually  exhibit 
more  or  less  characteristic  stratified  or  bedded  structure,  which  is 
apt  to  be  less  distinct  in  the  coarser  types.  They  are  deposited  in 
shallow  water  close  into  shore,  and  represent  either  coarse  material 
dropped  by  a  stream,  or  the  products  of  wave  action  along  the  shore. 
When  forming  extensive  areas  they  usually  indicate  an  advance  of  the 
sea  over  the  land,  and  they  become  important  guides  to  the  geologist 
in  the  interpretation  of  past  geological  conditions,  such  as  unconform- 
ities, etc.  They  usually  mark  the  lower  member  of  a  sedimentary 
series,  and  are  of  widespread  occurrence  among  sedimentary  rocks. 

Some  conglomerates  may  furnish  durable  building  material,  and 
in  some  localities,  especially  in  the  vicinity  of  Boston,  they  have  had 
a  limited  use,  but  on  account  of  their  heterogeneous  character  and 
general  coarseness,  they  have  not  been  employed  to  any  extent  either 
as  a  building  or  ornamental  stone.  The  harder  and  denser  conglom- 
erates have  sometimes  been  used  for  making  millstones. 

Sandstone 

Composition  and  texture.  —  Sandstones  are  sedimentary  rocks 
composed  of  grains  of  sand  bound  together  by  a  cementing  material. 
Many  sandstones  contain  little  if  any  cement,  but  owe  their  tenacity 
to  the  pressure  to  which  they  were  subjected  at  the  time  of  their 
consolidation  (Merrill).  The  component  grains  of  sandstone  are 
chiefly  quartz,  but  many  other  minerals  occur,  such  as  feldspar, 
mica,  garnet,  magnetite,  etc.  Size  of  the  individual  grains  varies 
within  rather-  wide  limits,  the  coarser-grained  sandstones  passing 
into  conglomerates  on  the  one  hand,  and  the  finer-grained  ones  into 
shales  on  the  other.  The  sand  particles  of  the  fine-grained  rocks 
are  commonly  angular  or  subangular,  but  are  usually  well-rounded 
from  water  action  in  the  coarser  ones. 


PLATE  X,  FIG.  1.  —  Medium-grained  sandstone. 


FIG.  2.  —  Coarse  conglomerate  with  little  cement,  Frank,  Alberta. 
(H.  Ries,  photo.) 


(104) 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC. 


105 


Color.  —  Sandstones  exhibit  a  variety  of  color,  the  various  shades 
of  gray,  white  to  buff,  brown,  and  red  being  the  most  common.  Like- 
wise, the  cementing  materials  in  sandstones,  as  in  conglomerates, 
vary,  being  usually  silica,  iron  oxide,  calcite,  or  clay,  but  sometimes 
minerals  of  secondary  character  (p.  96).  The  color  of  the  rock  and 
its  adaptability  depend  more  perhaps  upon  the  character  of  the 
cementing  material  than  upon  the  individual  grains.  If  silica  alone 
is  present  the  rock  is  light-colored,  hard,  and  among  the  most  durable, 
but  difficult  to  work.  When  the  cementing  substance  is  iron  oxide 
the  rock  is  some  shade  of  red,  brown,  or  yellow,  and  usually  works 
readily.  A  calcium  carbonate  cement  produces  a  light-colored  rock, 
nearly  white  or  some  shade  of  gray  in  color,  generally  softer,  less 
resistant  to  the  weather,  but  easy  to  work.  Clay  cement  if  abundant 
is  objectionable  because  of  the  readiness  with  which  it  absorbs  water, 
rendering  the  rock  subject  to  injury  by  frost.  The  color  of  the  sand- 
stone is  often  one  of  the  factors  governing  its  use  as  a  building  stone. 

Porosity.  —  The  porosity  of  a  sandstone  is  a  matter  of  practical 
importance  for  several  reasons.  High  porosity  may  mean  high 
absorption  and  high  permeability.  A  very  porous  sandstone  might 
therefore  be  regarded  as  unsuitable  for  dam  construction  or  for  use 
in  moist  situations.  If  the  absorbed  water  completely  fills  the  pores 
there  is  then  danger  of  the  stone  disintegrating  when  exposed  to  re- 
peated freezing. 


FIG.  65.  —  Section  showing  stratification  and  lamination;   (a)  laminated  beds. 

Porous  sandstones  under  favorable  structural  conditions  often  serve 
as  reservoirs  for  artesian  water. 


106 


ENGINEERING  GEOLOGY 


Structure.  —  Aqueous  sandstones  are  deposited  in  beds  or  layers  of 
varying  thicknesses  and  may  be  referred  to  as  thin-bedded  or  thick- 
bedded.  In  many  sandstones  laid  down  in  shallow  water,  rapid 
changes  in  currents  or  eddies  produce  cross-bedded  structure  (Fig.  66). 

Varieties  of  sandstone.  —  Many  varieties  of  sandstone  are  recog- 
nized, based  chiefly  upon  character  of  cementing  material,  composition, 
structure,  etc.  Named  according  to  the  character  of  cement  which 
binds  the  grains  together  we  may  recognize  siliceous  sandstones,  fer- 
ruginous sandstones,  calcareous  sandstones,  and  clayey  or  argillaceous 
sandstones.  Other  varieties  are:  Arkose,  a  sandstone  containing  much 
feldspar;  sometimes  called  feldspathic  sandstone,  derived  from  weath- 
ering of  feldspathic  rocks,  especially  granite,  the  products  having 


FIG.  66.  —  Section  illustrating  cross-bedding. 

been  moved  only  short  distances.  Graywacke,  a  compact,  usually  gray 
sandstone  (fine  conglomerate)  composed  of  rounded  or  angular  frag- 
ments of  various  kinds  of  rocks  in  addition  to  quartz  and  feldspar. 
Grit,  a  term  sometimes  applied  to  coarse  sandstones  composed  of  angu- 
lar grains  cemented  by  silica.  Flagstone,1  a  variety  of  thin-bedded 
sandstone  which  splits  readily  along  the  bedding  planes  into  slabs 
that  may  be  used  for  flagging.  Freestone,  a  variety  of  sandstone, 
usually  thick-bedded,  that  works  easily  or  freely  in  any  direction. 
Micaceous  sandstone,  a  variety  containing  much  mica. 

In  age  sandstones  range  from  the  Algonkian  down  to  the  most 
recent,  but  those  quarried  in  this  country  for  building  purposes  do 
not  include  any  of  later  age  than  Tertiary.  Sandstones  rank  among 
the  most  important  of  natural  building  materials.  For  the  proper- 
ties, mode  of  occurrence,  and  distribution  of  sandstones  as  a  con- 
structional stone,  the  reader  is  referred  to  Chapter  XI  on  Building 
Stone. 

Shale 

Shales  are  compacted  clays,  muds,  or  silts  that  possess  a  finely 
stratified  or  laminated  structure  (Plate  XI,  Fig.  2).     The  structure  is 
true  stratification  or  bedding  which  has  resulted  from  deposition  of 
1  See  Dickinson,  N.  Y.  State  Museum,  Bull.  61,  1903. 


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108  ENGINEERING  GEOLOGY 

the  finely-divided  material  in  water.  Because  of  being  composed  of 
the  finest  particles  of  land  waste  they  are  capable  of  being  split  into 
very  thin  leaves;-  and  for  the  same  reason  the  component  minerals  of 
shales  cannot  be  determined  with  the  unaided  eye. 

Shales  exhibit  a  great  variety  of  colors,  gray,  buff,  yellow,  red, 
brown,  purple,  and  green  to  black,  being  frequently  observed.  They 
are  usually  soft  and  brittle  rocks,  which  crumble  readily  under  the 
hammer.  They  may  grade  into  clays  on  the  one  hand,  and  into  fine- 
grained sandstones  when  siliceous,  into  thin-bedded  limestones  when 
calcareous,  into  some  kinds  of  coal  when  carbonaceous,  etc.,  on  the 
other.  When  metamorphosed  they  may  pass  into  slates  and  schists. 

Many  varieties  of  shales  are  recognized,  the  distinction  being 
founded  chiefly  on  composition.  Thus,  we  may  have  argillaceous  or 
clay  (aluminous)  shales,  arenaceous,  sandy,  or  siliceous  shales,  calca- 
reous shales,  ferruginous  shales,  carbonaceous  or  bituminous  shales,  etc. 

According  to  Clarke,  shale  is  the  most  abundant  of  the  three  prin- 
cipal kinds  of  sedimentary  rocks,  their  values  being  rated  as  follows: 
Shales  4  per  cent,  sandstones  0.75  per  cent,  and  limestones  0.25  per  cent. 

Shales  are  not  as  strong  as  sandstones  or  hard  limestones,  and 
for  this  reason,  if  unsupported  or  enclosed,  they  yield  to  the  pressure 
of  overlying  rocks.  This  is  occasionally  noticed  in  coal  mines,  where 
after  the  removal  of  the  coal  the  shale  rock  of  the  floor  and  roof  some- 
times squeeze  together.  For  the  same  reasons,  shale  rocks  which 
have  been  crushed  and  fractured  by  earth  movements  may  yield  to 
the  pressure  of  the  surrounding  rocks,  so  that  the  fractures  become 
healed  or  closed  up,  and  there  is  less  chance  for  the  circulation  of 
underground  waters.  This  fact  must  be  considered  in  the  construc- 
tion of  aqueduct  tunnels  to  avoid  danger  from  leakage. 

Shales,  because  of  their  thinly-bedded  character,  sometimes  cause 
trouble  in  tunneling,  the  material  becoming  dislodged  quite  easily. 

They  are  of  no  value  as  a  building  stone,  but  often  find  extended 
use  in  the  manufacture  of  brick,  tile,  and  pottery,  and  of  Portland 
cement.  (See  Chapters  XII  and  XIII,) 

Clays.  —  These  resemble  shales  chemically,  and  in  most  cases  are 
of  sedimentary  origin.  The  typical  ones  are  unconsolidated,  but 
grade  into  shales.  Their  uses  and  general  characters  are  discussed  in 
detail  in  Chapter  XIII. 

Variation  in  shale  and  sandstone  deposits.  —  Shales  when  followed 
along  the  bed  sometimes  grade  into  sandstones  and  vice  versa,  more- 
over the  two  kinds  of  rock  may  alternate,  sometimes  in  rapid  suc- 
cession. Plate  XI,  Fig.  2,  shows  a  heavy  sandstone  bed  underlain 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       109 

and  overlain  by  shale.  There  are,  however,  many  localities  in  which 
large  deposits  of  either  shale  or  sandstone  alone  are  found. 

The  possibility  of  variation  in  sedimentary  rocks,  especially  shales 
and  sandstones,  is  an  important  point  for  engineers  to  bear  in  mind, 
when  searching  for  a  convenient  site  to  open  a  quarry  for  road  ma- 
terial or  dimension  stone. 

The  case  of  the  Ashokan  dam  referred  to  above  can  again  be  taken 
to  illustrate  our  point.  The  dam  is  located  in  a  region  of  sedimentary 
rocks  consisting  of  sandstones  and  shales.  The  former  are  in  part 
thinly  bedded  and  used  for  flagstones,  and  many  quarries  have  been 
opened  up  in  the  thinly  bedded  or  "reedy"  rock.  In  other  parts  of 
the  formation  in  the  same  district  more  massive  beds  were  found, 
which  were  suitable  for  the  extraction  of  dimension  blocks.  As  a 
matter  of  practical  interest,  it  may  be  mentioned  that  the  reeds  or 
thin  bedding  were  due  to  the  presence  of  numerous  small  sized,  elon- 
gated gains,  lying  in  a  more  or  less  parallel  position.1 

WIND  (^EOLIAN)  DEPOSITS 

Under  this  head  are  included  only  two  kinds  of  material,  namely, 
loess  and  dune  sand.  Probably  the  first  of  these,  loess,  should  be  in- 
cluded here  only  in  part,  since  it  is  not  agreed  by  all  that  wind  has 
been  the  principal  agent  involved  in  the  formation  of  all  deposits. 

Loess 

Loess  is  the  name  given  to  a  very  fine,  homogeneous,  silty  or  clay- 
like  material  that  is  largely  siliceous  in  composition,  but  contains 
some  calcareous  matter,  which  sometimes  forms  nodules  and  small 
vertical  tubes.  It  is  usually  characterized  by  complete  absence  of 
stratification,  but  cleaves  vertically,  so  that  when  eroded  it  forms 
very  steep  precipitous  cliffs.  It  is  composed  chiefly  of  clay-like  mate- 
rial and  angular  grains  of  quartz,  tiny  flakes  of  mica,  and  more  or  less 
carbonate  of  lime,  which  has  been  reported  in  some  cases  to  reach 
30  per  cent  in  amount. 

Loess  covers  vast  areas  in  many  parts  of  the  world,  reaching  a 
thickness  of  hundreds  of  feet  in  some  cases,  and  for  this  reason  is 
of  some  importance  to  the  engineer.  Some  of  the  larger  and  more 
important  areas  of  loess  include  the  Mississippi  Valley  in  the  United 
States,  the  valleys  of  the  Rhine  and  its  tributaries  in  Europe,  and 
northern  central  China  in  Asia.  In  origin  it  is  claimed  by  some  to 
be  aeolian,  by  others  to  be  fluviatile  or  lacustrine,  and  by  still  others 
1  Berkey,  Sch.  of  M.  Quart.,  XXIX,  p.  154,  1908. 


PLATE  XII,  FIG.  1.  —  Front  slope  of  advancing  sand  dune.  Shows  edge  of 
forest  lying  in  its  path,  and  trees  already  partly  buried.  Cape  Henry,  Va.  (T.  L. 
Watson,  photo.) 


FIG.  2.  —  General  view  of  sand-dune  area. 
have  been  planted  to  stop  the  drifting  sand. 
photo.) 


Shows  grass  and  seedling  pines  which 
Baltic  coast  of  Germany.     (H.  Ries, 
(110) 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       Ill 

to  be  partly  seolian  and  partly  aqueous  (Chamberlin  and  Salisbury). 
The  loess  is  utilized  in  some  parts  of  the  West  for  common  brick 
manufacture,  but  the  product  is  not  always  satisfactory.  When  ex- 
posed to  rainstorms  the  loess  often  gullies  very  badly. 

A  similar  fine  clayey  material,  accumulated  in  basins  and  on  the 
plains  of  the  arid  regions  of  the  western  United  States,  probably 
formed  partly  by  wash  and  partly  by  wind  action  on  the  neighboring 
slopes,  has  been  called  adobe,  which  has  been  used  in  the  form  of  sun- 
dried  brick  for  building,  and  when  irrigated  forms  a  productive  soil. 

Sand  Deposits  (Dunes) 

Sand  relates  to  size  of  grain  and  not  to  mineral  composition,  but 
the  prevailing  kinds  are  composed  of  the  harder  varieties  of  rocks  and 
minerals,  since  the  softer  ones  tend  to  break  up  by  abrasion  and  de- 
composition into  finer  particles  known  as  dust.  The  size  of  indi- 
vidual sand  grains  may  vary  within  the  limits  of  1  to  0.05  millimeter 
in  diameter;  above  this  size  sand  grades  into  gravel  and  below  into 
silt  and  clay.  The  constituent  grains  may  consist  of  any  kind  of 
mineral  or  rock,  the  former  being  more  common,  and  the  composition 
of  the  sand  will  depend  upon  the  kind  of  rock  in  any  given  region 
from  which  it  was  derived.  Because  of  its  hardness,  resistance  to 
chemical  agents,  and  abundance  in  rocks,  quartz  is  the  commonest 
mineral  in  sand,  but  many  other  minerals  may  be  present.  Thus 
we  have  sand  deposits  of  sedimentary  origin,  composed  almost  entirely 
of  such  minerals  as  magnetite,  gypsum,  lime  carbonate,  dolomite, 
glauconite,  etc. 

The  fragmentary  material  resulting  from  the  disintegration  of  rocks 
may  be  removed  from  its  original  site  by  (1)  streams,  (2)  glacial  ice, 
or  (3)  wind.  Of  these  only  the  wind-blown  sands  or  seolian  ones  are 
treated  here. 

The  coarse  stream  or  sea  sands,  depending  upon  the  amount  of 
transportation,  are  frequently  more  or  less  rounded  from  wear,  while 
the  finer  particles  protected  by  a  film  of  water  are  likely  to  be  angular 
or  subangular;  glacial  sands,  when  not  subsequently  modified  by 
water  action,  are  angular;  while  wind-blown  sands  are  apt  to  be 
rounded.  However,  beach  sands  formed  by  the  sea  and  carried 
inland  by  the  wind  may  be  angular  or  subangular,  and  do  not,  as  a 
rule,  show  the  well-rounded  form  of  desert  sands. 

Sand  dunes.  —  In  arid  and  semi-arid  regions,  and  in  humid  regions 
where  the  loose  sand  is  not  protected  by  vegetation,  especially  the 
beaches  of  sea  and  lake  shores,  the  sand  is  piled  up  by  the  driving 


112  ENGINEERING  GEOLOGY 

action  of  the  wind  into  mounds  and  ridges  called  dunes  (PL  XII, 
Figs.  1  and  2).  The  sand  particles  are  lifted  only  a  slight  distance 
above  the  land  surface,  hence  their  movement  is  often  interfered 
with  by  surface  obstacles,  such  as  a  tree,  shrub,  building,  fence,  etc., 
which  results  in  deposition  and  accumulation.  The  ridges  commonly 
lie  transverse  to  the  direction  of  the  wind,  but  may  sometimes  be  lon- 
gitudinal or  parallel  as  illustrated  by  the  dunes  in  the  desert  of  north- 
west India.  They  are  usually  not  more  than  10  or  20  feet  high,  but 
sometimes  reach  heights  of  200  or  300  feet.1 

Dunes  commonly  show  a  long,  gentle  slope,  on  the  windward  side, 
up  which  the  sand  grains  can  be  readily  moved,  and  a  steep  slope 
(angle  of  rest  for  the  sand  grains)  on  the  leeward  side  (Fig.  67).  The 


FIG.  67.  —  Section  of   a  dune  showing  long,  gentle,  windward  slope    (ab)    and 
steeper  leeward  slope  (be}. 

slopes  may  be  very  irregular  when  the  dunes  are  partly  covered  by 
vegetation.  Dunes  migrate  by  the  transfer  of  sand  from  their  wind- 
ward to  their  leeward  side,  and  may  invade  forests  and  fertile  fields, 
and  even  bury  villages,  which  may  result  in  either  case  in  much  loss. 
Planting  of  vegetation  on  dunes  to  prevent  their  encroachment  on 
areas  is  resorted  to  in  some  countries  and  is  probably  the  simplest 
method  by  which  migration  may  be  stopped. 

Wind-blown  sands,  like  water  deposits,  are  stratified,  since  they 
are  transported  and  deposited  by  air  currents  of  varying  velocities, 
the  size  of  sand  grain  moved  being  dependent  upon  the  strength  of 
current.  The  composition  of  the  sand  varies,  but  is  usually  siliceous, 
sometimes  calcareous  as  in  the  Bahamas  and  Bermudas.  Many  of  the 
calcareous  sands  of  the  Bermudas,  through  partial  solution  and  re- 
deposition  of  the  lime  by  percolating  waters,  are  cemented  into  solid 
rock  of  considerable  extent. 

Dunes  are  not  confined  to  arid  regions,  but  are  likely  to  be  developed 
wherever  loose  sand  is  exposed  to  the  wind,  such  as  the  sandy  shores 
of  lakes  and  seas,  in  sandy  valleys  and  even  along  rivers. 

1  As  along  the  shores  of  Lake  Michigan. 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.      113 

Distribution.1  —  Sand  dunes  are  abundant  along  many  parts  of  the 
Atlantic  and  Pacific  coasts,  along  the  shores  of  the  Great  Lakes,  and 
in  many  parts  of  the  arid  regions  of  the  west.  They  are  not  unknown 
in  some  of  the  sandy  inland  areas  of  the  United  States. 

Wherever  found  they  are  often  a  source  of  trouble  if  the  region  is 
an  inhabited  one,  as  in  their  march  across  the  country  they  bury 
houses,  forests,  orchards,  railroads  or  anything  in  their  path.  Along 
the  Oregon  Railway  and  Navigation  Company,  the  sand  which  drifts 
across  the  tracks  near  the  Dalles,  Ore.,  has  to  be  removed  daily. 

The  practice  of  "  fixing"  dunes,  to  prevent  troubles  such  as  those 
mentioned,  is  a  problem  for  engineers  and  others,  which  has  been 
but  little  dealt  with  in  some  parts  of  the  United  States,  although  it 
has  gone  on  in  Europe  for  more  than  50  years. 

The  preliminary  methods  used  for  "fixing "  the  dunes  are:  (1)  Trans- 
planting with  beach  grass;  (2)  covering  with  heather;  and  (3)  cov- 
ering with  a  network  of  sand  hedges.  Any  one  of  these  methods 
serves  to  hold  the  sand  temporarily,  after  which  young  trees,  usually 
conifers,  are  transplanted,  and  the  danger  of  shifting  is  soon  removed. 

Some  railroads  have  adopted  the  plan  of  temporarily  fixing  the 
dunes  by  spraying  them  with  crude  oil. 

II.   SEDIMENTS  OF  CHEMICAL  ORIGIN  FORMED  FROM  SOLUTION 

Under  this  head  is  included  a  series  of  deposits  which  owe  their 
origin  to  processes  that  are  chemical  in  character,  and  formed  chiefly 
by  concentration  of  aqueous  solutions,  changes  of  temperature,  loss  of 
carbon  dioxide,  etc.,  aided  more  or  less  in  some  cases  by  the  action 
of  organic  life  (plants  and  animals),  and  resulting  in  the  precipitation 
of  insoluble  salts. 

Sulphates:    Gypsum  and  Anhydrite 

Gypsum  and  anhydrite,  the  hydrous  and  anhydrous  sulphates  of 
lime,  have  been  described  as  minerals  on  pages  37  to  39.  Since  they 
are  usually  intimately  associated  in  origin  and  mode  of  occurrence 
they  can  be  discussed  together.  They  may  be  readily  distinguished 
from  each  other  by  the  common  megascopic  properties  described  under 
each  mineral  in  Chapter  I. 

Gypsum  and  anhydrite  may  occur  as  separate  and  independent 
masses,  but  are  often  found  in  the  same  deposit  in  those  regions 
where  anhydrite  is  abundant,  when  their  association  is  frequently 
irregular  and  puzzling.  Under  such  conditions  the  two  are  found 

1  Hitchcock,  Nat.  Geog.  Mag.,  XV,  p.  43,  1904;  Kellogg,  Cal.  Jour.  Tech.,  HI, 
p.  156,  1904;  Stuntz  and  Free,  U.  S.  Bur.  Soils,  Bull.  68,  1911. 


114  ENGINEERING  GEOLOGY 

interbedded,  or  in  irregular  masses,  or  the  one  may  form  veins  travers- 
ing the  other.  Under  certain  conditions  each  may  be  transformed 
into  the  other.  In  their  most  extensive  and  important  occurrences, 
they  form  beds  or  lenticular  sheets  and  masses,  interstratified  usually 
with  clays,  shales,  sandstones,  and  limestones,  and  in  some  regions 
are  often  associated  with  rock  salt. 

Large  deposits  of  gypsum  and  anhydrite  are  known  at  a  number 
of  localities  in  the  United  States  and  Canada.  The  former  is  used 
for  plaster,  and  its  applications  are  discussed  in  Chapter  XII,  but 
the  latter  is  of  little  or  no  commercial  importance. 

In  some  regions,  the  solubility  of  the  gypsum  produces  a  hum- 
mocky  topography  and  even  sink  holes.  The  change  of  anhydrite 
to  gypsum  may  occur  on  exposure  of  the  former  to  moisture,  and  in 
Europe  at  least  one  case  is  known  where  a  tunnel  was  driven  through 
a  deposit  of  anhydrite  and  thrown  out  of  alignment  caused  by  the 
swelling  of  the  material  when  changed  to  gypsum,  the  alteration 
being  brought  about  by  trickling  water.  They  have  been  formed  on  a 
large  scale  from  concentration  of  oceanic  waters  by  evaporation  and 
in  inland  lakes  in  which  evaporation  equals  or  exceeds  the  amount 
of  inflow.  Less  extensive  deposits  are  formed  in  other  ways. 

Chlorides:    Rock  Salt 

The  mineral  halite  (NaCl)  occurs  in  many  localities  in  massive  granular  form  as 
beds  of  rock  salt  of  varying  thickness,  interstratified  with  clay,  marl,  and  sand- 
stones, usually  associated  with  gypsum,  anhydrite,  and  dolomite.  The  celebrated 
salt  deposits  of  Stassfurt,  Germany,  associated  with  gypsum,  anhydrite,  and  the 
chlorides  and  sulphates  of  potassium  and  magnesium,  are  the  most  extensive  in  the 
world,  having  a  known  thickness  of  4000  feet.  Beds  of  rock  salt  are  known  in 
the  United  States,  in  New  York,  Kansas,  Michigan,  Louisiana,  Virginia,  and  many 
other  states.  The  formation  of  rock  salt  has  in  many  cases  been  similar  to  that 
of  gypsum.  Some  of  the  principal  deposits  owe  their  origin  to  the  evaporation 
of  arms  of  the  sea  cut  off  from  the  main  body  of  water,  and  to  the  desiccation  of 
inland  lakes,  but  may  be  formed  in  other  ways. 

Salt  deposits  are  of  no  special  importance  to  the  engineer.  Owing  to  their  ready 
solubility  they  are  rarely  found  outcropping  on  the  surface  except  in  arid  regions. 
Their  presence  is  sometimes  noted  in  another  way,  because  surface  waters  in  some 
regions  may  show  an  abnormal  chlorine  content  caused  by  rock  drainage  from  salt- 
bearing  formations  entering  surface  streams.  Saline  water  in  a  deep  well,  however, 
does  not  necessarily  indicate  the  presence  of  salt  beds. 

Siliceous  Deposits 

Under  siliceous  materials  are  included  those  deposits  of  silica  (Si02), 
which  form  by  deposition  from  evaporation  of  aqueous  solutions,  and 
by  the  action  of  organic  life.  Some  may  form  at  times  as  the  result 


ROCKS,   THEIR  GENERAL  CHARACTERS,   ETC.  115 

of  direct  chemical  reactions.  The  deposits  so  formed  have  not  the 
widespread  occurrence  and  importance  as  sediments  formed  by 
other  processes,  but  are  sometimes  of  considerable  interest  locally. 
Those  included  under  this  head  are  flint,  siliceous  sinter  (geyserite}, 
and  diatomaceous  earth. 

Flint.  —  Flint,  known  also  as  chert  and  hornstone,  has  been  re- 
ferred to  as  a  variety  of  quartz  in  Chapter  I.  It  is  a  hard,  dark 
gray  to  black  rock,  breaking  with  conchoidal  fracture,  and  composed 
of  amorphous  or  chalcedonic  silica.  Its  dark  color  is  due  to  car- 
bonaceous matter,  which  disappears  on  strong  heating,  and  it  is  trans- 
lucent on  thin  edges,  resembling  many  felsites  of  igneous  origin. 
Chert  occurs  chiefly  as  nodules,  layers  or  lenses  in  chalk  and  lime- 
stone (Plate  XIII,  Fig.  2).  In  the  United  States  it  is  especially 
common  in  the  Cambro-Ordovician  limestones  of  magnesian  compo- 
sition along  the  Appalachian  region,  and  is  also  found  in  other  lime- 
stones. The  disadvantages  of  chert  in  building  stone  are  referred 
to  in  Chapter  XI.  The  chert  used  in  ball  and  tube  mills  is  obtained 
from  the  chalk  formations  of  Germany,  England,  etc. 

Jasper,  a  ferruginous  opaline  silica,  occurring  as  large  masses  in  the  iron  ore  for- 
mations of  the  Lake  Superior  region  and  known  as  jaspilite,  is  also  a  variety  of  flint, 
and  the  novaculite,  occurring  in  extensive  beds  in  Arkansas,  and  used  in  the  manu- 
facture of  whetstones  and  hones,  is  still  another  one.  Its  origin  is  still  a  mooted 
question. 

Geyserite.  —  This,  known  also  as  siliceous  sinter,  is  a  product  of  amorphous 
silica  deposited  from  solutions  of  evaporating  hot  waters  in  volcanic  regions,  and 
by  silica  secreting  algae.  The  rock  may  be  loose  or  unconsolidated  and  porous, 
or  dense  and  compact,  and  when  free  from  impurities,  of  white  color,  though  some- 
times stained  shades  of  yellow  and  red.  It  is  most  extensively  developed  in  the 
United  States  in  the  hot-spring  and  geyser  region  of  the  Yellowstone  National 
Park.  At  Steamboat  Springs,  Nevada  (Plate  XIII,  Fig.  1),  there  are  extensive 
deposits  of  siliceous  sinter  which  contain  traces  of  antimony  and  mercury. 

Diatomaceous  earth.  —  Diatomaceous  earth,  known  also  (but  incorrectly)  as 
infusorial  earth  or  tripolite,  is  a  soft,  pulverulent,  siliceous  clay-like  material,  very 
fine  and  porous  in  texture,  somewhat  resembling  chalk,  bog-lime  or  kaolin  in  its 
physical  properties,  and  of  white,  yellow,  or  gray  color.  It  can  be  readily  distin- 
guished from  chalk  and  bog-lime  by  not  effervescing  in  acid  and  from  kaolin 
by  its  distinct  gritty  feel  and  lighter  weight.  It  is  formed  from  the  shells  or 
tests  of  certain  aquatic  microscopic  forms  of  plant  life  known  as  diatoms,  which 
have  the  power  of  secreting  silica  in  the  same  manner  as  mollusks  secrete  lime 
carbonate. 

Diatomaceous  earth  occurs  in  Virginia  and  Maryland  in  beds  of  varying  thick- 
ness up  to  30  feet  and  more,  and  in  California  in  deposits  several  thousand  feet 
thick.  It  sometimes  accumulates  in  the  bottom  of  ponds  and  is  occasionally  mis- 
taken for  bog-lime. 

Diatomaceous  earth  is  used  to  a  small  extent  as  a  polishing  powder  and  as  a  pack- 


116  ENGINEERING  GEOLOGY 

ing  for  insulating  heated  pipes.  When  mixed  with  a  small  amount  of  clay  it  can 
also  be  made  into  hollow  blocks  for  partitions,  for  which  purpose  it  serves  as  an 
insulator  against  heat  and  sound. 

Ferruginous  Rocks  (Iron  Ores) 

The  iron  ores,  including  the  oxides  (hematite,  limonite,  and  mag- 
netite) and  the  carbonate  (siderite  or  spathic  iron  ore,  clay-ironstone, 
and  black  band  ore),  are  discussed  under  Ore-Deposits  in  Chapter 
XVII,  and  are  described  as  minerals  in  Chapter  I.  They  form  the 
source  of  iron  in  the  trades,  and  are  only  briefly  referred  to  here  under 
rocks. 

The  iron  ores  represent  a  variety  of  occurrence,  and  different  modes 
of  origin  must  be  attributed  to  them.  In  general  they  may  be  either 
of  chemical  or  of  organic  origin,  or  partly  of  both.  Variations  in  prop- 
erties, mode  of  occurrence,  etc.,  according  to  variety,  are  shown. 
Further  description  of  these  may  be  had  in  Chapters  I  and  XVII. 

Carbonate  Rocks 

The  group  of  rocks  considered  under  this  head  is  composed  essen- 
tially of  carbonate  of  lime,  or  of  this  substance  with  carbonate  of 
magnesia.  They  vary  greatly  as  to  purity,  color,  and  texture;  are 
readily  soluble  in  cold  or  hot  hydrochloric  acid;  and  are  easily  scratched 
with  the  knife,  their  hardness  being  under  4.  In  mode  of  formation 
they  are  partly  organically  and  partly  chemically  derived  rocks. 
Fragmental  calcareous  deposits  result  from  the  mechanical  breaking 
down  of  original  masses  and  redeposition  of  the  debris,  such  as  coral 

sands,  etc. 

Limestone 

This  is  the  most  common,  important,  and  widely  distributed  of  the 
carbonate  rocks.  It  is  composed  of  calcium  carbonate  of  varying 
degrees  of  purity  (see  table  of  analyses  in  Chapter  XII),  the  more 
common  impurities  being  magnesia,  silica,  clay,  iron,  and  bituminous 
or  organic  matter.  These  may  be  present  in  amounts  sufficient  to 
give  character  to  the  rock,  when  it  may  be  designated  magnesian 
or  dolomitic,  siliceous,  argillaceous,  ferruginous,  or  bituminous  lime- 
stone. Limestone  usually  shows  a  wide  range  of  color  due  to  the 
character  and  amount  of  impurities  present.  When  pure  the  rock 
is  white,  but  the  various  shades  of  gray  to  black  are  the  most  common 
colors,  while  many  others  are  known.  Those  of  black  or  dark  gray 
color  sometimes  fade  slightly  on  prolonged  exposure  to  the  atmos- 
phere. 


PLATE  XIII,  FIG.  1.  —  Deposit  of  siliceous  sinter  (white  material),  Steamboat 
Springs,  Nev.  The  small  cone,  which  is  filled  with  boiling  water,  has  formed  around 
one  of  the  hot  spring  vents.  (H.  Ries,  photo.) 


FIG.  2.  —  Cherty  limestone,  6  miles  west  of  Lexington,  Va.     The  chert  nodules 
stand  out  in  relief  on  the  weathered  surface.     (After  Bassler,  Bull.  II- A,  Va.  Geol. 

Surv.)  (117) 


118  ENGINEERING  GEOLOGY 

Chemical  composition.  —  The  limestones  show  equally  great 
variation  in  composition.  Magnesium  carbonate  may  be  present  from 
traces  up  to  the  percentage  amount  required  to  form  dolomite;  and 
silica  may  range  from  a  trace  up  to  the  limit  where  the  rock  becomes 
a  calcareous  sandstone.  Similar  gradations  of  limestones  into  cal- 
careous shales  occur,  according  to  the  amount  of  clayey  material 
present. 

Where  a  limestone  is  to  be  used  for  lime  or  cement  manufacture, 
or  as  a  flux  in  the  smelting  of  ores,  its  chemical  composition  needs 
to  be  considered.  Variation  in  composition  is  sometimes  found 
from  layer  to  layer,  and  an  appreciable  variation  is  not  always  visi- 
ble to  the  naked  eye.  Careful  sampling  of  a  limestone  quarry  for 
chemical  analysis  is  therefore  necessary.  A  good  case  of  this  is  to 
be  seen  in  the  Trenton  limestone  of  the  Lehigh  cement  district  in 
eastern  Pennsylvania,  where  analyses  of  twenty-nine  beds  aggre- 
gating 75  feet  in  thickness  showed  percentages  of  calcium  carbonate 
ranging  from  96.60  per  cent  to  51.30  per  cent,  and  of  magnesium 
carbonate  from  22.09  per  cent  to  1.51  per  cent.1 

Physical  properties.  —  The  compact  calcitic  varieties  vary  in  spe- 
cific gravity  from  2.5  to  2.8,  effervesce  freely  in  cold  dilute  acid,  and 
can  be  readily  scratched  with  a  knife. 

Variation  of  limestones  in  texture,  strength,  and  durability  is  as 
great  as  in  composition.  They  may  vary  from  very  fine-grained  and 
compact  rocks  to  those  composed  of  coarse  fragments  of  shells  and 
coral.  They  are  found  in  beds  of  all  thicknesses  up  to  100  feet  and 
more.  Most  of  the  limestones  used  for  building  purposes  belong 
either  to  the  Cambrian,  Silurian,  Devonian,  or  Carboniferous  horizons. 
Limestones  weather  chiefly  through  solution,  the  soluble  calcium 
carbonate  being  removed,  and  the  insoluble  material  (impurities) 
left  in  place  to  form  residual  soils  (see  Chapter  IV).  The  properties, 
occurrences,  and  uses  of  limestone  as  a  building  stone  are  described 
in  Chapter  XI. 

Solubility.2  —  The  solubility  of  limestone  is  not  alone  a  matter 
to  be  considered  in  connection  with  its  resistance  to  weather,  but  also 
in  engineering  operations  where  limestone  and  water  are  in  constant 
contact. 

The  question  of  imperviousness  of  a  limestone  may  be  closely 
related  to  its  solubility,  as  has  been  demonstrated  on  several  occa- 

1  Peck,  Econ.  Geol.,  Ill,  p.  43,  1908. 

2  For  other  effects  of  water  dissolving  limestone  see  Weathering,  Chap.  IV,  Under- 
ground Waters,  Chap.  VI,  and  Iron  Ores,  Chap.  XVII. 


ROCKS,   THEIR  GENERAL  CHARACTERS,  ETC.  119 

sions  in  aqueduct  construction.  Where  an  aqueduct  has  to  cross 
under  a  valley  by  a  pressure  tunnel,  more  or  less  loss  may  take  place 
through  the  crevices  in  the  rock,  but  if  the  rock  is  a  soluble  one  like 
limestone,  any  crevices  in  it  may  become  enlarged  by  solution  with 
an  increasing  leakage.  This  was  noticed,  for  example,  in  the  case  of 
a  3-mile  section  of  the  Thirlmere  (England)  aqueduct,  where  a  local 
limestone  was  used  for  concrete  aggregate.  A  leakage  amounting 
to  1,250,000  imperial  gallons  per  day  developed  in  a  year,  due  to 
the  limestone  fragments  becoming  corroded  by  the  water.  Another 
instance  was  that  of  the  limestone  blocks  used  in  building  the  old 
Delaware  and  Hudson  canal,  which  showed  the  effect  of  contact  with 
water.  Here  the  blocks  that  had  been  in  contact  with  the  water  for 
approximately  35  to  40  years  had  been  etched  until  the  fossils  and 
other  cherty  constituents  stood  out  from  one  eighth  to  one  half  inch 
beyond  the  general  surface  of  the  stone,  and  in  some  cases  pits  are  an 
inch  deep.1 

Varieties  of  limestone.  —  Many  varieties  of  limestone  are  recog- 
nized, based  chiefly  upon  differences  of  composition,  texture,  etc. 
Most  of  these  are  used  for  structural  purposes,  and  this  is  to  be  as- 
sumed unless  otherwise  mentioned  below. 

Dolomite.  —  The  name  applied  by  many  to  those  limestones  which 
approximate  the  mineral  dolomite  in  composition.  Unfortunately 
the  usage  is  not  uniform  and  any  magnesian-rich  limestone  is  re- 
ferred to  under  the  above  name.  Between  a  straight  calcic  limestone 
and  a  pure  dolomite,  there  may  occur  all  gradations.  A  dolomite 
is  similar  in  color,  texture,  and  other  physical  characters  to  lime- 
stone, except  that  it  is  slightly  harder,  somewhat  more  resistant, 
because  it  is  less  soluble,  and  does  not  effervesce  except  feebly  in 
cold  acid.  It  is  not  always  an  original  rock,  but  has  sometimes  been 
derived  from  straight  calcic  limestones  by  the  substitution  of  mag- 
nesium carbonate  for  a  part  of  the  calcium  carbonate  —  a  process 
known  as  dolomitization.  It  is  also  used  for  flux  and  lime  making. 

Bog  lime.  —  A  white,  powdery,  calcareous  deposit,  precipitated  through  plant 
action  on  the  bottom  of  many  ponds,  and  used  in  Portland  cement  manufacture. 
It  is  often  erroneously  called  marl,  a  term  which  properly  belongs  to  a  calcareous 
clay.  Shell  marl  is  an  aggregate  of  shells  of  various  organisms  usually  admixed 
with  some  clay  or  sand,  and  formed  either  in  fresh  or  salt  water. 

Chalk.  —  A  soft,  porous,  fine-grained  variety  of  limestone  composed  chiefly 
of  the  minute  shells  of  foraminifera;  and  when  pure  is  white,  though  a  variety  of 
colors  may  be  shown  owing  to  the  presence  of  impurities.  It  forms  extensive  de- 
posits in  France,  Germany,  and  England,  but  is  less  abundant  hi  the  United  States. 

1  Berkey,  N.  Y.  State  Museum,  Bull.  146,  p.  138,  1911. 


PLATE  XIV,  FIG.  1.  —  Weyers  Cave,  Va.,  showing  stalactites  of  lime  carbonate 
suspended  from  the  roof. 


FIG.  2.  —  Same  cave,  showing  coalescence  of  stalactites  and  stalagmites,  to  form 
(120)  column  from  roof  to  floor. 


ROCKS,   THEIR  GENERAL  CHARACTERS,  ETC.  121 

Chalk  is  rarely  used  for  structural  purposes,  but  in  some  regions  where  it  occurs 
has  been  used  as  an  ingredient  of  Portland  cement.  Coquina.  —  This  term  is  ap- 
plied to  a  loosely  cemented  shell  aggregate,  like  that  found  near  St.  Augustine, 
Fla.  The  stone  does  not  have  a  high  strength  nor  is  it  of  good  durability  if  exposed 
to  severe  weather  conditions.  It  was  used  by  the  Spaniards  for  constructional 
work,  and  hi  the  mild  Florida  climate  has  stood  well.  Coral  rock.  —  A  calcareous 
deposit  consisting  of  coral  reefs,  coral  fragments  and  shells,  the  entire  mass  being 
cemented  by  lime  carbonate.  Hydraulic  limestone.  —  A  clayey  limestone,  used  in 
cement  making,  but  usually  of  no  value  as  a  building  stone.  Lithographic  limestone. 
—  This  is  a  very  fine-grained,  homogeneous  limestone,  which  because  of  its  peculiar 
physical  properties  is  of  special  value  for  lithographic  but  not  structural  work.  Oolite 
or  oolitic  limestone.  —  A  variety  of  the  rock  made  up  of  small  spherical  or  rounded 
grams  of  calcium  carbonate,  resembling  fish  roe  in  appearance,  hence  the  name. 
When  of  coarser  texture,  the  term  pisolitic  (Plate  XIV,  Fig.  1)  is  employed.  Trav- 
ertine, calcareous  tufa  or  calc  sinter.  —  A  name  applied  to  the  less  compact,  cellular 
or  porous  forms  of  limestone  deposited  by  springs  or  streams.  In  this  country  no 
deposits  of  sufficient  size  for  building  purposes  are  known,  but  the  stone  is  quar- 
ried in  Italy.  Small  deposits  are  common  in  many  parts  of  the  United  States, 
and  some  interesting  ones  are  found  around  the  Mammoth  Hot  Springs  of  the  Yel- 
lowstone National  Park.  Stalactites  and  stalagmites.  —  Deposits  of  compact  crys- 
talline limestone,  formed  respectively  on  the  roof  and  floors  of  caves,  are  forms 
of  travertine  (Plate  XIV,  Figs.  1  and  2).  Deposits  formed  on  the  floor  of  sufficiently 
massive  character  and  extent  to  be  cut  are  called  cave  onyx,  although  most  of  the 
onyx  marble  of  commerce  is  a  spring  formation.  Limestones  may  sometimes  exhibit 
more  or  less  conspicuously  their  fossiliferous  character,  when  they  may  be  named 
for  the  chief  organic  remains  in  them,  such  as  crinoidal,  shell  limestone,  etc. 

Phosphate  Rocks1 

These  rocks,  composed  chiefly  of  calcium  phosphate  and  known  by  the  general 
term  phosphorite,  are  of  great  value  as  a  source  of  phosphoric  acid  in  the  manufac- 
ture of  commercial  fertilizers.  While  not  uncommon,  extensive  beds  are  very 
much  more  limited  in  distribution  than  those  of  the  common  types  of  sedimentary 
rocks.  They  are  of  organic  origin,  formed  from  animal  remains,  and  in  most  cases 
have  probably  suffered  further  concentration  of  the  phosphatic  material  by  solution 
and  removal  of  calcium  carbonate  from  the  phosphatic  limestone.  They  may  be 
either  compact,  earthy,  or  concretionary,  with  pebble  and  nodular  forms  common. 
Some  shade  of  gray  is  the  commonest  color.  Large  deposits  are  found  in  the  United 
States  in  Florida,  South  Carolina,  and  Tennessee,  and  in  Idaho,  Wyoming,  and  Utah. 

Carbonaceous  Rocks 

Under  this  group  of  rocks  are  included  all  accumulations  of  vege- 
table matter  that  have  undergone  partial  or  complete  decay  under 
water.  Decay  of  vegetable  matter  out  of  contact  with  air  results 
in  a  greater  concentration  of  carbon  and  removal  of  the  gaseous 
constituents,  as  the  process  advances.  The  conditions  of  vegetable 

1  For  fuller  discussion  and  bibliography,  see  Ries,  "  Economic  Geology,"  3rd  ed., 
1910. 


122  ENGINEERING  GEOLOGY 

accumulation  and  its  transformation  into  coal  are  explained  in  Chapter 
XIV.  The  different  varieties  of  carbonaceous  rocks  grade  into  each 
other,  and  there  is  a  well-graded  or  transitional  series  from  the  un- 
altered plant  remains  at  one  end  to  graphite  at  the  other.  The 
principal  varieties  usually  recognized  are  peat,  lignite  or  brown  coal 
bituminous  or  soft  coal,  and  anthracite  or  hard  coal.  The  formation, 
properties,  mode  of  occurrence,  distribution,  and  uses  of  these  varie- 
ties are  fully  described  in  Chapter  XIV,  to  which  the  reader  is  referred. 
Coals  occur  in  beds  of  varying  thicknesses  interstratified  with  clays, 
shales,  sandstones,  and  limestones,  and  are  subject  to  the  same  struc- 
tures as  the  inclosing  rocks.  In  the  United  States  the  coal-bearing 
formations  range  in  age  from  Carboniferous  to  Tertiary.  Between 
the  coals  and  the  sandstones  and  shales,  many  intermediate  types  are 
recognized. 

METAMORPHIC  ROCKS 

Introduction.  —  The  term  metamorphic,  when  broadly  applied,  in- 
cludes any  kind  of  change  or  alteration  that  any  rock  has  undergone. 
It  involves  changes  that  are  both  physical  and  chemical,  and  the 
rock  so  altered  may  have  been  originally  of  sedimentary  or  igneous 
origin. 

The  alteration  includes  change  in  mineral  composition  or  texture, 
or  both,  and  is  often  so  complete  as  to  obscure  the  primary  characters 
of  the  original  rock,  rendering  it  extremely  difficult,  if  not  impossible, 
in  many  cases,  to  say  with  certainty  whether  the  metamorphic  product 
was  derived  from  an  original  igneous  or  sedimentary  rock.  All 
gradations  exist  between  sedimentary  rocks  and  their  metamorphic 
equivalents  on  the  one  hand,  and  between  igneous  rocks  and  their 
metamorphic  products  on  the  other. 

As  discussed  in  Chapter  III,  the  alteration  of  the  rocks  may  be  a 
deep-seated  change  or  a  superficial  one,  the  resulting  products  in  the 
two  cases  being  widely  different  in  general  characters.  The  products 
(soils,  etc.)  derived  by  the  action  of  atmospheric  or  weathering  agen- 
cies (superficial)  are  not  included  in  this  chapter  but  are  discussed 
under  Weathering  in  Chapter  IV,  so  that  what  follows  here  applies  to 
rocks  metamorphosed  by  deep-seated  processes. 

Agents  of  metamorphism.  —  The  principal  agents  involved  in 
the  alteration  of  igneous  and  sedimentary  rocks  and  the  production 
of  their  metamorphic  equivalents  are:  (1)  Earth  movements  and 
pressure;  (2)  liquids  and  gases,  chiefly  water;  and  (3)  heat.  The 
effect  of  the  first  is  mechanical;  of  the  second  and  third,  chemical, 


PLATE  XV,  FIG.  1.  —  Pisolitic  structure. 


FIG.  2.  —  Fossiliferous  limestone,  showing  longitudinal  and  transverse  sections  of 

crinoid  stems. 


(123) 


124  ENGINEERING  GEOLOGY 

usually  indicated  by  the  production  of  new  minerals.  These  are 
discussed  under  Metamorphism  in  Chapter  III. 

Chemical  composition  of  metamorphic  rocks.  —  The  chemical 
composition  of  metamorphic  rocks  varies  greatly,  because  of  the  wide 
variations  in  composition  of  the  numerous  types  among  igneous  and 
sedimentary  rocks  yielding,  when  altered,  metamorphic  ones.  The 
chemical  composition  of  many  rocks  is  not  greatly  changed  during 
the  process  of  metamorphism;  hence,  metamorphosed  igneous  and 
sedimentary  rocks  frequently  show  the  composition  characteristic 
of  their  class.  Thus,  as  remarked  by,  Van  Hise,  "  the  metamorphosed 
sedimentary  rocks,  with  minor  modifications,  have  the  chemical 
composition  of  mud,  grits,  sandstones,  etc.;  the  metamorphosed 
igneous  rocks  have  the  composition  of  granites,  diorites,  etc."  Chem- 
ical analysis  therefore  frequently  forms  an  important  criterion  for 
discriminating  between  metamorphosed  sedimentary  and  igneous 
rocks. 

Mineral  composition  of  metamorphic  rocks.  —  Mineral  compo- 
sition is  dependent  on  chemical  composition,  and  hence,  in  meta- 
morphic rocks,  is  subject  to  wide  variation.  It  has  been  shown  that 
certain  minerals,  such  as  the  feldspathoids  (nepheline  and  sodalite), 
are  characteristic  of  igneous  rocks.  Likewise,  there  are  certain  min- 
erals which  are  considered  to  be  characteristic  of  metamorphic  rocks, 
such  as  staurolite,  cyanite,  sillimanite,  zoisite,  chlorite,  talc,  etc. 
Again  there  are  minerals  which  are  common  to  both  groups  of  rocks, 
such  as  quartz,  feldspar,  mica,  amphibole,  pyroxene,  etc.  The  mica, 
amphibole  and  pyroxene  groups  contain  many  species  of  minerals 
(Chapter  I)  some  of  which  are  found  in  igneous  rocks,  others  in  meta- 
morphic ones,  while  still  others  may  occur  in  both  groups. 

Staurolite,  andalusite,  sillimanite,  and  cyanite  are  usually  consid- 
ered to  be  very  characteristic  of  metamorphic  rocks  that  are  of  sedi- 
mentary origin. 

Texture  and  structure  of  metamorphic  rocks.  —  Texturally,  the 
metamorphic  rocks  resemble  most  the  igneous  ones,  in  being  highly 
crystalline,  and  hence  they  are  sometimes  referred  to  as  crystalline 
schists. 

Metamorphic  rocks  frequently  exhibit  conspicuously  developed 
minerals  in  size,  distributed  through  a  groundmass  of  smaller  min- 
erals, closely  resembling  the  porphyritic  texture  of  igneous  rocks. 
Since  it  can  be  often  shown  that  these  strongly  developed  minerals 
are  the  result  of  metamorphic  processes  subsequent  to  the  formation 
of  the  original  rock,  they  are  conveniently  referred  to  as  pseudo- 


ROCKS,   THEIR  GENERAL  CHARACTERS,  ETC.  125 

phenocrysts,  and  the  metamorphic  rocks  containing  them  as  pseudo- 
porphyritic  in  texture.  This  texture  should  not  be  confused  with 
the  augen  (eye)  texture,  which  is  also  porphyritic  in  appearance  but 
represents  remnants  of  the  original  texture,  such  as  phenocrysts  of 
some  igneous  rocks,  or  pebbles  of  conglomerates,  etc. 

Metamorphism  as  explained  in  Chapter  III  frequently  results  in 
the  production  of  a  secondary  structure  in  rocks  known  as  foliation 
(rock  cleavage)  which  resembles  more  or  less  closely  bedding  or  strati- 
fication planes  in  sedimentary  rocks.  Hence  metamorphic  rocks 
resemble  sedimentary  ones  in  structure,  and  at  times  some  igneous 
rocks,  for  a  similar  primary  structure  is  often  shown  in  lavas  and  to 
some  extent  in  plutonic  types.  Bastin  has  suggested  the  term  "foli- 
ates "  as  a  convenient,  comprehensive  one  to  include  all  rocks  showing 
foliated  structure  other  than  bedding  planes. 

The  foliated  structure  in  metamorphic  rocks,  due  to  the  arrange- 
ment of  the  constituent  minerals,  is  entirely  secondary,  and  is  not 
connected  with  bedding  in  sediments,  although  the  two  may  coincide 
at  times.  The  terms  bedding  and  stratification,  therefore,  should 
not  be  applied  to  foliation  in  metamorphic  rocks. 

Varieties  of  structure. — We  may  recognize  the  following  three  prin- 
cipal structures  in  metamorphic  rocks :  (1)  Banded  (Plate  XVII,  Fig.  1) , 
in  which  lithologically  unlike  layers  of  minerals  arranged  in  more  or 
less  parallel  bands  are  shown,  as  in  gneisses;  (2)  Schistose,  represent- 
ing the  development  of  a  rather  evenly  foliated  structure,  as  a  result 
of  which  the  rock  often  splits  easily,  but  not  always  very  regularly, 
as  in  schists;  and  (3)  Slaty  (Fig.  68),  in  which  the  mineral  grains  are 
very  small,  and  the  rock  dense,  but  having  the  property  of  splitting 
(slaty  cleavage)  into  thin,  even  slabs.  Gradations  between  any  two 
may  occur.  Thus  a  schistose  rock  may  pass  into  a  banded  one  by 
the  like  mineral  grains  becoming  more  segregated  into  definite  bands, 
or  it  may  on  the  other  hand  grade  into  a  slaty  structure  by  the  grains 
becoming  finer  and  more  uniform. 

Criteria  for  the  discrimination  of  metamorphosed  igneous  and 
sedimentary  rocks.  —  In  the  study  of  a  metamorphic  rock,  it  is 
desirable  but  not  always  easy,  to  determine  whether  the  rock  was 
derived  from  an  original  igneous  or  sedimentary  one.  The  evidence 
upon  which  the  geologist  depends  is  gained  partly  from  careful  field 
study  of  the  mode  of  occurrence,  general  characteristics  and  relation- 
ships of  the  rocks,  and  partly  from  both  microscopical  and  chemical 
laboratory  study,  of  rock  specimens  collected  in  the  field. 

In  those  cases  where  metamorphism  has  not  been  extreme,  the 


126  ENGINEERING   GEOLOGY 

original  textures  and  structure  of  the  igneous  and  sedimentary  rocks 
have  not  been  completely  obscured,  and  the  discrimination  of  the 
derived  rock  is  not  so  difficult.  In  many  cases,  however,  the  meta- 
morphism  has  been  so  complete  as  to  entirely  obliterate  all  trace  of  the 
general  character  of  the  original  rock,  and  discrimination  becomes  ex- 
tremely difficult.  Various  criteria  have  been  proposed  in  such  cases, 
but  probably  chemical  analysis  forms  one  of  the  most  important. 
From  the  chemical  standpoint  Bastin  has  summarized  the  evidence 
as  follows: 

I.  Dominance  of  MgO  over  CaO  is  strongly  indicative  of  sedimen- 
tary origin. 

II.  Dominance  of  E^O  over  Na20  is  of  lesser  critical  value,  but  is 
nevertheless  suggestive  of  sedimentary  origin. 

III.  The  presence  of  any  considerable  excess  of  A12O3  in  the  analy- 
sis over  and  above  the  1  :  1  ratio  necessary  to  satisfy  the  lime  and 
alkalies  is  also  suggestive  of  sedimentary  origin. 

IV.  High  silica  content  may  be  indicative  of  sedimentary  origin 
when  supported  by  other  criteria.     This  criterion  must,   however, 
be  used  with  caution,  since  silicification  probably  takes  place  in  the 
dynamic  metamorphism  of  certain  igneous  rocks. 

V.  When  three  or  all  of  the  above  relationships  hold  good,  the  evi- 
dence of  sedimentary  origin  may  be  regarded  as  practically  conclusive. 

Mineral  composition  often  becomes  an  important  criterion  in  dis- 
tinguishing metamorphosed  sedimentary  from  metamorphosed  igneous 
rocks,  as  indicated  under  these  heads,  above. 

Classification  of  Metamorphic  Rocks 

Since  in  most  cases  it  is  not  possible  on  megascopic  grounds  to 
group  metamorphic  rocks  according  to  origin,  whether  derived  from 
original  sedimentary  or  original  igneous  masses,  some  other  basis  of 
classification  that  is  practical  must  be  sought.  Probably  the  classi- 
fication of  metamorphic  rocks  which  best  meets  the  needs  of  the  en- 
gineer, and  the  one  followed  in  this  book,  is  based  chiefly  on  mineral 
composition,  texture,  and  structure.  It  follows. 

CLASSIFICATION  OF  METAMORPHIC  ROCKS 

I.  Gneisses  of  various  kinds. 

II.  Crystalline  schists  of  various  kinds. 

III.  Quartzites. 

IV.  Slates  and  phyllites. 

V.   Crystalline  limestones  and  dolomites  (marbles). 
VI.   Ophicalcites,  serpentines  and  soapstones. 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC. 


127 


Other  kinds  of  metamorphic  rocks  are  known,  but  they  are  of  little 
or  no  importance  to  the  engineer,  and  hence  are  not  considered.  The 
six  groups  given  above  are  treated  below  in  the  order  named. 

It  is  both  possible  and  helpful  to  illustrate  in  a  general  way  the 
metamorphic  equivalent  of  each  of  the  principal  types  of  sedimentary 
and  igneous  rocks,  as  shown  in  the  following  tables.1 

TABLE  OF  SEDIMENTARY  ROCKS  AND  THEIR  METAMORPHOSED  EQUIVALENTS 


Loose  sediments. 

Compacted  sedimen- 
tary rocks. 

Metamorphic  rocks. 

Gravel 
Sand 
Silt  and  clay 
Lime  deposits 

Conglomerate 
Sandstone 
Shale 
Limestone 

Gneiss  and  schist 
Quartzite 
Slate 
Marble 

TABLE  OF  IGNEOUS  ROCKS  AND  THEIR  METAMORPHIC  DERIVATIVES 


Igneous  rocks. 

Metamorphic  rocks. 

Coarse-grained      feldspathic      rocks,  1 
such  as  granite,  syenite,  etc.             } 

Fine-grained  feldspathic  rocks,  such  I 
as  felsite,  tuffs,  etc.                             J 

Ferruginous  rocks,  such  as  dolerites  ) 
and  basalts.                                           ) 

Gneiss 
Schists,  etc. 
Schists,  etc. 

Gneiss 

Definition.  —  Gneiss  2  is  an  old  word  that  originated  among  the 
early  Saxon  miners,  and  has  had  rather  loose  geological  usage.  It 
was  applied  more  particularly  to  laminated  rocks  having  the  mineral 
composition  of  granite,  but  was  later  extended  by  many  to  include 
other  laminated  types  of  plutonic  igneous  rocks.  It  has  thus  had  a 
dual  meaning  by  many,  comprising  both  structural  and  mineral- 
ogical  factors.  In  Germany  the  word  gneiss  has  been  employed  to 
apply  to  those  laminated  rocks  containing  quartz  and  feldspar  with 
one  or  more  minerals. 

The  term  gneiss,  following  Van  Hise,  is  used  in  this  book  strictly 
in  the  structural  sense.  It  may  be  defined  as  any  banded  meta- 
morphic rock,  whether  originally  of  igneous  or  of  aqueous  origin, 

1  Pirsson,  Rocks  and  Rock  Minerals,  p.  348. 

2  Quarrymen  usually  but  erroneously  apply  the  name  granite  to  gneisses. 


128 


ENGINEERING  GEOLOGY 


the  bands  of  which  are  mineralogically  unlike  and  consist  of  inter- 
locking mineral  particles  which,  for  the  most  part,  are  large  enough 
to  be  visible  to  the  naked  eye.  The  bands  may  vary  in  regularity 
(Plate  XVI,  Figs.  1  and  2),  and  in  thickness  may  range  from  a  frac- 
tion of  a  centimeter  to  many  centimeters.  Likewise  a  similar  range 
in  thickness  of  the  different  bands  of  the  same  gneiss  may  be  noted. 

Mineral  composition.  —  The  most  important  gneisses  correspond 
in  mineral  composition  to  plutonic  igneous  rocks,  but  they  are  not 
necessarily,  as  defined  above,  of  igneous  origin,  since  many  gneisses 
are  known  to  be  metamorphosed  sediments.  Feldspar,  both  the 
alkalic  and  soda-lime  varieties;  quartz,  mica,  either  biotite  or  musco- 
vite,  or  both;  and  hornblende  are  the  commonly  occurring  minerals 
in  gneiss. 

Besides  these,  many  other  minerals  may  occur,  such  as  garnet,  epidote,  silliman- 
ite,  tourmaline,  chlorite,  etc.,  and  any  one  of  these  may  be  present  in  large  enough 
quantity  to  give  specific  or  varietal  name  to  the  rock. 

Chemical  composition.  —  The  chemical  composition  of  gneisses  is  necessarily  widely 
variable,  the  variation  being  of  the  same  order  as  that  of  the  original  rocks  (igneous 
and  sedimentary)  from  which  they  were  derived.  To  the  petrographer  the  chemical 
analysis  of  a  gneiss  is  often  of  great  value  in  affording  a  clue  as  to  the  kind  of  orig- 
inal rock  from  which  it  was  derived,  whether  igneous  or  sedimentary. 
|  The  range  in  chemical  composition  of  the  gneisses  is  illustrated  in  the  table  of 
analyses  below,  arranged  in  order  of  descending  silica: 

ANALYSES  OF  GNEISSES 


I. 

II. 

in. 

IV. 

V. 

VI. 

VII. 

VIII. 

SiO2 

77  53 

70  21 

69  29 

66.13 

61.04 

48.68 

46.63 

38.05 

A12O3 

13.60 

13.95 

14.07 

15.11 

16.97 

14.39 

19.47 

24.73 

Fe2O3  

0.23 

1.05 

2.59 

2.52 

4.00 

3.26 

5.65 

FeO  
MgO  

0.16 
Trace 

3.08 
1.26 

2.03 
1.32 

3.19 

2.42 

5.58 
3.62 

10.09 
6.32 

6.63 
5.37 

6.08 
11.58 

CaO... 

0.73 

3.10 

2.76 

1.87 

5.99 

9.23 

9.15 

1.25 

Na2O 

6.65 

3.27 

2.89 

2.71 

1.96 

2.31 

3.19 

2.54 

K2O  

1.20 

2.69 

2.87 

2.86 

0.55 

0.47 

1.55 

1.94 

H2O  

0.33 

0.67 

0.43 

1.79 

0.43 

2.49 

1.71 

7.53 

Rest  

0.19 

1.02 

0.84 

1.27 

3.73 

2.20 

3.12 

0.93 

100.62 

100.30 

99.09 

99.87 

98.87 

100.18 

100.08 

100.28 

I.  Losee  gneiss,  northeast  of  Berkshire  Valley,  N.  J.;  II.  Baltimore  gneiss,  near  Philadelphia,  Penn.; 
III.  Biotite  granite-gneiss,  near  Manchester,  Chesterfield  County,  Virginia;  IV.  Mica  (muscovite) 
gneiss,  near  Philadelphia,  Penn.;  V.  Quartz  norite-gneiss,  Odessa,  Minnesota;  VI.  Hornblende 
gneiss  near  Philadelphia,  Penn.;  VII.  Plagioclase  gneiss,  north  fork  of  Mokelumne  River,  Amador 
County,  California;  VIII.  Gabbro-diorite  gneiss,  below  Quinnesec  Falls,  Wisconsin. 

Varieties.  —  Varietal    distinctions    of    gneisses    may   be   based  on 
(1)  structural  differences,  such  as  banded  gneiss,  foliated  or  lenticular 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       129 

gneiss,  augen  gneiss,  etc.;  (2)  character  of  the  prevailing  accessory 
mineral,  as  in  granite,  such  as  biotite  gneiss,  muscovite  gneiss,  horn- 
blende gneiss,  etc.;  and  (3)  on  composition  and  origin,  such  as  gran- 
ite-gneiss, syenite-gneiss,  diorite-gneiss,  gabbro-gneiss,  etc. 

Granulite  is  the  name  applied  to  a  finely  banded  rock  composed  chiefly  of  quartz 
and  feldspar,  and  sometimes  the  accessory  minerals  garnet,  cyanite,  etc.  The 
name  was  originally  applied  to  rocks  in  Germany  where  they  were  first  studied, 
but  the  usage  since  has  not  been  uniform,  and  is  seldom  employed  at  present  in  the 
United  States. 

General  properties.  —  In  texture,  gneisses  are  compact  holocrys- 
talline  rocks,  and  may  range  from  even-granular  to  pseudo-porphyritic, 
in  which  the  principal  minerals  though  variable  in  size  (ranging  from 
fine  through  medium  to  coarse)  are  distinguishable  by  the  naked  eye. 
Porphyritic  texture  is  common  among  the  feldspathic  gneisses  in  which 
feldspar  is  the  porphyritically  developed  mineral. 

In  structure,  gneisses  are  banded  rocks,  in  which  the  lines  may  be 
straight  or  regular,  or  curved  and  contorted.  The  lines  may  be  con- 
tinuous or  short  and  lenticular,  and  the  individual  bands  may  be 
extremely  thin  or  thick.  In  color,  great  variation  is  shown,  depending 
chiefly  on  the  kinds  and  proportion  of  the  principal  minerals.  Hence, 
variation  may  range  from  nearly  white  through  shades  of  red,  gray, 
brown,  green,  to  nearly  black. 

Other  physical  properties,  such  as  hardness,  specific  gravity,  ab- 
sorption, etc.,  are  similar  to  their  equivalent  igneous  types,  and  are 
dependent  chiefly  on  mineral  composition,  size  and  shape  of  grain. 
(See  Chapter  on  Building  Stone.) 

Uses.  —  On  account  of  the  banded  structure,  gneiss  cannot  be 
worked  so  uniformly  as  granite,  hence  its  use  is  more  restricted.  On 
the  other  hand,  the  banded  structure  permits  of  the  rock  being  split 
into  more  or  less  parallel  flat  surfaces,  and  of  use  in  the  construction 
of  rough  walls  and  for  street  work.  When  used  for  constructional 
purposes  the  rock  should  be  placed  like  sedimentary  ones,  so  that 
the  foliation  lies  in  the  mortar  bed  and  not  on  edge,  in  order  to  avoid 
splitting  and  scaling.  (See  further  under  Chapter  on  Building  Stone.) 

Occurrence  and  distribution.  —  Gneiss  is  one  of  the  most  common 
and  widely  distributed  of  rocks.  It  is  especially  abundant  in  the 
older  geological  formations,  more  particularly  in  the  pre-Cambrian 
horizons,  but  may  occur  in  formations  as  late  as  Mesozoic.  It  forms 
extensive  areas  in  Canada,  the  Appalachians,  Cordilleran,  and  upper 
Great  Lakes  regions  in  the  United  States;  and  has  similar  wide  dis- 
tribution over  other  parts  of  the  world. 


PLATE  XVI,  FIG.  1.  —  Hornblende  gneiss,  showing  irregular  banding.  Dark 
patches,  hornblende;  light  areas,  mixed  quartz  and  feldspar.  (From  Ries,  Build- 
ing Stones  and  Clay  Products.) 


FIG.  2.  —  Biotite  gneiss,  showing  folding  of  the  bands. 


(130) 


PLATE  XVII,    FIG.    1.  —  Magnetite   gneiss,    showing   distinct   banding.     The 
bands  are  also  broken  by  small  faults,  Temagami,  Ont.     (H.  Hies,  photo.) 


FIG.  2.  —  Gneiss  quarry,  near  Lynchburg,  Va.     Shows  regular  banding  of  the 
gneiss.     (T.  L.  Watson,  photo.) 

(161) 


132  ENGINEERING   GEOLOGY 

Crystalline  Schists 

Definition.  —  The  term  schist,  like  gneiss,  has  loose  geological 
usage  and  by  many  has  been  employed  in  a  dual  sense,  —  structure 
and  mineral  composition.  Following  Van  Hise  in  the  definition  of 
gneiss,  schist,  as  used  in  this  book,  is  defined  to  include  those  foliated 
metamorphic  rocks,  whose  individual  folia  are  mineralogically  alike, 
and  whose  principal  minerals  are  so  large  as  to  be  visible  to  the 
naked  eye.  This  definition  is  uniform  with  that  of  gneiss  and 
slate,  into  either  of  which  a  schist  may  grade.  Because  of  this 
fact,  it  frequently  happens  that  no  hard  and  fast  line  can  be  drawn 
between  schists  and  gneisses,  and  by  becoming  finer  in  grain  and 
texture,  the  schists  may  grade  into  slates.  By  decrease  in  mica 
and  increase  in  quartz,  mica  schists  may  pass  into  quartz  schists 
and  quartzites. 

Mineral  composition.  —  Mineralogically  the  crystalline  schists  in- 
clude a  large  and  extremely  variable  group  of  rocks.  They  differ  from 
the  gneisses  in  mineral  composition  chiefly  in  the  lack  of  feldspar  as 
an  essential  mineral,  although  they  may  be  and  are  sometimes  feldspar- 
bearing.  Quartz  is  the  most  frequent  and  abundantly  occurring 
essential  constituent,  with,  in  the  more  common  varieties  of  the 
rocks,  one  or  more  minerals  of  the  mica,  chlorite,  talc,  amphibole,  or 
pyroxene  group. 

The  schists  are  especially  rich  in  accessory  minerals,  among  the  common  ones 
being  feldspar,  garnet,  cyanite,  andalusite,  sillimanite,  staurolite,  ottrelite,  epidote, 
tourmaline,  magnetite,  pyrite,  etc.  Any  one  of  these  may  be  present  to  the  extent 
of  giving  varietal  name  to  the  rock.  Many  other  minerals  occur  in  schists  and 
at  times  are  locally  important,  but  they  are  of  less  general  importance  than  the  ones 
mentioned  above. 

Chemical  composition.  —  Considered  as  a  group,  the  crystalline  schists  vary 
indefinitely  in  chemical  composition,  and  even  for  the  same  variety,  such  as  the 
common  one,  mica  schist,  'wide  variations  are  shown.  Practically  all  gradations 
may  be  found  ranging  from  the  most  acid  (quartz  schists)  to  the  most  basic  (am- 
phibolite  schists). 

The  wide  range  in  composition  is  shown  in  the  table  below,  in  which  are  assembled 
analyses  of  some  of  the  different  varieties  of  schist,  arranged  in  order  of  descending 
silica. 

Varieties.  —  The  varietal  names  given  to  the  more  important 
kinds  of  schists  are  based  chiefly  upon  the  character  of  the  prevail- 
ing ferromagnesian  mineral  present.  Thus  we  have  mica  schists, 
chlorite  schists,  hornblende  and  actinolite  schists,  talc  schists,  etc.  Of 


ROCKS,   THEIR  GENERAL  CHARACTERS,   ETC. 
ANALYSES  OF  CRYSTALLINE  SCHISTS 


133 


I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

IX. 

SiO2                 ...   • 

91.65 
1.59 
3.57 
0.21 
0.17 
None 
0.07 
1.93 
0.60 
0.13 

90.91 
4.18 
0.22 
1.27 
0.37 
0.22 
0.77 
0.58 
0.80 
0.74 

75.54 

18.65 
0.35 
0.06 
None 
0.03 
None 
None 
4.77 
0.64 

70.40 
14.70 
0.65 
2.57 
1.47 
1.63 
3.17 
3.46 
1.10 
0.88 

64.77 
14.45 
1.84 
4.54 
2.34 
2.33 
1.37 
5.03 
1.99 
1.92 

64.28 
17.28 
1.10 
5.34 
2.57 
1.19 
0.91 
2.93 
2.92 
0.54 

57.24 
23.48 
3.19 
4.87 
0.93 
0.09 
1.18 
3.55 
4.98 
0.27 

34.92 
32.31 
10.21 
8.46 
1.13 
0.36 
2.12 
1.87 
5.29 
3.60 

12.35 
0.10 
58.68 
21.34 
4.08 
1.91 
Trace 
None 
0.19 
1.59 

A12O3          

Fe2O3 

FeO 

MgO               

CaO  
Na2O 

KoO 

H2O                  

Rest  

99.92 

100.06 

100.04 

100.03 

100.58 

100.04 

99.68 

100.27 

100.24 

I.  Quartz  schist,  near  Stevenson  station,  Maryland;  II.  Quartz-sericite  schist,  Mount  Ascutney,  Ver- 
mont; III.  Sillimanite  schist,  San  Diego  County,  California;  IV.  Feldspathic  mica  schist,  Mari- 
posa  County,  California;  V.  Mica  schist,  near  Gunflint  Lake,  Minnesota;  VI.  Andalu  siteschist, 
Mariposa  County,  California;  VII.  Sericite  schist,  Ladiesburg,  Maryland;  VIH.  Chloritoid  phyl- 
lite,  Liberty,  Maryland;  IX.  Actinolite-magnetite  schist,  Mesabi  Range,  Minnesota.  All  analyses 
are  quoted  from  "  The  Data  of  Geochemistry  "  by  Clarke,  Bull.  491,  U.  S.  Geol.  Survey,  1911. 

these,  the  mica  schists  are  the  most  common  and  widely  distributed. 
The  mica  may  be  biotite  or  muscovite,  or  both. 

Frequently  the  hydrous  mica,  sericite,  prevails,  giving  sericite  schist;  less  often 
the  soda  mica,  paragonite,  is  present  producing  a  more  restricted  type  known  as 
paragonite  schist.  The  mineral  ottrelite  occurs  in  the  rocks  of  some  localities,  which 
gives  rise  to  the  variety  ottrelite  schist. 

Among  the  principal  accessory  minerals  that  may  be  sufficiently  developed  at 
times  as  to  give  rise  to  modified  varietal  names  are  garnet,  staurolite,  sillimanite, 
andalusite,  cyanite,  magnetite,  tourmaline,  etc. 

Greenstone  schists,  sometimes  called  "green  schists,"  has  been  applied  to  schists 
of  green  color  rather  than  to  those  of  definite  mineral  composition,^and  both  horn- 
blende schists  and  chlorite  schists  have  been  included  under  it. 

General  properties.  —  All  schists  are  alike  structurally  in  having 
more  or  less  pronounced  schistosity  or  foliation  as  a  common  feature. 
Hence,  they  split  readily  in  the  direction  of  foliation,  sometimes 
with  smooth  and  even  surfaces,  but  they  break  with  more  or  less 
difficulty,  and  often  with  irregular  surfaces,  at  right  angle  directions 
to  the  schistosity. 

On  account  of  the  slippery  character  of  the  foliation  planes,  they 
will  sometimes  if  unsupported  cause  rock  slips  in  quarries,  railway 
cuts  and  underground  workings. 

In  many  schists,  especially  in  some  of  the  common  mica  varieties, 
quartz  is  distributed  through  the  rock  in  the  form  of  eyes  or  small 
lenses  about  which  the  mica  folia  are  wrapped,  so  that  when  parted 


134  ENGINEERING  GEOLOGY 

along  the  direction  of  foliation  an  uneven  or  lumpy  surface  is  shown. 
Because  of  their  foliated  structure  schists  are  not  desirable  rocks  for 
use  as  building  stone. 

Schists  resemble  each  other  texturally  in  being  holocrystalline 
rocks,  whose  principal  minerals  are  sufficiently  large  to  be  visible  mega- 
scopically,  and  are  graded  according  to  the  size  of  individual  mineral 
grains  into  fine-,  medium-,  and  coarse-grained  rocks. 

In  color,  schists  exhibit  a  very  wide  range,  dependent  chiefly  upon 
the  kind  and  proportions  of  their  principal  minerals.  Mica  schists 
usually  vary  from  very  light,  through  gray  and  brown,  to  very  dark, 
depending  on  the  proportion  of  light-  and  dark-colored  micas  present. 
Chlorite  schists  are  usually  some  shade  of  green;  common  hornblende 
schists  vary  from  green  to  black;  and  talc  schists  are  usually  light, 
white  to  pale  green,  yellowish,  or  gray;  sometimes  dark  gray. 

Other  physical  properties,  such  as  hardness,  specific  gravity,  etc., 
also  show  much  variation,  dependent  mainly  on  mineral  composition, 
and  the  proportions  of  the  principal  constituents. 

Uses.  —  The  structural  peculiarities  of  schists  described  above 
make  them  undesirable  for  use  as  building  stone.  When  sufficiently 
solid,  they  are  extensively  employed,  however,  for  purposes  of  rough 
construction,  such  as  foundations,  bridges,  flagging,  etc. 

Occurrence  and  distribution.  —  The  crystalline  schists  being  meta- 
morphic  rocks,  derived  either  from  original  sedimentary  or  igneous 
masses,  have  great  areal  distribution  and  are  the  common  types  in 
regionally  metamorphosed  areas.  Mica  schists  form  the  country 
rock  over  much  of  the  eastern  crystalline  belt  including  New  England 
and  extending  southwestward  to  middle  northern  Alabama.  They 
also  occur,  though  to  a  less  extent,  around  Lake  Superior  and  in  the 
West.  Hornblende  schists  are  very  common  rocks  in  metamorphic 
regions,  where  they  form  belts,  less  often  independent  large  areas, 
in  the  midst  of  other  metamorphic  rocks,  especially  gneisses  and 
mica  schists.  Many  of  their  occurrences  in  the  form  of  long  bands 
and  belts,  and  as  large  areas  about  igneous  masses,  suggest  derivation 
from  igneous  rocks;  although  they  are  known  to  have  been  formed 
in  places  from  impure  sedimentary  beds  by  metamorphism. 

Chlorite  schists  and  talc  schists  are  common  types  in  New  England, 
the  crystalline  region  of  the  Appalachians,  and  around  Lake  Superior. 
The  chlorite  schists  have  been  derived  chiefly  from  rocks  containing 
abundant  ferromagnesian  silicate  minerals,  while  the  talc  schists  have 
been  formed  from  the  metamorphism  of  rocks  rich  in  magnesian  sili- 
cates that  were  poor  or  lacking  in  iron. 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.  135 

Quartzite 

Definition.  —  Quartzites  in  general  are  the  metamorphosed  equiva- 
lents of  sandstones,  into  which  they  may  grade  with  frequently  no 
sharp  line  of  demarcation  noted  between  them.  They  are  hard  and 
compact  crystalline  rocks  which  break  with  a  splintery  or  conchoidal 
fracture. 

Quartzites  differ  from  sandstones  mainly  in  their  greater  hardness, 
denseness,  and  crystalline  character,  properties  which  result  from 
metamorphism.  A  practical  distinction  that  may  often  be  made 
between  the  two  rocks  is  that,  when  sandstones  are  fractured,  the 
fracture  passes  between  the  individual  sand  grains  and  not  across 
them,  whereas  in  quartzites  the  fracture  passes  through  rather  than 
between  the  component  grains. 

Mineral  and  chemical  composition.  —  Some  quartzites  are  re- 
markably pure,  composed  almost  entirely  of  quartz,  with  such  other 
minerals  as  may  occur  present  only  in  microscopic  size  and  propor- 
tion. The  chemical  analysis  of  such  a  rock  will  yield  nearly  all 
silica,  with  scarcely  more  than  traces  of  other  oxides.  Many  quartz- 
ites, however,  contain  other  minerals  besides  quartz,  some  of  which 
have  resulted  from  the  metamorphism  of  the  clay,  lime,  and  iron 
oxide  cement  which  bound  the  sand  grains  together  in  the  original 
rock. 

Besides  quartz,  there  may  be  present  in  variable  amounts,  feld- 
spar, mica  (muscovite  or  biotite),  chlorite,  cyanite,  epidote,  mag- 
netite, hematite,  graphite,  and  sometimes  calcite.  One  or  more  of 
these  minerals  sometimes  occur  in  such  amounts  as  to  exercise  some 
control  over  the  properties  of  the  rock.  The  chemical  composition, 
therefore,  of  the  quartzites  will  vary  in  accordance  with  that  of 
mineral  composition. 

Varieties.  —  The  distinction  between  quartzites  may  be  made  on  the 
basis  of  the  presence  of  certain  accessory  minerals,  such  as  chloritic 
quartzite,  micaceous  quartzite,  feldspathic  quartzite,  epidotic  quartzite, 
etc.  Other  varieties,  based  on  differences  in  texture  and  structure 
are  known.  Buhrstone  is  a  cellular  but  hard  and  tough  quartz- 
ite representing,  in  some  cases  at  least,  a  silicified  limestone,  and 
formerly  used  as  a  millstone.  Itacolumite,  known  also  as  flexible 
sandstone,  is  the  name  given  to  a  more  or  less  micaceous  variety, 
whose  grains  are  loosely  interlocked  and  have  the  power  of  slight 
movement  on  one  another.  Quartzite-schist  is  a  variety  in  which 
foliated  structure  has  been  developed,  the  surface  of  the  foliation 


PLATE  XVIII.  —  Beds  of  slate,  showing  cleavage,  overlain  by  quartzite.  The 
bedding  of  the  slate  which  does  not  show  in  the  view  is  parallel  with  that  of  the 
quartzite.  Field,  B.  G.  (H.  Hies,  photo.) 


(136) 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       137 

planes  being  coated  with  scales  of  mica.  Quartzites  which  have 
formed  from  pebbly  sandstones  or  conglomerates  are  known  as  con- 
glomerate quartzites.  The  pebbles  in  some  of  these  have  been  stretched 
and  flattened  from  dynamic  metamorphism. 

General  properties.  —  Texturally,  quartzites  are  hard  and  tough, 
usually  firm  and  compact,  granular  rocks,  whose  individual  grains 
may  range  from  fine  to  coarse  in  size.  Quartzites  may  form  thin  or 
thick  massive  beds  in  the  midst  of  other  metamorphic  rocks  (Plate 
XVIII),  especially  schists.  They  may  be  white,  gray,  yellowish, 
greenish,  or  reddish  in  color.  The  dense  and  compact  varieties  have 
low  porosity  and  absorption,  and  high  compressive  strength.  These 
properties  together  with  that  of  high  siliceous  composition  render 
quartzite  a  resistant  and  durable  rock.  They  are  usually  hard  to 
drill  and  also  to  dress. 

Uses.  —  On  account  of  their  great  durability  and  resistance  to 
atmospheric  agents  and  high  temperatures,  quartzites,  whose  joint 
planes  are  sufficiently  spaced  to  permit  the  extraction  of  dimension 
stone,  may  be  used  to  advantage  as  a  building  stone.  Hardness 
is  their  principal  drawback,  both  in  quarrying  and  dressing  the  stone. 
In  the  form  of  crushed  stone,  quartzites  are  admirably  suited  for 
railroad  ballast,  concrete  work,  etc.  The  purer  varieties  are  some- 
times ground  for  glass  sand. 

Occurrence  and  distribution.  —  Quartzites  occur  in  association 
with  schists  and  other  metamorphic  rocks  in  masses  up  to  hundreds 
of  feet  in  thickness.  They  are  widely  distributed  rocks,  occurring 
in  nearly  all  areas  of  metamorphosed  sediments,  but  have  their  great- 
est development  in  the  older  geological  formations,  especially  in  the 
Cambrian  and  pre-Cambrian.  Quartzites  are  common  in  the  eastern 
metamorphic  region,  including  New  England  and  the  Appalachians, 
around  Lake  Superior,  and  in  many  places  in  the  West. 

Slate  and  Phyllite 

Definition.  —  Slate  may  be  defined  as  a  thinly  cleavable  rock,  the 
cleavage  pieces  of  which  are  mineralogically  alike,  and  the  mineral 
grains  so  small  in  size  as  not  to  be  distinguishable  by  the  eye.  It  [is 
a  dense  or  aphanitic,  homogeneous  rock  of  very  fine  texture.  As 
pointed  out  below  the  cleavage  (Fig.  68)  of  slate  is  a  secondary  struc- 
ture produced  by  metamorphism  and  not  an  original  one  in  the  sense 
of  bedding,  stratification,  or  lamination,  as  in  shales  and  similar  sedi- 
ments; hence,  the  distinction  between  slate  and  shale. 

Slates  are  the  metamorphic  equivalents  of  muds  and  shales  and 


138 


ENGINEERING  GEOLOGY 


less  often  of  volcanic  ash  and  tuffs;  hence,  they  represent  the  finest 
particles  of  mineral  matter.  Shales,  slates,  phyllites,  and  mica 
schists  form  a  continuous  series  of  rocks  derived  chiefly  from  clay 


QUARRY'FLOCF 


Fig.  68.  —  Section  showing  relation  of  cleavage  to  stratification. 
(After  Dale.) 

or  mud  by  progressive  metamorphism  (dehydration  and  crystalliza- 
tion). Gradations  exist  between  shales  and  slates  on  the  one  hand, 
and  between  slates,  phyllites,  and  mica  schists  on  the  other. 

Mineral  and  chemical  composition.  —  Megascopically,  the  mineral 
composition  of  slates  is  of  no  importance,  since  the  constituent  grains 
of  the  rock  are  too  small  in  size  to  be  distinguished  by  the  eye.  When 
examined  in  thin  section  under  the  microscope,  however,  slates 
reveal  a  great  variety  of  minerals,  the  principal  ones  of  which  are 
quartz  and  mica  (biotite  and  muscovite,  including  sericite). 

Besides  these  occur  chlorite,  feldspar,  magnetite,  hematite,  pyrite,  carbonates 
of  lime,  iron,  and  magnesia,  carbonaceous  matter  and  graphite,  zircon,  tourmaline, 
rutile  needles,  andalusite,  ottrelite,  staurolite,  anatase,  etc. 

Slates  are  normally  clayey  or  argillaceous  rocks  in  composition,  but  are  subject 
to  considerable  variation  chemically.  The  range  in  essential  chemical  composition 
of  commercial  slate  of  aqueous  sedimentary  origin,  as  shown  by  Dale  in  29  analyses, 
is  as  follows: 


RANGE  OF  COMPOSITION  OF  SLATE 


Per  cent 

55-67 
11-23 


Silica. 

Alumina 

Ferric  oxide 0 . 52-  7 

Ferrous  oxide 0 . 46-  9 

Potash 1.76-5.27 

Soda 0.50-3.97 

Magnesia 0.88-4.57 

Lime 0.33-5.20 

Water  above  110°  C 2. 82-4.09 

Chemical  analyses  of  commercial  slates  have  economic  importance  in  their  bear- 
ing on  the  question  of  the  cause  of  fading  observed  in  some  slates. 

Varieties.  —  A  convenient  grouping  of  commercial  slates  based  on 
origin  and  composition,  as  developed  by  Dale,  is  into  (A)  aqueous 
sedimentary,  subdivided  into  (1)  clay  slates  and  (2)  mica  slates,  in- 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.      139 

eluding  (a)  fading  and  (6)  unfading;  and  (B)  igneous,  subdivided  into 
(1)  ash  slates  and  (2)  dike  slates. 

General  properties.  —  Slates  are  texturally  dense  and  compact  very 
fine-grained  rocks,  whose  component  minerals  are  not  distinguish- 
able megascopically.  Their  most  important  structural  feature  is  cleav- 
age, by  virtue  of  which  the  rock  readily  splits  into  thin  sheets  or 
slabs,  and  the  regularity  and  perfection  of  which  renders  the  slate  of 
value  for  roofing  purposes.  Slaty  cleavage  as  discussed  on  page  191 
is  a  secondary  structure,  developed  by  metamorphism,  which  may 
or  may  not  coincide  with  the  original  bedding;  usually  it  does  not, 
but  may  cut  it  at  almost  any  angle.  The  original  bedding  planes 
of  the  rock  usually  become  closed  during  the  process  of  metamorphism 
and  when  visible  in  the  slate  they  appear  as  lines  or  bands  known  as 
ribbons,  which  may  be  of  different  color  or  of  different  mineral  compo- 
sition (siliceous  and  calcareous  material  being  the  most  common) ,  and 
which  are  often  plicated.  When  irregular  and  numerous,  ribbons 
may  render  the  slate  worthless. 

The  cleavage  surfaces  may  be  quite  lustrous,  but  are  usually  dull. 
They  may  be  very  smooth  or  may  show  extremely  fine  plications. 
Sometimes  the  cleavage  surfaces  are  spotted,  and  in  some  slates  are 
even  knotty  from  the  presence  of  certain  minerals. 

The  cleavage  of  the  slate  is  often  responsible  for  the  rock  slips  which 
occur  in  many  excavations  made  in  this  kind  of  rock. 

The  usual  color  of  slate  ranges  from  gray  to  dark  or  bluish-black 
but  red,  green,  and  purple  shades  are  also  known.  The  gray  and 
black  slates  owe  their  color  to  the  presence  of  variable  amounts  of 
carbonaceous  matter;  the  red  and  purple  ones  to  iron  oxide;  and  the 
green  ones  sometimes  to  the  presence  of  chlorite. 

The  average  specific  gravity  of  slate  is  about  2.75,  but  may  be 
affected  by  the  presence  of  such  minerals  as  magnetite,  pyrite,  etc. 
Slates  are  rather  soft  rocks  and  may  be  readily  cut,  a  property  which 
is  of  considerable  economic  importance.  For  other  properties  and 
uses  of  slate,  see  Chapter  on  Building  Stone. 

Occurrence  and  distribution.  —  Slates  are  common  rocks  in  meta- 
morphic  areas  and  have  a  wide  range  geologically.  Those  of  the  eastern 
United  States  are  chiefly  of  Cambrian  and  Ordovician  age.  Slates 
have  rather  extensive  distribution  in  the  Lake  Superior  region,  and  in 
many  places  in  the  West,  especially  along  the  western  slopes  of  the 
Sierra  Nevada  Mountains. 

The  principal  production  of  slate  in  the  United  States  is  from  the 
eastern  states  (see  Chapter  on  Building  Stone). 


140  ENGINEERING  GEOLOGY 

Phyllite 

Phyllite  is  the  name  given  to  a  group  of  thinly  cleavable,  finely 
crystalline,  micaceous  rocks  intermediate  between  the  mica  schists 
and  slates,  into  which  they  may  grade.  They  probably  represent 
a  more  advanced  stage  of  metamorphism  than  slates.  Quartz  and 
usually  sericite  are  the  principal  minerals,  but  others,  such  as  garnet, 
pyrite,  etc.,  are  frequently  present  in  small  amounts.  Probably  the 
so-called  hydromica  schists,  described  by  the  older  geologists  in  this 
country,  are  for  the  most  part  phyllite. 

Phyllite  differs  from  slate  in  containing  a  larger  amount  of  mica 
which  is  visible  to  the  naked  eye,  and  the  rock  is  more  brittle  but 
not  so  tough.  It  is  usually  light  in  color,  sometimes  nearly  pure 
white,  but  frequently  of  various  darker  shades,  even  black  in  some 
cases.  It  is  apt  to  be  soft  and  has  a  rather  greasy  feel. 

Crystalline  Limestones  and  Dolomites  (Marbles) 

Introduction.  —  Under  this  head  are  included  all  rocks  composed 
essentially  of  calcium  carbonate  (limestone)  or  a  mixture  of  calcium 
and  magnesium  carbonates  (magnesian  limestone  and  dolomite)  that 
have  a  crystalline  or  granular  texture.  They  have  been  formed  from 
ordinary  limestones  and  dolomites  described  on  pages  128  to  135,  by 
the  processes  of  metamorphism,  either  of  contact  or  regional  character 
(pages  206,  208).  Such  crystalline  limestones  and  dolomites  are  the 
metamorphic  equivalents  of  the  ordinary  carbonate  rocks,  and  are 
known  geologically  as  marbles;  but  in  the  trade  the  term  marble  is 
applied  to  any  limestone  that  will  take  a  polish,  whether  crystalline 
or  not.  The  serpentinous  marbles  are  separately  discussed  under 
"  ophicalcites  "  as  a  member  of  the  next  group  of  metamorphic  rocks 
(pages  143). 

Composition.  —  Since  the  crystalline  limestones  and  dolomites  are 
the  metamorphic  equivalents  of  the  ordinary  carbonate  rocks,  they 
naturally  show  the  same  range  in  chemical  composition.  Most  lime- 
stones contain  impurities,  such  as  silica,  carbonaceous  matter,  iron 
oxides,  argillaceous  or  clayey  material,  etc.,  so  that  when  subjected 
to  metamorphism,  the  change  involves  not  only  crystallization  but 
the  development  of  new  minerals;  hence  the  crystalline  limestones 
and  dolomites  may  show  great  diversity  in  mineral  composition, 
ranging  from  essentially  pure  crystalline  carbonate  rocks  on  the  one 
hand  to  an  aggregate  of  nearly  all  silicates  on  the  other. 

From  carbonaceous  material  will  develop  graphite  which  causes 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       141 

dark  spotting  or  streaking,  or  in  some  cases  a  uniformly  dark  color. 
Other  impurities  of  the  character  mentioned  above  will  develop, 
under  conditions  of  metamorphism,  various  silicate  minerals. 

These  include  phlogopite  and  biotite  among  the  micas,  wollastonite  and  diopside 
among  the  pyroxenes,  tremolite  and  actinolite  among  the  amphiboles,  grossularite 
among  the  garnets,  and  many  others.  In  addition  to  these  quartz,  magnetite,  spinel, 
titanite,  and  pyrite,  etc.,  sometimes  occur.  Clarke  states  that  "  the  list  of  minerals 
now  known  as  existing  in  metamorphosed  limestones  must  comprise  at  least  70  species, 
and  possibly  more." 

General  properties.  —  Marbles,  when  pure,  are  compact  crystalline 
granular  rocks  composed  of  calcite  or  dolomite,  or  a  mixture  of  the 
two.  The  texture  may  range  from  exceedingly  fine-grained,  in  which 
the  individual  grains  are  so  small  in  size  as  not  to  be  distinguishable, 
to  very  coarse-grained,  in  which  the  grains  may  attain  a  size  of  a 
quarter  of  an  inch  and  more  in  diameter.  All  gradations  between 
these  two  extremes  occur.  The  texture  affects  the  weathering  quali- 
ties, ornamental  value,  and  to  some  extent  the  working  qualities  of 
the  stone. 

Unlike  many  metamorphic  rocks,  marble,  when  pure,  is  apt  to  be 
massive  and  without  indication  of  schistose  structure,  but  when  im- 
pure from  the  presence  of  other  minerals  (such  as  mica)  these  may  be 
so  arranged  as  to  produce  schistosity.  This  is  especially  true  of  the 
impure  marbles  of  the  Piedmont  region  in  the  Atlantic  states,  where 
they  are  frequently  found  grading  into  true  calcareous  (calcite) 
schists.  Marbles  which  are  strongly  banded  by  mica  are  not  as 
durable  in  a  severe  climate,  nor  do  they  take  a  continuous  polish. 
Some  marbles  show  a  brecciated  structure  (Plate  IX,  Fig.  1),  and 
these  though  often  of  highly  ornamental  character  are  not  adapted 
for  exterior  work. 

Marbles  show  a  wide  range  of  color,  dependent  chiefly  upon  their 
purity.  The  pure  ones  are  white,  others  gray  to  black,  and  still 
others  may  show  varying  shades  of  red,  pink,  yellow,  green,  brown, 
etc.  The  principal  impurities  which  act  as  a  pigment  influencing 
color  are  carbonaceous  matter  and  the  oxides  of  iron,  as  well  as  finely- 
divided  mica.  The  color  may  be  entirely  uniform  in  the  pure  marbles, 
but  more  often  it  is  spotted,  blotched,  or  streaked.  Absorption  is 
low,  usually  less  than  one  per  cent,  but  even  fine-grained  apparently 
dense  marbles  may  be  relatively  permeable.*  The  specific  gravity 

*  This  may  be  tested  by  soaking  the  dry  stone  for  24  hours  in  a  4  per  cent  alco- 
holic solution  of  nigrosine,  then  splitting  the  marble,  and  noting  how  deep  the  dye 
has  penetrated. 


142  ENGINEERING  GEOLOGY 

generally  averages  between  2.66  and  2.79.  The  hardness  of  the  cal- 
cite  marbles  is  3,  and  for  the  dolomitic  ones  3.5  to  4,  but  all  are  readily 
scratched  by  the  knife.  The  calcite  marbles  may  be  distinguished 
from  the  dolomitic  ones  by  effervescing  in  cold  dilute  acid  (see  pages 
35  and  36). 

Alteration.  —  Marbles  like  ordinary  limestones  are  soluble  rocks 
and  weather  with  comparative  readiness,  the  calcareous  material  being 
dissolved  and  removed  in  solution  with  such  insoluble  impurities  as 
may  have  been  present  in  the  rock  left  in  place  to  form  the  mantle  of 
residual  decayed  material.  In  some  quarries  these  solution  fissures 
penetrate  the  stone  to  some  depth,  causing  waste  in  quarrying.  They 
may  also  serve  as  entrance  channels  for  surface  waters  to  reach  mine 
workings  or  tunnels.  Sometimes  the  coarser  textured  marbles, 
especially  those  of  dolomitic  composition,  weather  through  physical 
causes,  breaking  down  into  a  coarse  sand  or  gravel  as  in  the  Adiron- 
dacks  and  western  New  England. 

Occurrence  and  distribution.  —  Since  the  crystalline  limestones 
(marbles)  are  the  result  of  metamorphism  they  are  necessarily  found 
in  metamorphic  regions  in  association  with  gneisses,  schists,  slates, 
etc.  They  form  interstratified  masses  or  lenses  with  schists,  slates, 
etc.,  which  vary  greatly  in  size.  On  account  of  the  variation  in 
texture  and  purity  of  the  different  beds  in  a  given  section,  all 
may  not  be  of  equal  commercial  value.  They  have  extensive 
development  and  economic  importance  throughout  the  metamorphic 
crystalline  region  of  the  eastern  United  States,  where  quarries  have 
been  opened  in  most  of  the  states,  with  Vermont,  Tennessee,  Georgia, 
Alabama,  Massachusetts,  Pennsylvania,  and  New  York,  in  the  order 
named  as  the  principal  producers.  Marbles  are  found  in  places  in 
the  West,  being  strongly  developed  in  Colorado,  California,  and 
Washington.  They  have  extensive  development  in  Eastern  Canada, 
and  in  similar  metamorphic  regions  of  other  countries. 

The  uses  and  properties  of  marble  for  structural  purposes  are  dis- 
cussed in  the  Chapter  on  Building  Stone.  Marbles  can  be  employed 
for  all  purposes  to  which  limestones  are  put. 

Ophicalcite,  Serpentine,  and  Soapstone 

In  general  characters  and  origin  this  group  of  rocks  has  many 
points  of  resemblance,  and  for  convenience  may  therefore  be  treated 
together.  It  is  a  series  whose  members  range  in  mineral  composition 
from  a  mixture  of  silicate  and  carbonate  minerals  as  in  ophicalcite  to 
essentially  all  silicate  components  as  in  the  pure  serpentine  and  soap- 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC. 


143 


stone.  Through  the  first  member  of  the  series,  ophicalcite,  the  group 
as  regards  composition  (containing  in  part  carbonates)  and  texture 
may  be  considered  as  related  to  the  preceding  one,  marbles  including 
crystalline  limestones  and  dolomite.  In  composition  the  soapstones 
are  rocks  related  closely  to  the  talc  schists  into  which  they  may  grade 
through  the  development  of  foliated  structure  by  dynamic  meta- 
morphism. 

The  following  table  of  analyses  serves  in  a  general  way  to  indicate  the  chemical 
relationships  as  well  as  points  of  difference  between  the  members  of  this  group 
of  rocks. 

ANALYSES  OF  SERPENTINE  AND  SOAPSTONE 


I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

SiO2 

44.14 

43.87 
0.31 

42.52 

40.42 
1.86 
2.75 
4.27 
35.95 
0.66 

0  16 

39.14 
2.08 
4.27 
2.04 
39.84 
Trace 

62.00 
) 

58.40 
7.44 
29.19 

38.85 
C  12.77 
j  12.86 

22*58 
6.12 

0.30 
6.52 

A12O3  

Fe2O3... 

1.96 

•-f 

FeO 

7  17 

) 

MgO. 

42.97 

38.62 
0.02 

42.16 

33.10 

CaO 

Na2O  I 

K2O    (  
H2O 

12.89 

9.55 
0.27 

14.22 

10.72 

2.68 

12.70 
0.11 

4.90 

4.97 

Rest  

100.00 

99.81 

100.86 

99.47 

100.18 

100.00 

100.00 

100.00 

I.  Theoretical  composition  of  pure  serpentine;  II.  Serpentine,  Webster,  North  Carolina;  III.  Serpentine, 
Montville,  New  Jersey;  IV.  Dark  green  serpentine,  Rowe,  Massachusetts;  V.  Serpentine,  Green- 
ville, California;  VI.  Theoretical  composition  of  pure  talc;  VII.  Soapstone,  Fairfax  County, 
Virginia;  VIII.  Soapstone,  Albemarle  County,  Virginia. 

Ophicalcite 

Ophicalcite,  known  also  as  ophiolite,  is  the  name  given  to  marbles 
(crystalline  limestones)  streaked  and  spotted  with  serpentine.  The 
name  is  usually  restricted  to  a  mixture  of  green  serpentine  and  white 
calcite,  magnesite,  or  dolomite  in  variable  proportions.  The  serpen- 
tine occurs  as  irregular  large  and  small  stringers  and  masses,  and  may 
contain  a  core  of  the  original  silicate  mineral  from  which  it  was 
derived.  Verde  antique  is  a  general  name  applied  to  green  serpen- 
tinous  marble. 

It  seems  probable  that  the  ophicalcites  were  derived  from  originally  impure 
limestones  by  metamorphism,  which  rendered  the  rock  entirely  crystalline  and  the 
impurities  crystallized  out  in  the  form  of  silicate  minerals,  such  as  pyroxene,  horn- 
blende, olivine,  etc.  The  silicate  minerals  were  later  secondarily  converted  by 
hydration  into  serpentine.  It  has  been  shown  by  Merrill  that  the  serpentine  in 
dolomite  of  Montville,  New  Jersey,  and  that  of  the  ophicalcites  of  Warren  County, 
New  York,  was  derived  from  pyroxene. 


144  ENGINEERING  GEOLOGY 

Ophicalcites  are  not  very  abundant  rocks  but  are  highly  prized  for  use  as  a  deco- 
rative stone.  They  are  soft  rocks  and  can  be  easily  polished,  but  as  a  rule  they 
weather  readily  and  unequally  on  exposure.  Another  defect  in  the  rock  is  the 
presence  of  numerous  joints  and  fractures  so  closely  spaced  that  stone  of  more  than 
a  few  feet  in  size  can  rarely  be  obtained. 

Ophicalcite  occurs  in  Quebec,  Canada,  in  the  northern  Green  Mountains,  and  in 
the  Adirondacks  of  New  York  State. 


Serpentine 

General  properties.  —  Pure  serpentine  is  a  hydrous  silicate  of 
magnesia,  but  as  masses  forming  the  rock  serpentine  it  is  usually 
more  or  less  impure  from  the  varying  quantities  of  other  minerals 
mixed  with  it.  These  may  include  among  the  silicates  olivine, 
pyroxene,  and  hornblende;  the  oxides  magnetite  and  chromite;  the 
sulphide  pyrite;  and  the  carbonates  of  lime  and  magnesia,  through 
the  increase  of  which  serpentine  proper  grades  into  the  serpentinous 
marbles  or  ophicalcites.  Garnet  of  the  variety  pyrope  and  biotite 
or  a  magnesian  mica  sometimes  occur.  Other  secondary  minerals 
may  and  sometimes  do  accompany  serpentine. 

Some  of  the  associated  minerals  such  as  olivine,  pyroxene,  and 
hornblende  are  the  remains  of  the  original  magnesian  silicates  from 
which  the  serpentine  was  derived.  Others,  like  serpentine  itself,  are 
secondary,  having  separated  out  and  formed  during  the  process  of 
alteration.  Because  of  the  variety  and  varying  quantities  of  asso- 
ciated minerals  serpentine  may  show  great  diversity  in  chemical  com- 
position, as  represented  in  the  table  of  analyses  on  page  161. 

When  reasonably  pure,  the  rock  serpentine  is  compact,  though  a 
variety  of  texture  may  be  shown.  It  is  dull  to  waxy  in  luster,  breaks 
usually  with  a  smooth  to  splintery  fracture,  and  is  soft  enough  to  be 
cut  by  the  knife,  but  from  the  presence  of  silica  it  may  be  much 
harder.  The  usual  color  is  green  to  yellowish  green,  sometimes  yel- 
low, with  the  more  impure  forms  exhibiting  various  shades  of  brown, 
red,  and  black. 

Origin  and  occurrence.  —  The  serpentine  rocks  are  secondary, 
having  been  formed  from  pre-existing  ones  through  processes  of  al- 
teration. They  may  be  formed  through  alteration  of  any  basic 
rock  composed  essentially  of  magnesian  silicates,  especially  olivine, 
pyroxene,  or  amphibole.  As  such  most  of  the  serpentines  have  prob- 
ably been  formed  by  the  alteration  of  basic  igneous  rocks,  such  as 
peridotites,  pyroxenites,  etc.  Hornblende  schists  may  also  yield 
them. 


ROCKS,  THEIR  GENERAL  CHARACTERS,  ETC.       145 

Serpentine  is  a  common  and  widely  distributed  rock  in  metamor- 
phic  regions,  occurring  as  an  alteration  from  both  igneous  and  meta- 
morphic  rocks.  It  seldom  forms  large  and  extensive  masses,  but 
occurs  in  places  in  the  metamorphic  crystalline  region  of  the  eastern 
United  States  extending  from  New  England  to  Georgia;  in  several 
of  the  western  states,  especially  California,  Oregon,  and  Washington; 
and  in  eastern  Canada.  Many  serpentine  deposits  show  an  abun- 
dance of  slipping  planes,  which  cause  trouble  by  rock  slides  or  slips 
in  quarries,  railroad  cuts,  and  other  excavations.  Indeed  engineers 
in  laying  out  a  railroad  may  try  to  avoid  this  kind  of  rock  if  they  are 
familiar  with  its  characteristics. 

Uses.  —  Serpentine  is  used  chiefly  as  an  ornamental  stone,  but  as 
a  rule  is  of  such  low  weathering  resistance  as  to  often  make  it  un- 
satisfactory for  exterior  use.  (See  Chapter  on  Building  Stone.) 

Soapstone 

General  properties.  —  Soapstone,  called  also  steatite,  is  composed  essentially  of 
the  mineral  talc  as  shown  in  the  table  of  analyses  on  page  161.  It  is  closely  related 
to  the  talc  schists  into  which  it  grades  on  the  development  of  foliated  structure  by 
dynamic  metamorphism.  Like  serpentine,  it  is  a  hydrous  silicate  of  magnesia,  but 
contains  more  silica  and  less  water.  These  differences  are  indicated  in  the  table  of 
analyses  below  of  the  theoretic  percentages  of  constituents  in  the  chemically  pure 
minerals. 


SiO2 

MgO 

H20 

Serpentine        

44.14 

42.97 

12  89 

Talc  (soapstone) 

62  00 

53  10 

4  90 

Soapstone  is  never  chemically  pure,  but  contains  varying  quantities  of  the  min- 
erals, mica,  chlorite,  amphibole  (tremolite),  pyroxene  (enstatite),  together  with 
quartz,  magnetite,  pyrrhotite,  and  pyrite.  Carbonates  may  be  present  in  some 
cases. 

Soapstone  is  a  massive  rock  of  bluish-gray  to  green  color,  sometimes  dark,  and 
is  soft  enough  to  be  readily  cut  with  the  knife,  hence  it  can  be  easily  worked.  It 
has  a  pronounced  soapy  or  greasy  feel,  and  resists  to  a  marked  degree  heat  and  the 
action  of  acids,  properties  which  make  the  stone  of  especial  value  for  use  in  the  trades, 
and  for  which  it  is  extensively  quarried. 

Origin  and  occurrence.  —  Soapstone  is  a  secondary  rock  derived  from  the 
alteration  of  magnesian  silicate  minerals,  such  as  tremolite,  enstatite,  etc.,  in  the 
same  general  way  as  serpentine  (page  162).  It  is  found  therefore  in  metamorphic 
regions  in  association  with  talcose  and  chloride  rocks,  sometimes  with  serpentine 
and  beds  of  crystalline  limestones.  It  is  a  common  rock  in  many  localities  in  the 
metamorphic  region  of  the  eastern  United  States.  It  has  wide  distribution  in  the 
metamorphic  crystalline  area  of  Virginia,  which  is  the  principal  producing  state  in 
the  United  States,  the  common  rock  associates  being  schists  of  varying  composition. 


146  ENGINEERING  GEOLOGY 

Uses.  —  Soapstone  is  a  very  durable  rock,  but  on  account  of  its  somber  color, 
soapy  feel  and  softness,  it  is  undesirable  for  general  constructional  purposes.  Be- 
cause of  its  ready  workability  due  to  softness,  insolubility  and  heat-resisting  qual- 
ities, it  is  suited  to  a  considerable  range  of  applications.  Most  of  the  product 
quarried  at  the  present  time  is  used  in  the  manufacture  of  wash  or  laundry  tubs, 
electric  switchboards  and  insulators,  and  laboratory  sinks.  Some  of  the  harder 
material  quarried  in  Virginia  makes  excellent  stair  treads,  being  preferred  by  some 
to  slate.  It  was  formerly  used  to  some  extent  in  the  manufacture  of  stoves  for  heat- 
ing purposes,  and  for  fire  brick,  but  in  recent  years  its  use  for  these  purposes  has  not 
been  so  great.  The  waste  from  quarrying,  and  in  some  cases  the  entire  output  from 
a  single  quarry,  is  pulverized  and  used  as  a  lubricant. 

References  on  Rocks 

1.  Clarke,  F.  W.     The  Data  of  Geochemistry.    Bulletin  491,  U.  S. 
Geological  Survey,  1911,  2d  edition,  782  pages. 

2.  Finlay,   G.   I.     Introduction   to  the  Study  of   Igneous  Rocks. 
McGraw-Hill  Book  Co.,  New  York,  1913,  228  pages. 

3.  Harker,  A.     The  Natural  History  of  Igneous  Rocks.     The  Mac- 
millan  Co.,  New  York,  1909. 

4.  Hatch,   F.   H.     Text-Book  of  Petrology.     The  Macmillan  Co., 
New  York,  1909,  5th  edition,  404  pages. 

5.  Hatch,  F.  H.,  and  Rastall,  R.  H.     The  Petrology  of  the  Sedi- 
mentary Rocks.     George  Allen  &  Co.,  Ltd.,  London,  1913,  425  pages. 

6.  Iddings,  J.  P.     Igneous  Rocks:    Composition,  Texture  and  Classi- 
fication, Description  and  Occurrence.     John  Wiley  &  Sons,  New  York, 
1909,  Vol.  I,  464  pages;  1913,  Vol.  II,  685  pages. 

7.  Kemp,  J.  F.     A  Handbook  of  Rocks  for  Use  without  the  Micro- 
scope.    D.  Van  Nostrand  Co.,  New  York,  1908,  4th  edition,  238  pages. 

8.  Merrill,  G.  P.     Stones  for  Building  and  Decoration.     John  Wiley 
&  Sons,  New  York,  1903,  3d  edition,  551  pages. 

9.  Pirsson,  L.  V.     Rocks  and  Rock  Minerals:  A  Manual  of  the  Ele- 
ments of  Petrology  without  the  Use  of  the  Microscope.     John  Wiley  & 
Sons,  New  York,  1908,  414  pages. 

For  books  dealing  with  the  economic  uses  of  rocks,  see  references  in 
Chapters  XI-XIV  and  XVI. 


CHAPTER  III 
STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS 

Introductory 

MANY  of  the  rock  formations  of  the  earth's  crust  have  been  consider- 
ably disturbed  since  their  time  of  origin.  In  some  cases  this  has  simply 
involved  a  change  in  position  of  extensive  rock  masses,  without  affecting 
their  structure,  as  when  the  sea  bottom  with  its  sediments  was  up- 
lifted without  warping;  but  hi  others  the  rocks,  as  a  result  of  stresses 
to  which  they  have  been  subjected,  incident  to  movements  of  the  earth's 
crust,  have  been  more  or  less  seriously  disturbed,  and  their  structure 
more  or  less  changed.  We  thus  find  that  rocks  are  bent  or  folded  to  a 
variable  degree,  and  usually  traversed  by  fractures,  along  which  move- 
ment or  slipping  may  have  taken  place. 

The  chief  structures  produced  then  as  a  result  of  the  above  are  folds, 
joints,  faults,  and  cleavage. 

FOLDS 

Introduction.  —  Beds  of  sedimentary  rock  are  usually  laid  down  in 
a  horizontal  position,  but  a  departure  from  this  attitude  is  sometimes 
noted,  especially  where  deposition  takes  place  on  steeply  sloping  shores 
and  in  deltas.  Examination  of  the  beds  over  much  of  the  earth's 
surface  reveals  the  fact  that  they  no  longer  preserve  a  persistent 
horizontal  attitude,  but  show  all  degrees  of  inclination  to  the  plane  of 
the  horizon,  because  of  the  folding  which  they  have  undergone.  These 
modifications  of  the  original  attitude  of  the  beds  have  resulted  from 
earth  movements,  and  are  recorded  from  field  study  in  terms  of  dip  and 
strike. 

Dip.  —  By  dip  is  meant  the  inclination  of  the  beds  to  a  horizontal 
plane  (Fig.  69),  and  is  measured  in  degrees  by  an  instrument  known 
as  a  clinometer,  which  consists  of  a  pendulum  with  a  graduated  arc. 
For  convenience  the  clinometer  is  usually  combined  with  the  compass, 
so  that  from  the  former  the  inclination  or  amount  of  dip  may  be 
ascertained,  and  from  the  latter  the  direction.  In  measuring  the  dip, 
the  direction  as  well  as  the  amount  of  inclination  is  taken.  Thus, 

147 


148  ENGINEERING  GEOLOGY 

24°  S.  30°  E.  expresses  the  exact  position  of  the  particular  bed.     The 
maximum  angle  of  inclination  of  the  bed  is  always  taken  as  the  dip. 

Strike.  —  This  is  the  direction  of  the  line  of  intersection  of  the 
dipping  bed  with  the  plane  of  the  horizon,  and  is  necessarily  measured 

at  right  angles  to  the  dij)  (Fig.  69)"". 
Like  dip,  the  direction  of  strike  is 
read  with  the  compass  from  the 
north  point;  thus,  N.  60°  W.  If 
the  direction  of  the  dip  remains 
constant,  the  strike  is  a  straight 

,.     ,  «      line,  but  with  change  in  direction 
FIG.  69.  —  Diagram  showing  dip  (cd) 

and  strike  (aft).  of  dip  there  also  follows  a__ehange. 

of  strike.  Since,  therefore,  the  di- 
rection of  strike  is  always  at  right  angles  to  that  of  dip,  if  the  latter  is 
measured  it  is  unnecessary  to  record  that  of  strike.  Thus,  a  bed  with 
an  east  dip  has  a  north  and  south  strike.  Beds  having  the  same  strike 
might  show  different  angles  of  dip. 

By  accurate  measurement, and  correlation  of  dip  and  strike  observa- 
tions in  land  areas  that  have  suffered  considerable  erosion,  folds  may 
usually  be  determined. 

Parts  of  folds.  —  The  line  of  prolongation  of  a  fold  is  its  axis,  which 
may  be  miles  long  or  only  a  small  fraction  of  a  mile,  but  whether  long 
or  short,  the  dip  decreases  and  the  fold  finally  dies  away.  This  crest 
or  trough  line  is  usually  not  horizontal,  but  inclined  at  varying  angles 
with  the  plane  of  the  horizon,  the  angle  of  inclination  being  defined  as 
the  pitch  of  the  fold.  The  plane  which  bisects  the  angle  between  the 
limbs  of  a  fold  is  known  as  the  axial  plane  (Fig.  70),  and  may  be  curved 
from  complex  movements.  The  axial  plane  divides  the  fold  into  two 
parts  known  as  limbs. 

Kinds  of  folds.  —  All  folds  may  be  regarded  as  modifications  of 
three  principal  types,  namely,  anticlines,  syndines,  and  monoclines. 
Folds  may  be  simple,  composite,  or  complex,  but  as  they  occur  in 
nature  most  of  them  are  complex,  since  they  are  usually  cross-folded 
-that  is  to  say  their  axial  lines  are  folded.  A  single  fold  without 
crenulations  may  sometimes  occur  when  it  is  described  as  a  simple 
fold.  If  crenulations  are  superposed  on  a  simple  fold  it  is  said  to  be 
composite. 

Anticlines.  —  These  are  folds  produced  by  the  arching  of  beds,  so 
that  the  limbs  dip  away  from  the  crest  on  the  two  sides  of  the  axial 
plane  (Fig.  70).  The  arch  may  be  broad  or  gentle,  or  sharp  and  angu- 
lar with  steep  dips,  all  gradations  between  the  two  being  observed. 


STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS      149 


Synclines.  —  These  are  folds  produced  by  the  beds  being  bent  in  a 
downward  flexure,  so  that  they  dip  from  both  sides  towards  the  bottom 
of  the  trough  (Fig.  70).  They  vary  in  the  same  manner  as  anticlines. 


FIG.  70.  —  A,  anticlinal  fold;  B,  synclinal  fold.     (Modified  from  Willis.) 

A  single  or  isolated  fold  sometimes  occurs  (as  in  some  West  Virginia 
oil  districts),  but  as  a  rule  the  area  of  disturbed  strata  will  show  a 
group  or  series  of  connecting  anticlines  and  synclines,  which  are  either 
broad  and  open,  or  narrow  and  compressed,  the  beds  in  the  latter  case 
being  frequently  twisted  and  contorted  in  the  most  complex  manner. 

Anticliue 


FIG.  71.  —  Section  showing  anticline  and  syncline. 

The  Appalachian  Mountains  of  the  eastern  United  States  form  a  typical 
example  of  this  type  of  structure  (Fig.  71  and  Plate  XXI). 

Monocline.  —  A  monoclinal  fold  is  a  single  bend  or   curvature  in 
strata  which  lie  at  different  levels  on  opposite  sides  of  the  bend,  but 


150 


ENGINEERING  GEOLOGY 


have  the  same  general  direction  of  dip  (Fig.  72).  It  is  the  simplest  kind 
of  flexure,  and  is  generally  observed  in  regions  of  horizontal  or  gently 
dipping  beds.  Folds  of  the  monoclinal  type  are  developed  on  a  large 


FIG.  72.  —  Monoclinal  fold. 


FIG.  73.  —  Section  and  block  showing  monoclinal  attitude  of  beds. 


FIG.  74.  —  Plan  and  section  of  quaqua- 
versal  fold. 


FIG.  75.  —  Plan  and  section  of  centro- 
clinal  fold. 


scale  in  the  high  plateau  region  of  the  West,  and  the  gently  dipping  beds 
of  the  Coastal  Plain  province  in  the  eastern  United  States  furnish  a 
good  illustration  of  the  monoclinal  attitude  of  strata. 

Other  types  of  folds.  —  The  quaquaversal  or  dome-shaped  fold  is  a  special  case 
of  the  anticline,  in  which  the  beds  dip  outwards  in  all  directions  from  a  central 
point  (Fig.  74). 


STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS      151 

The  centrodinal  fold  or  structural  basin  is  a  special  case  of  a  syncline,  in  which 
the  beds  dip  inward  from  all  sides  towards  a  central  point  (Fig.  75). 

Both  domes  (quaquaversal  folds)  and  basins  (centroclinal  folds)  are  regarded  as 
modifications  of  normal  anticlines  and  synclines,  and  are  not  very  common  struc- 
tural forms. 

When  the  disturbed  beds  over  any  considerable  area  have  been  raised  into  a 
great  or  broad  arch  composed  of  numerous  minor  folds  and  flexures,  such  a  complex 
of  folds  is  known  as  an  anticlinorium  (Plate  XIX,  Fig.  1).  Conversely,  when  the 
beds  have  been  depressed  into  a  broad  trough  composed  of  subordinate  folds,  it  is 
termed  a  syndinorium  (Plate  XIX,  Fig.  2).  In  other  words,  the  terms  anticlinorium 
and  syndinorium  refer  to  composite  arches  and  troughs,  to  which,  when  simple, 
Dana  has  applied  the  terms  geanticline  and  geosyncline. 

Folds  whose  beds  have  been  so  compressed  that  the  limbs  are  parallel  are  known 
as  isoclines  (Fig.  76,  A-C).  When  eroded  to  a  general  level  the  beds  of  isoclinal  folds 


FIG.  76.  —  (A}  Isoclinal  folds,  upright;    (#)  isoclinal  folds,  inclined;    (C)  isoclinal 
folds,  recumbent;   (D)  fan  structure,  upright.     (Willis.) 


present  a  continuous  and  uniform  dip,  so  that  they  appear  as  a  single  succession 
of  inclined  beds.  In  such  a  region  of  folded  and  eroded  rocks  the  same  bed  may 
be  repeated  many  times  at  the  surface,  and  unless  carefully  studied  the  observer 
may  readily  be  deceived  in  the  number  of  independent  beds.  In  regions  of  complex 
folding  like  the  Alps,  a  double  series  of  isoclinal  folds  has  developed,  so  that  the 
axial  planes  of  the  minor  folds  converge  downward  on  the  two  sides  of  a  central 
anticline,  producing  a  type  of  convoluted  structure  known  as  fan  structure  or  fold 
(Plate  XIX,  Fig.  3).  The  Mont  Blanc  range  is  a  good  example. 

Minor  folds  are  frequently  developed  in  weak  beds  such  as  slate  or  shale  by 
shearing  between  two  more  competent  beds  like  quart zite;  the  folds  are  conveniently 
designated  drag  folds  (Leith).  Parallel  folds  show  no  thickening  or  thinning  of 
the  beds;  the  bedding  surfaces  are  parallel,  but  the  curvature  is  not  exactly  the 
same  in  any  two  beds.  Similar  folds  show  thickening  and  thinning  of  the  beds,  the 
bedding  surfaces  are  not  parallel,  but  the  curvature  is  the  same  for  all  beds 
(Leith). 


152 


ENGINEERING   GEOLOGY 


PLATE  XIX,  FIG.  1.  —  Ideal  section  of  an  upright  normal 
anticlinorium.     (After  Van  Hise.) 


FIG.  2.  —  Ideal  section  of  an  upright  normal  synclinorium. 
(After  Van  Hise.) 


FIG.  3.  —  Generalized  fan  fold  of  the  central  massif  of  the  Alps. 
(After  Heim.) 


FIG.  4.  —  General  section  of  roof  structure  in  the  central  massif 
of  the  Alps.     (After  Heim.) 


PLATE  XX,  FIG.  1.  —  Contorted  strata  in  Chickamauga  limestone  near  Ben  Hur, 
Va.     (Va.  Geol.  Survey,  BuU.  II-A.) 


FIG.  2.  — Folded  quartzite,  Eagle  Mountain,  Botetourt  Co.,  Va.     (Va.  Geol.  Survey, 

Bull.  II-A.) 

(153) 


154 


ENGINEERING  GEOLOGY 


The  principal  kinds  of  folds  as  defined  above  may  be  classified  (1)  with  refer- 
ence to  the  relation  of  the  limbs  to  each  other,  and  (2)  the  amount  of  compression 
they  have  suffered.  According  to  the  first  principle,  each  kind  of  fold  may  be  up- 
right  or  symmetrical  (Fig.  71),  inclined  or  asymmetrical  (Plate  XXI,  Fig.  2),  over- 
turned (Plate  XXI,  Fig.  4),  or  recumbent  (Fig.  76),  dependent  upon  the  position  of 
the  axial  plane,  whether  vertical,  inclined,  overturned,  or  recumbent.  According 
to  the  degree  of  compression  to  which  the  folds  have  been  subjected,  we  may  group 
them  into  (a)  open  folds  whose  limbs  are  widely  spaced  (Plate  XXI,  Fig.  1),  in 
which  the  amount  of  compression  has  been  moderate,  resulting  in  the  production  of 
somewhat  gentle  flexures;  and  (6)  close  folds  whose  limbs  are  in  contact  (Plate 
XXI,  Fig.  3),  characterized  usually  by  sharp  flexures  with  steep  slopes,  resulting 
from  a  high  degree  of  compression. 

Folds  modified  by  erosion.  —  Folds  are  rarely  found  in  nature 
with  their  original  forms,  but  are  modified  by  denudation  (Fig.  80),  for 
as  soon  as  they  are  lifted  above  sea  level,  folds  become,  by  reason  of 
their  position,  subject  to  more  rapid  erosion  than  their  surrounding 
areas.  The  erosion  of  folded  rocks  develops  characteristic  topographic 
features.  Ordinarily  anticlines  are  eroded  more  rapidly  than  synclines 
which  seem  to  offer  greater  resistance  to  the  forces  of  denudation. 
Hence  in  folded  strata  that  have  been  exposed  to  denudation  for  a  long 
period  of  time,  the  greatly  eroded  anticlines  form  the  lower  belts  or 

areas,  and  the  more  resistant  syn- 
clines of  trough-shaped  strata  the 
higher  ones. 

In  some  areas  of  folded  rocks 
that  are  of  great  geologic  age, 
such  as  the  Piedmont  region  of 
the  eastern  United  States,  the 
folds  have  been  completely  trun- 
cated by  erosion  and  the  surface  everywhere  reduced  approximately 
to  a  common  general  level.  In  such  regions  the  determination  of 


FIG.  77.  —  Stretch  thrust  developed 
from  an  overturned  fold  by  stretch- 
ing of  the  middle  limb.  (Heim.) 


FIG.  78.  —  Tilted  folds. 


folded  structure   cannot  be  based  on  topography,  but  is  determined 
by  careful  records  made  of  dips  and  strikes  in  field  study. 


STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS     155 


FIG.  79.  —  Monocline  showing  thinning  of  beds  in  the  fold. 


FIG.  80.  —  Eroded  fold,  showing  igneous  rock  (a), 
and  shales  (6). 


Relation  of  Folding  to  Engineering  Operations 

Tunneling.  —  Folded  rocks  sometimes  show  considerable  fractur- 
ing along  the  axis  of  the  fold.  In  the  case  of  an  anticline  these  fractures 
diverge  upward  (Fig.  81),  while  in  a  syncline  they  diverge  downward. 


FIG.  81.  —  Ideal  section  of  bent  rock  stratum  showing 
fracturing  along  convex  surface  and  compression  along 
concave  surface.  (After  Van  Hise,  U.  S.  Geol.  Survey, 
16th  Ann.  Rept.) 

Where  a  tunnel  is  driven  along  the  crest  of  a  fold  (Fig.  82),  much  trouble 
may  be  experienced  from  shattered  rock,  and  it  may  be  necessary  to 
line  it  from  end  to  end.  In  the  case  of  a  syncline  additional  trouble 
may  be  caused,  even  with  moderate  fracturing,  because  here  the  blocks 


PLATE  XXI. —  Types  of  folds:  (1)  Symmetrical  or  upright  open  fold;  (2)  un- 
symmetrical  or  inclined  fold,  open;  (3)  symmetrical  or  upright  fold,  closed; 
(4)  unsymmetrical  fold,  closed,  and  overturned;  (5)  syncline  showing  a 
keel;  a  carinate  syncline;  (6)  carinate  anticline,  the  lower  strata  remaining 
flat;  (7)  carinate  anticline,  overturned;  (8)  carinate  anticline,  or  recumbent 
fold.  (Willis.) 

(156) 


STRUCTURAL  FEATURES  AND   METAMORPHISM   OF  ROCKS     157 

bounded  by  fracture  planes  are  like  inverted  keystones  and  liable  to 
drop  out. 

The  fr  ctures  along  the  crest  of  a  fold  may  cause  additional  trouble 
by  serving  as  channel  ways  for  surface  waters. 

In  driving  tunnels  in  areas  of  folded  rocks  the  engineer  must  needs 
give  careful  attention  to  the  geologic  structure,  and  neglect  to  do  so 
has  sometimes  led  to  costly  mis- 
takes. Let  us  take  the  case  of 
a  tunnel  that  was  to  be  driven 
through  horizontal  or  undisturbed 
rocks.  Then  we  might  assume 
that  the  kind  of  rock  would  be 
the  same  throughout  the  tunnel, 
unless  the  section  were  penetrated  **muaK 

by  intrusive  rocks.     If,   however, 

FIG.  82.  —  Section  snowing  relation   of 
the  rocks  are  folded  the  problem  tunnel  to  anticlinai  fold. 

is  different,   and  it  then  becomes 

necessary  to  work  out  the  geological  structure  (see  p.  211)  and  kind 
of  rocks  in  the  hill  to  be  penetrated,  so  as  to  calculate  approxi- 
mately the  yardage  of  each  kind  of  rock  to  be  removed.  An  anticlinal 
ridge  might  appear  on  rapid  inspection  to  be  composed  of  but  one  kind 
of  rock,  whereas  the  central  portion  of  the  arch  might  be  rock  of  a 
totally  different  nature,  firmer  or  looser,  than  the  outer  shell. 

Quarrying.  —  The  position  of  folded  beds  likewise  affects  quarrying 
operations.  If  the  beds  dip  into  a  hill,  the  overburden  will  increase  with 
the  distance  from  the  outcrop,  even  though  the  hill  surface  itself  does 
not  rise;  but  should  the  dip  rise  with  the  hill,  the  thickness  of  the  over- 
burden does  not  necessarily  increase.  With  beds  of  very  steep  dip  it 
is  often  possible  to  work  the  quarry  as  a  steep-walled  cut,  removing  the 
desired  beds  and  leaving  the  worthless  ones  standing.  This  is  done  in 
some  marble,  natural  cement  rock,  and  clay  deposits.  With  intense 
folding  the  rock  may  also  be  so  fractured  that  the  deposit  contains  few 
or  no  large  blocks. 

Ore  deposits.  —  The  crushed  rocks  along  the  crests  of  folds  some- 
times play  an  important  role  in  the  formation  of  ore  deposits,  since  the 
cavities  between  the  crushed  fragments  sometimes  serve  as  spaces  for 
the  deposition  of  ore.  (Lead  and  zinc  ores  of  southwest  Virginia.) 

Mining.  —  The  position  of  folded  beds  may  influence  the  method  of 
mining  to  be  employed,  as  in  the  anthracite  region  of  Pennsylvania. 
Intense  folding  may  also  shatter  the  rocks  to  such  an  extent  as  to  make 
the  roof  unsafe,  and  require  much  timbering. 


158  ENGINEERING  GEOLOGY 


Field  Observations1 

It  is  frequently  necessary  for  the  engineer  to  make  field  observations 
in  order  to  work  out  the  geologic  structure,  to  determine  the  thickness 
and  cubic  contents  of  a  series  of  beds,  or  to  calculate  the  depth  of  a 
given  bed  below  the  surface  at  a  given  point.  This  involves  making 
certain  measurements,  the  method  of  doing  so  being  explained  below. 

"  Angular  measurements.  —  Determinations  of  dip  and  strike  should  be  made 
upon  practically  every  outcrop,  except  in  regions  of  horizontal  rocks  or  massive 
crystalline  ones.  This  requires  measuring  vertical  angles  and  can  be  done  with  a 
clinometer,  or  with  a  spirit  level  and  vertical  circle  (Abney  hand  level  or  Brunton 
compass). 

In  determining  dip  angles  the  edge  of  the  clinometer  may  be  placed  directly 
upon  the  sloping  surface  to  be  determined,  but  care  must  be  exercised  that  the 
part  of  the  surface  selected  actually  represents  the  average  slope  of  the  beds,  and 
that  the  measurement  of  the  angle  is  not  influenced  by  local  irregularities.  Also, 
to  obtain  the  correct  dip  the  edge  of  the  clinometer  must  be  placed  on  a  line  exactly 
at  right  angles  to  the  strike.  Where  the  exposure  is  such  as  to  permit  it,  better 
results  can  be  secured  by  sighting  to  the  edge  of  the  beds  across  the  edge  of  the 
clinometer  at  such  a  distance  that  several  feet  will  be  covered  and  the  average  dip 
obtained.  Care  must  be  taken  to  have  the  eye  as  near  as  possible  in  the  extension 
of  the  plane  whose  inclination  is  being  measured,  and  to  sight  on  a  horizontal  line. 

Land  slopes  may  be  measured  with  a  clinometer  when  they  can  be  seen  in  profile, 
but  elsewhere  by  means  of  a  vertical  circle,  approximately  with  the  Abney  level  or 
Brunton  compass,  and  accurately  with  a  transit  or  telescopic  alidade. 

Vertical  measurements.  —  The  means  employed  for  determining  differences  in 
elevation  will  be  varied  according  to  the  conditions  and  degree  of  accuracy  required. 
The  instruments  most  used  are  (a)  the  aneroid,  (6)  the  hand  level,  (c)  the  wye  level, 
and  (d)  the  telescope  with  vertical  circle. 

Determination  of  thickness  of  beds.  —  In  the  study  of  areal,  stratigraphic,  and 
structural  geology,  the  thickness  of  beds  must  be  determined  at  many  points.  The 
character  of  the  topography  and  of  the  outcrops,  and  inclination  of  the  beds,  will 
determine  the  method  employed. 

The  simplest  case  is  where  the  beds  are  approximately  horizontal  and  the  slopes 
are  steep.  Under  such  conditions  it  is  necessary  only  to  measure  the  vertical  dis- 
tances between  upper  and  lower  limits  of  the  stratigraphic  units  by  aneroid,  hand 
level,  or  wye  level,  depending  on  the  degree  of  accuracy  required.  If  the  slope  on 
which  the  section  is  made  is  very  steep  —  30°  or  more  —  dips  of  3°  or  less  may  be 
neglected. 

If  the  beds  dip,  three  factors  must  be  determined  —  (1)  dip  angle,  (2)  slope 
angle,  and  (3)  distance  across  the  beds  normal  to  the  strike;  and  three  cases  occur  — 
(a)  with  surface  horizontal,  (6)  with  surface  sloping  and  beds  dipping  into  the  slope, 
and  (c)  with  surface  sloping  and  beds  dipping  with  the  slope.  These  three  cases 
are  shown  in  Fig.  83,  from  which  it  is  seen  that: 

(a)  Thickness  of  beds  A  to  B  =  aB  =  AB  X  sin  BAa. 

(b)  Thickness  of  beds  B  to  C  =  bC  =  BC  X  sin  (CBe'+  eBb). 

(c)  Thickness  of  beds  C  to  D  =  cD  =  CD  X  sin  (fCc  -fCD). 

1  The  authors  have  quoted  freely  from  Hayes,  Handbook  for  Field  Geologists, 
Wiley  and  Sons,  N.  Y.,  2d  ed.,  1909. 


STRUCTURAL  FEATURES  AND  METAMORPHISM   OF  ROCKS     159 

The  dip  angle  (BAa  =  eBb=fCc)  is  measured  directly  with  the  clinometer;  the 
slope  angles  (CBe  and/CZ))  are  either  measured  directly  or  obtained  from  the  differ- 
ence in  elevation,  which  is  the  slope  distance  into  the  sine  of  the  slope  angle,  i.e., 


Sin  CBe 


Sin /CD  = 


Ce 
BC 


CD 


FIG.  83.  —  Diagram  illustrating  determination  of  thickness  of  beds  by 
trigonometric  method.     (After  Hayes.) 

These  results  may  be  expressed  in  the  following  rules: 

(1)  Where  the  surface  is  horizontal,  the  thickness  equals  the  distance  across  the 
dipping  beds  multiplied  by  the  sine  of  the  dip  angle.  (2)  Where  the  surface  slopes 
and  beds  dip  into  the  slope  the  thickness  equals  the  distance  across  the  beds  multi- 
plied by  the  sine  of  the  sum  of  dip  and  slope  angles.  (3)  Where  the  surface  slopes 
and  beds  dip  with  the  slope  the  thickness  equals  the  distance  across  the  beds  multi- 
plied by  the  sine  of  the  difference  of  dip  and  slope  angles. 

To  facilitate  calculations  a  table  of  natural  sines  and  tangents  is  given  on  page  180. 

With  increasing  dip  the  horizontal  measurement  becomes  relatively  more  im- 
portant than  the  vertical,  and  where  the  dip  becomes  approximately  90°  the  differ- 
ence in  elevation  between  limits  of  the  beds  may  be  neglected  and  the  true  thickness 
will  be  represented  by  the  horizontal  distance  measured  at  right  angles  to  the  strike 
of  the  beds. 

A  convenient  method  of  determining  the  thickness  of  beds,  without  'calculation, 
when  the  angle  of  dip  and  horizontal  distance  across  the  outcrop  normal  to  the 
strike  are  known,  is  by  the  use  of  the  diagram  shown  in  Fig.  84.  The  horizontal 
rulings  correspond  to  degrees.  Any  convenient  scale  may  be  adopted  for  the 
spaces  between  vertical  rulings,  as  1,  10,  50,  or  100  feet.  To  determine  the  thick- 
ness of  beds,  find  the  horizontal  line  corresponding  to  the  dip  angle  and  follow  it  to 
the  right  for  a  distance  corresponding  to  the  measured  distance  across  the  outcrop 
on  the  scale  selected.  If  the  distance  coincides  with  a  curved  line  the  latter  is  fol- 
lowed to  the  top  of  the  diagram,  where  the  thickness  is  determined  directly  by  the 
distance  between  it  and  the  left  margin,  the  same  scale  being  used.  If  the  point 
falls  between  two  curved  lines,  the  measurement  is  made  to  a  point  at  the  top  of 
the  diagram  having  the  same  relation  to  these  lines. 

A  convenient  method  for  the  direct  measurement  of  thickness  in  making  de- 
tailed sections,  particularly  on  steep  slopes  or  with  steeply  dipping  beds  and  where 
exposures  are  nearly  continuous,  is  as  follows:  To  the  upper  end  of  a  rod  of  con- 
venient length  —  5  feet  is  about  right  for  a  man  of  ordinary  height  —  is  fastened  a 
short  arm  to  form  a  right-angled  T.  A  zigzag  jointed  5-foot  rule  may  be  used  in- 
stead of  the  rod.  In  addition  to  the  rod  either  (a)  a  hinged  clinometer  with  level 
on  one  arm,  or  (6)  an  Abney  level,  or  (c)  a  Brunton  compass  is  used.  The  dip  of 
the  beds  is  determined,  and  if  a  clinometer  is  used  the  arms  are  opened  so  that  the 
angle  between  them  is  equal  to  the  dip  angle.  If  then,  the  lower  limb  of  the  clinom- 


160 


ENGINEERING   GEOLOGY 


TABLES  AND  FORMULAS 


B              Cotangent              , 

TABLE  II.  —  NATURAL  CIR- 
CULAR FUNCTIONS 

XXR^^E  ^\cy^ 

f 

\;^13¥ 

• 

Sine. 

Tang. 

Cosine. 

Cotang. 

| 

H!G      /zf\     D  " 

Diagram  illustrating  circular  functions. 

SOLUTION  OF  TRIANGLES 
c 

.s^                   a 

L^^                                                               n 

0 
1 
2 
3 
4 
5 
6 
7 

8 
9 

10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
31 
32 
33 
34 
35 
36 
37 
38 
39 
40 
41 
42 
43 
44 
45 

0.0000 
0.0175 
0.0349 
0.0523 
0.0698 
0.0872 
0.1045 
.1219 
0.1392 
0.1564 
0.1737 
0  1908 
0.2079 
0.2250 
0.2419 
0.2588 
0.2756 
0.2924 
0.3090 
0.3256 
0.3420 
0.3584 
0.3746 
0.3907 
0.4067 
0.4226 
0.4384 
0.4540 
0.4695 
0.4848 
0.5000 
0.5150 
0.5300 
0.5446 
0.5592 
0.5736 
0.5878 
0.6018 
0.6157 
0.6293 
0.6428 
0.6560 
0.6691 
0.6820 
0.6947 
0.7071 

0.0000 
0.0175 

0.0349 
0.0524 
0.9699 
0.0875 
0.1051 
0.1228 
0.1405 
0.1584 
0.1763 
0.1944 
0.2126 
0.2309 
0.2493 
0.2680 
0.2868 
0.3057 
0.3249 
0.3443 
0.3640 
0.3839 
0.4040 
0.4245 
0.4452 
0.4663 
0.4877 
0.5095 
0.5317 
0.5543 
0.5774 
0.6009 
0.6249 
0.6494 
0.6745 
0.7002 
0.7265 
0.7536 
0.7813 
0.8098 
0.8391 
0.8693 
0.9004 
0.9325 
0.9657 
1.0000 

1.0000 

0.9999 
0.9994 
0.9986 
0.9976 
0.9962 
0.9945 
0.9926 
0.9903 
0.9877 
0.9848 
0.9816 
0.9782 
0.9744 
0.9703 
0.9659 
0.9613 
0.9563 
0.9511 
0.9455 
0.9397 
0.9336 
0.9272 
0.9205 
0.9136 
0.9063 
0.8988 
0.8910 
0.8830 
0.8746 
0.8660 
0.8572 
0.8480 
0.8387 
0.8290 
0.8192 
0.8090 
0.7986 
0.7880 
0.7772 
0.7660 
0.7547 
0.7431 
0.7314 
0.7193 
0.7071 

Infin. 
57.2900 
28.6363 
19.0811 
14.3007 
11.4301 
9.5144 
8.1444 
7.1154 
6.3138 
5.6713 
5  1446 
4.7046 
4.3315 
4.0108 
3.7321 
3.4874 
3.2709 
3.0777 
2.9042 
2.7475 
2.6051 
2.4751 
2.3559 
2.2460 
2.1445 
2.0503 
1.9626 
1.8807 
1.8041 
1.7321 
1.6643 
1.6003 
1.5399 
1.4826 
1.4282 
1.3764 
1.3270 
1.2799 
1.2349 
1.1918 
1  .  1504 
1.1106 
1.0724 
1.0355 
1.0000 

90 
89 

88 
87 

86 

85 
84 
83 
82 
81 
80 
79 
78 
77 
76 
75 
74 
73 
72 
71 
70 
69 
68 
67 
66 
65 
64 
63 
62 
61 
60 
59 
58 
57 
56 
55 
54 
53 
52 
51 
50 
49 
48 
47 
46 
45 

s'mA=b 
C=90°-A.    6 

C 

Right  triangle. 

c                   a 
cos  A  =  r.     tanA  =  -. 
6                    c 

21 

A^                  c                   B 

Oblique  triangle. 

Gi™-       qultd. 

Formula. 

A,  5,  a     6 
A,  a,  b       B 
C,  a,  6       B 

a,b,c       A 

a,  6,  c     Area 
A,  J5,  c  Area 

,      asinJ? 

sin  A 
HnR     6sinA 

a 
P         6sinC 

a—  6  cos  C 
If  s  =  £  (a+6+c). 

V           6c 
1/1     t  /s(s—  a) 

Area  =  Vs(s-a)  (s  —  6)  (s—  c) 
Area=|6csinA. 

CIRCLES 
Circumference=27rR.     Area=7rR2.    7r=3.1416 

o 

Cosine. 

Cotang. 

Sine. 

Tang. 

.-• 

•STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS     161 

eter  is  held  firmly  on  the  top  of  ^the  T  rod  and  the  rod  is  inclined  until  the  upper 
limb  is  horizontal,  the  lower  limb  will  be  in  the  plane  of  bedding  projected  upward 
toward  the  observer.  By  sighting  down  the  limb  the  bed  in  whose  plane  it  lies  is 
determined  and  the  beds  between  this  plane  and  the  foot  of  the  rod  have  a  thickness 
equal  to  its  length.  The  foot  of  the  rod  is  now  moved  up  to  this  bed  and  again 
brought  into  position  so  that  the  upper  limb  of  the  clinometer  is  horizontal  and  the 


FIG.  84.  —  Diagram  for  use  in  determination  of  thickness  of  beds  by  graphic  method. 

(After  Hayes.) 


rod  is  at  right  angles  to  the  bedding,  and  a  new  point  is  obtained  by  sighting  down 
the  lower  limb.  Count  is  kept  of  the  unit  thicknesses  and  the  total  thickness  between 
determined  limits  is  obtained  with  no  calculation  except  a  multiplication  of  the 
length  of  the  rod  into  the  number  of  sights  taken.  The  method  is  very  similar  to 
the  use  of  the  hand  level  for  obtaining  elevations,  and  becomes  identical  with  it 
when  the  dip  becomes  zero. 

When  the  Abney  level  or  the  Brunton  compass  is  used  the  method  is  the  same, 
except  that  the  vernier  arm  carrying  the  level  is  set  at  a  point  on  the  divided  circle 
corresponding  to  the  dip  angle. 

Where  surface  exposures  are  nearly  or  quite  continuous,  so  that  it  is  not  neces- 
sary to  follow  stream  channels,  and  where  dips  are  steep  and  variable,  sections 
should  be  measured  as  nearly  as  possible  at  right  angles  to  the  strike.  In  order  to 


162  ENGINEERING  GEOLOGY 

get  the  best  exposures  it  is  generally  necessary  to  make  occasional  offsets  along  the 
strike,  following  some  easily  identifiable  bed  or  contact.  Measurements  along  the 
strike  need  not  be  made  with  the  same  degree  of  accuracy  as  those  normal  to  the 
same.  The  notes  of  such  a  traverse  may  conveniently  be  kept  in  tabular  form,  a 
page  of  the  notebook  being  ruled  into  columns  for  (1)  number  of  the  station,  (2)  char- 
acter of  rocks,  (3)  distance  (measured  on  the  slope),  (4)  single  angle  (U  when  the 
slope  is  up  in  the  direction  of  traverse  and  D  when  it  is  down),  (5)  altitude  (or  ele- 
vation with  reference  to  any  assumed  datum),  (6)  dip  angle  (F  when  the  dip  is  in 
the  direction  of  the  traverse,  and  B  when  the  reverse),  (7)  strike,  and  (8)  thickness. 
All  columns  except  the  last  should  be  filled  as  the  traverse  proceeds,  and  where 
direct  measurements  can  be  made  the  thickness  should  be  recorded  also.  Columns 
3  to  6  contain  the  necessary  data  for  computing  thicknesses  by  the  methods  given 
above,  if  they  cannot  be  measured  directly. 

In  case  it  is  necessary  to  make  surface  measurements  diagonally  across  the  strike, 
the  distance  normal  to  the  strike  is  determined  by  the  solution  of  a  right-angled 
triangle,  the  line  traversed  being  the  hypothenuse  (h)  and  the  angle  which  this  line 
makes  with  the  strike  being  an  adjacent  angle  (c).  The  side  (B)  opposite  this 
known  angle  will  be  the  distance  on  the  slope  normal  to  the  strike  —  that  is, 


B  = 


sin  c 

In  making  sections  of  steeply  inclined  and  poorly  exposed  beds,  the  observed 
dips  at  the  nearest  exposures  often  show  wide  variation.  A  convenient  method  of 
obtaining  approximate  thicknesses  under  such  conditions  is  as  follows:  Measure 
horizontal  distances  as  nearly  as  possible  at  right  angles  to  the  strike,  locating  and 
measuring  as  many  dips  as  possible.  Construct  a  normal  profile  to  scale  and  plot 
upon  it  all  dips  projected  in  their  proper  horizontal  relations,  as  in  Fig.  85.  Extend 
the  dips  in  straight  lines  above  and  below  the  profile.  At  the  intersection  of  each 
dip  line  with  the  surface  profile  draw  a  line  at  right  angles  and  extend  it  until  it 
intersects  the  dip  lines  on  either  side.  The  thickness  of  the  beds  between  any  two 
observed  exposures,  as  A  and  B,  will  be  equal  to  one-half  the  sum  of  the  lines  inter- 
sected between  the  dip  lines  above  and  below  the  profile;  that  is, 

Thickness  of  beds  A  to  B  =  —    -  • 


Thickness  of  beds  C  to  D  =  --  ~-  ,  etc. 

These  values  can  be  scaled  off  directly  from  the  diagram.     The  construction  is 
based  on  the  assumption  that  the  dip  varies  uniformly  from  A  to  B,  C  to  D,  etc., 


9' 


FIG.  85.  —  Diagram  illustrating  determination  of  thickness  of  beds 
by  construction  method.     (After  Hayes.) 

which  may  or  may  not  be  the  case.  Moreover,  the  results  are  too  large  if  the  ob- 
served dips  are  at  different  elevations  and  converge  downward,  and  they  are  too 
small  if  they  diverge.  Thus  in  the  section  represented  by  Fig.  85  the  thicknesses  will 


STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS     163 


be  approximately  correct  from  A  to  E,  too  small  from  E  to  F,  and  too  large  from  F 
to  G.  The  method  is  applicable  therefore  only  where  the  profile  is  approximately 
horizontal  and  should  be  employed  only  where  the  exposures  are  not  sufficient  for 
accurate  measurement. 

Determination  of  depth  of  beds.  —  It  is  frequently  necessary  to  determine  in 
the  field  the  depth  of  a  particular  bed  or  horizon  at  a  distance  from  its  outcrop,  or 
to  determine  the  distance  from  the  outcrop  at  which  a  coal  bed  or  oil  sand  reaches 
a  given  depth.  The  problem  may  be  solved  by  graphic  or  trigonometric  methods. 

The  graphic  method  involves  the  construction  of  a  section  at  right  angles  to  the 
strike.  Dips  are  plotted  on  the  profile  drawn  to  scale  and  showing  the  thicknesses  of 
intervening  beds  as  determined  by  the  methods  given  in  paragraphs  1  to  8  above. 


FIG.  86.  — Diagram  illustrating  determination  of  depth  of  beds 
by  trigonometric  method.     (After  Hayes.) 

The  depth  of  a  bed  at  any  point,  or  the  distance  from  the  outcrop  at  which  any  bed 
reaches  a  given  depth,  can  then  be  scaled  off  directly  from  the  section. 

By  the  trigonometric  method  three  cases  occur:  (1)  where  the  surface  below 
which  the  depth  is  to  be  determined  is  horizontal;  (2)  where  the  surface  slopes  and 
the  beds  dip  into  the  slope;  and  (3)  where  the  surface  slopes  and  the  beds  dip  with 
the  slope.  The  three  cases  are  shown  in  Fig.  86,  from  which  it  is  seen  that: 


(1)  Depth  of  bed  Aa  at  B  =  Ba  = 

(2)  Depth  of  bed  Bb  at  C  =  Cb  = 

(3)  Depth  of  bed  Cc  at  D  =  DC  = 


AB  X  tan  BAa. 
BC  XsinCBb 

cos  eBb 
CD  X  swDCc 

cosfCc 


and  depth  of  bed  Aa"  at  D  =  Da"  =  Da  +  Cb  +  DC. 

In  this  figure  AB,  BC,  and  CD  are  the  surface  distances  normal  to  the  strike  of 
the  beds;  BAa,  EBb,  and/Cc  are  the  dip  angles;  CBb  is  the  sum,  and  DCc  is  the 
difference  of  dip  and  slope  angles. 

For  convenience  in  determinations  where  the  surface  is  approximately  horizontal, 
a  table  giving  depths  of  a  bed  for  various  angles  of  dip  and  distances  from  outcrop  is 
given  below." 

The  following  table,  condensed  from  Hayes,  gives  the  depth  of  a  stra- 
tum below  horizontal  surface  for  various  distances  and  depths.  For 
intermediate  distances  between  those  given,  interpolations  can  be 
easily  made. 


164  ENGINEERING  GEOLOGY 

DEPTH  OF  STRATUM  BELOW  HORIZONTAL  SURFACE  FOR  VARIOUS  DISTANCES  AND  DIPS 


Dip  angle, 
degrees 

Feet 

J  mile 
(1320  ft) 

Smile 
(2640  ft) 

1  mile 
(5280  ft) 

100 

200 

500 

1000 

1 

1.75 

3.50 

8.75 

17.5 

23.04 

46.08 

92.16 

2 

3.49 

6.98 

17.45 

34.9 

46.09 

92.18 

184.4 

3 

5.24 

10.48 

26.20 

52.4 

69.18 

138.4 

276.7 

4 

6.99 

13.98 

34.95 

69.9 

92.30 

184.6- 

369.2 

5 

8.75 

17.50 

43.75 

87.5 

115.5 

230.5 

461.9 

6 

10.51 

21.02 

52.55 

105.1 

138.7 

277.4 

555.0 

7 

12.28 

24.56 

61.40 

122.8 

162.1 

324.2 

648.3 

8 

14.05 

28.10 

70.20 

140.5 

185.5 

371.0 

742.0 

9 

15.84 

31.68 

79.20 

158.4 

209.1 

418.2 

836.3 

10 

17.63 

35.26 

88.15 

176.3 

232.8 

465.6 

931.0 

11 

19.44 

38.88 

97.20 

194.4 

256.6 

513.2 

1026 

12 

21.26 

42.52 

106.30 

212.6 

280.6 

561.2 

1123 

13 

23.09 

46.18 

115.45 

230.9 

304.7 

609.4 

1219 

14 

24.93 

49.86 

124.65 

249.3 

329.1 

658.2 

1316 

15 

26.80 

53.60 

134.00 

268.0 

353.7 

707.4 

1415 

16 

28.68 

57.36 

143.40 

286.8 

378.5 

757.0 

1514 

17 

30.57 

61.14 

152.85 

305.7 

403.6 

807.2 

1614 

18 

32.49 

64.98 

162.45 

324.9 

428.9 

857.8 

1716 

19 

34.43 

68.86 

172.15 

344.3 

454.3 

908.6 

1817 

20 

36.40 

72.80 

182.00 

364.0 

480.4 

960.8 

1923 

21 

38.39 

76.78 

191.95 

383.9 

506.7 

1012 

2027 

22 

40.40 

80.80 

202.00 

404.0 

533.3 

1067 

2133 

23 

42.45 

84.90 

212.25 

424.5 

560.3 

1121 

2241 

24 

44.52 

89.04 

222.60 

445.2 

587.7 

1175 

2351 

25 

46.63 

93.26 

233.15 

466.3 

615.5 

1231 

2462 

26 

48.77 

97.54 

243.85 

487.7 

643.7 

1287 

2575 

27 

50.95 

101.90 

254.75 

509.5 

672.6 

1345 

2690 

28 

53.17 

106.34 

265.85 

531.7 

701.8 

1404 

2807 

29 

55.43 

110.86 

277.15 

554.3 

731.7 

1463 

2927 

30 

57.74 

115.48 

288.70 

577.4 

762.1 

1524 

3048 

Where  the  slope  is  gentle  and  great  accuracy  is  not  required,  this  table  may  be 
used  by  adding  to  the  depths  given  the  difference  in  elevation  between  the  outcrop 
and  the  point  at  which  the  depth  is, desired,  the  difference  in  elevation  being 
positive  when  this  point  is  higher  than  the  outcrop  and  negative  when  it  is  lower. 
The  errors  will  generally  be  well  within  the  limits  of  accuracy  of  measurement,  and 
the  formulae  given  above  need  not  be  employed  except  with  steep  slopes." 

JOINTS 

Introduction.  —  All  hard  and  firm  rocks,  regardless  of  kind,  are 
traversed  by  fractures  called  joints.  These  may  be  observed  in  almost 
any  natural  or  artificial  exposure  of  hard  rock,  and  constitute  division 
planes  which  separate  the  rock  into  large  and  small  blocks  of  regular  or 
irregular  shape.  Jointed  structure  is  of  importance  in  many  ways 
because  of  its  relation  to  quarrying  and  general  engineering  operations. 

General  characters.  —  Joints  traverse  the  rocks  in  different  direc- 
tions and  at  various  angles,  and  in  most  areas  at  least  two  systems 
are  observed  (Plate  XXII,  Fig.  1),  the  fractures  of  each  system  being 


PLATE  XXII,  FIG.  1.  —  Limestone  showing  horizontal  bedding,  and  one  set  of 
vertical  joints.  The  flat  face  of  the  quarry  is  a  joint  surface  of  a  second  set. 
Cement  rock  quarry,  Milwaukee,  Wis.  (H.  Ries,  photo.) 


FIG.  2.  —  Faulted  pegmatite  dike  in  granite,  near  Boulder,  Colo. 


(H.  Ries,  photo.) 
(165) 


166  ENGINEERING  GEOLOGY 

roughly  parallel  to  each  other,  but  in  regions  of  great  disturbance  three 
or  more  sets  of  joints  are  not  uncommon.  The  spacing  of  joints  of  a 
single  set  may  vary,  being  measurable  at  times  in  yards,  at  others  in 
inches,  and  this  is  a  matter  of  practical  importance  since  it  governs  the 
sizes  of  dimension  blocks  that  can  be  extracted  from  a  given  quarry. 

Joints  may  be  either  vertical  or  horizontal,  and  even  intermediate 
positions  are  not  uncommon.  In  igneous  rocks,  horizontal  joints  are 
sometimes  mistaken  for  stratification  planes.  (See  granite,  Chapter  XI.) 
The  best  joint  exposures  are  commonly  seen  on  vertical  surfaces,  for  on 
horizontal  ones  the  overlying  mantle  of  residual  clay  or  other  uncon- 
solidated  material  may  often  conceal  them. 

Some  joints  are  closed,  others  are  open,  and  in  some  cases  they  are 
widened  by  solution,  this  being  especially  true  of  joints  in  limestones. 
In  such  cases  they  become  less  and  less  conspicuous  when  followed 
downward.  In  many  quarries  much  stone  bordering  the  joints  some- 
times has  to  be  rejected  because  of  its  unsound  or  weathered  character. 

Classification  of  joints.  —  Joints  are  sometimes  grouped  into 
(1)  strike  joints,  and  (2)  dip  joints  to  indicate  their  parallelism  in  direc- 
tion to  the  dip  and  strike  of  beds.  They  may  also  be  classified  as  ten- 
sion and  compression  joints  to  indicate  their  relation  to  stresses;  or 
as  major  joints  of  some  persistence,  and  .minor  joints  of  short  extent. 

Joints  in  sedimentary  rocks.  —  There  are  usually  developed  in 
bedded  rocks  two  systems  of  joints  intersecting  each  other  at  approxi- 
mately right  angles,  and  perpendicular  to  the  bedding  planes.  They 
may  be  of  slight  development  and  confined  to  individual  beds  or  they 
may  be  extensive  and  traverse  a  series  of  beds  of  considerable  thickness. 
They  frequently  end  at  the  contact  of  two  unlike  rocks;  thus  joints 
which  traverse  limestone  or  sandstone  may  end  where  shale  begins. 

Joints  in  igneous  rocks.  —  Joints  in  igneous  rocks  frequently  show 
less  regularity  than  those  in  sedimentary  ones,  and  their  arrangement 
at  times  is  very  irregular.  In  igneous  rocks  like  granite,  which  have 
extensive  use  as  building  stone,  two  systems  of  joints,  a  vertical  set  and 
a  horizontal  set,  and,  in  many  places,  a  third  or  diagonal  set,  are  de- 
veloped. These  may  be  widely  spaced  or  closely  spaced.  Consider- 
able variation  is  noted  in  the  development  of  the  vertical  joints,  which 
are  conspicuously  developed  in  most  cases  but  may  be  few  and  scarcely 
visible  in  others. 

Horizontal  joints  which  divide  the  rock  into  sheets  are  frequently 
strongly  developed  in  granite,  and  are  usually  parallel  to  the  rock  sur- 
face. In  flat  surface  exposures  they  approach  a  horizontal  position;  in 
gently  arched  exposures  they  have  approximately  the  same  degree  of 


STRUCTURAL  FEATURES  AND   METAMORPHISM   OF   ROCKS     167 

curvature  as  that  of  the  rock  surface;  and  in  steep  domes  they  are  cor- 
respondingly steep,  observing  parallelism  with  the  doming  surface. 
They  are  usually  more  conspicuous  at  and  near  the  surface,  and  become 
less  prominent  below.  Ordinarily  they  separate  the  rock  into  thinner 
sheets  at  or  near  the  surface,  and  into  thicker  sheets  at  greater  depth. 
(Plate  XXIX,  Fig.  1.) 

In  dense  and  compact  igneous  rocks  like  basalt,  which  occur  in  dikes 
and  lava  flows  (sheets),  there  is  often  developed  a  regular  form  of  pris- 
matic jointing  known  as  columnar  structure,  as  shown  in  Plate  VIII. 
The  columns  may  be  vertical  or  horizontal,  sometimes  bent  and  curved, 
and  may  vary  greatly  in  size  (length  and  diameter). 

Joints  in  Relation  to  Engineering  Work 

Few  people  perhaps  realize  the  important  bearing  which  joints  have 
on  engineering  problems,  hence  we  refer  to  some  of  the  important  ones 
below. 

Quarrying  operations.  —  Joints  facilitate  the  extraction  of  stone, 
and  the  quarrying  of  some  hard  ones  like  granite  would  be  considerably 
increased  were  it  not  for  their  presence;  but,  while  of  benefit  on  the 
one  hand,  they  will  on  the  other  serve  to  limit  the  size  of  the  dimension 
blocks  that  can  be  extracted.  An  otherwise  good  stone  may  be  so 
broken  up  by  jointing,  as  to  be  useless  for  any  purpose  except  road 
material.  Joints  also  permit  the  entrance  of  surface  water,  which  in 
some  cases  causes  more  or  less  weathering  of  the  rock  along  them. 

Rock  slides.  —  In  unsupported  rock  masses,  outcropping  on  hill- 
sides, or  exposed  hi  the  sides  of  quarries  or  underground  workings,  the 
joints  sometimes  act  as  slipping  planes,  causing  slides.  If  the  water 
gets  into  the  joint  cracks  and  freezes  the  action  is  sometimes  hastened. 

Reservoir  construction.  —  Since  joints  serve  as  water  passages, 
engineers  constructing  dams  or  reservoirs  should  see  that  where  the 
masonry  work  joins  the  rock,  the  joints  are  not  sufficiently  numerous 
to  permit  leakage.  Grouting  is  sometimes  necessary  to  close  them  up. 
Very  often,  the  joints  are  more  numerous  and  open  close  to  the  sur- 
face than  they  are  at  greater  depth.  The  danger  here  mentioned 
becomes  most  serious  in  limestone  formations  (Chapter  IV). 

Water  supply.  —  In  regions  of  igneous  and  metamorphic  rocks, 
that  are  usually  dense,  any  supply  of  underground  water  must  collect 
almost  exclusively  in  joint  fissures,  which  often  form  easy  channel 
ways  for  their  circulation.  But  even  here  there  are  limitations  as  to 
depth  to  which  we  may  obtain  a  reasonable  water  supply  (Chapter  VI). 


168  ENGINEERING  GEOLOGY 

For  example,  it  has  been  shown  in  the  Piedmont  region  of  crystalline 
rocks  that  the  conditions  favorable  to  water  supply  lessen  rapidly  be- 
low 250  or  300  feet,  for  the  reason  that  the  joints  above  this  depth  are 
more  open. 

Ore  deposits.  —  Because  joints  sometimes  serve  as  channels  for 
underground  waters  they  are  at  times  of  importance  as  structures 
(spaces)  for  the  deposition  of  mineral  matter  and  the  formation  of 
mineral  veins.  Recognition  of  this  occasional  relation  of  ore  veins  to 
joints  has  in  some  instances  facilitated  the  development  of  the  ore,  or 
search  for  further  ore  bodies  in  those  districts  where  it  applies. 

FAULTS 

Definition.  —  A  fault  may  be  denned  as  a  fracture  in  the  rocks 
along  which  displacement  of  one  side  with  respect  to  the  other  has 
taken  place,  parallel  to  the  fracture.  The  amount  of  displacement  may 
vary  from  a  few  inches  to  many  thousand  feet,  and  the  movement  may 
have  been  of  short  duration,  or  continued  for  a  long  period. 

Significance.  —  Faults  are  not  restricted  to  any  ^roup  or  kind  of 
rocks,  but  may  traverse  all,  and  are  structures  of  fundamental  impor- 
tance in  all  regions  where  they  occur.  They  may,  and  sometimes  do, 
greatly  affect  and  modify  the  surface  topography;  they  frequently  ex- 
ercise an  important  control  on  surface  and  underground  waters;  they 
may  become  fissure  veins  by  filling  and  replacement  along  their  courses, 
and  hence  are  of  great  economic  importance  in  mineralization  or  the 
formation  of  ore  deposits  (see  Chapter  XVII) ;  and  they  may  prove  to 
be  sources  of  great  disaster  in  loss  of  time  and  money,  in  mining  oper- 
ations, unless  properly  interpreted  and  understood. 

Fault  terms.1  —  For  clearness  of  discussion  it  is  desirable  to  have 
terms  to  indicate  the  several  characteristics  of  faults.  These  are  as 
follows : 

A  closed  fault  is  one  in  which  the  two  walls  of  a  fault  are  in  contact. 

An  open  fault  is  one  in  which  the  two  walls  of  a  fault  are  separated. 
The  same  fault  may  be  closed  in  one  part  and  open  in  another. 

The  fault  space  is  the  space  between  the  walls  of  an  open  fault. 

A  fault  surface  is  the  surface  of  a  fracture  along  which  dislocation 
takes  place,  and  if  without  notable  curvature  it  is  called  a  fault  plane 
(Fig.  91). 

1  The  terminology  of  faults  has  been  much  confused,  and  that  used  in  this  work 
has  been  proposed  by  a  committee  appointed  by  the  Geological  Society  of  America 
to  report  on  a  proper  nomenclature  of  faults.  Their  conclusions,  which  will  no 
doubt  be  adopted  by  American  Geologists  are  given  in  full  in  Bull.  Geol.  Soc.  Amer., 
1913,  vol.  24,  pp.  163-186. 


STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS     169 

A  fault  line  is  the  intersection  of  a  fault  surface  wi#i  the  earth's 
surface,  or  with  any  artificial  surface  of  reference,  such  as  the  floor  of  a 
tunnel.  When  a  fault  is  made  up  of  slips  on  closely  spaced  surfaces, 
with  more  or  less  deformation  of  the  intervening  rock,  it  is  called  a 
shear  zone.  The  name  would  also  be  applicable  to  breccia  zones  (Fig. 
88)  which  characterize  some  faults,  especially  those  of  the  thrust  type. 

The  fault  breccia  (Fig.  88)  is  the 
breccia  frequently  found  in  the  shear 
zone,  and  more  especially  in  the  case  of 
thrust  faults.  Gouge  is  the  fine-grained 
impervious  clay-like  material,  which  is 
often  found  between  the  walls  of  a  fault. 
A  horse  (Fig.  87)  is  a  mass  of  rock  broken 
from  one  wall  and  caught  between  the 
walls  of  the  fault. 

The  fault  strike  is  the  direction  of  the 
intersection  of  the  fault  surface,  or  the 
shear  zone,  with  a  horizontal  plane.  The 
fault  dip  (Fig.  91)  is  the  inclination  of 

the  fault  surface,  or  shear  zone,  measured 
,  ,    -  ,      .        .    !      i  T,    FIG.  87. — Section  showing "  horse 

downward  from  a  horizontal  plane.     It  developed  by  faulting. 

is   never  greater  than  90  degrees.     The 

hade  (Fig.  91)  is  the  inclination  of  the  fault  surface,  or  shear  zone, 
measured  from  the  vertical;  it  is  the  complement  of  the  dip.  A  fault 
hades  to  the  side  towards  which  it  dips.  The  hanging  wall  (Fig.  91)  is 
the  upper  wall  of  the  fault.  The  foot  wall  (Fig.  91)  is  the  lower  wall 
of  the  fault. 

A  multiple  fault  is  used  to  designate  a  groiip  of  parallel  faults  of  fairly 
close  spacing,  with  the  intervening  rock  not  distorted.  Shear  zones 
would  not  be  applicable  in  this  case.  An  auxiliary  fault  is  a  minor 
fault  ending  against  the  main  fault.  It  is  often  the  boundary  of  a 
dropped  wedge. 

Criteria  for  faulting.  —  It  is  of  first  importance  perhaps  to  de- 
termine the  existence  of  a  fault,  and  then  to  discover  the  direction  and 
amount  of  the  movement.  Various  criteria  can  be  used,  but  one  alone 
seldom  proves  conclusive,  and  some  may  be  developed  under  conditions 
other  than  faulting.  The  criteria  which  may  be  applied  are:  (1)  Dis- 
placement of  dikes  (Plate  XXII,  Fig.  2),  veins  or  beds;  (2)  breccia- 
tion  along  line  of  fracture  (Fig.  88,  and  Plate  XXIII,  Fig.  1) ;  (3)  stria- 
tions  (slickensides)  on  fracture  surfaces;  (4)  the  presence  of  gouge; 
(5)  the  presence  frequently  of  a  shear  zone  or  division  of  the  rock 


•    V    •   *      .',v   .^•C'jp;  -..v"' 


PLATE  XXIII,  FIG.  1.  —  Fault  in  Ordovician  slates  near  mouth  of  Slate  River,  Va. 
The  two  hammers  mark  boundary  of  fault  breccia.     (T.  L.  Watson,  photo.) 


FIG.  2.  —  View  from  Mount  Stephen,  near  Field,  B.  C.,  looking  towards  pass  at 
Hector.  On  right  slope  are  seen  the  two  ends  of  the  upper  tunnel  crossing 
fault  zone  in  mountain  on  right.  On  extreme  left,  slope  of  Mt.  Ogden,  where 
the  lower  spiral  tunnel  is  in  massive  limestone. 

(170) 


STRUCTURAL  FEATURES  AND  METAMORPHISM   OF  ROCKS     171 


into  slices  parallel  to  the  plane  of  the  fault;  (6)  fault  scarps  (Fig.  91), 
seen  where  faults  are  recent,  and  erosion  has  not  had  tune  to  reduce 
them;  (7)  drainage  lines  sometimes  developed  along  fault  lines. 


FIG.  88.  —  Faulting  accompanied  by 
brecciation. 


FIG.  89.  —  Normal  faulting  showing 
distortion  of  shale. 


It  must  not  be  assumed  that  in  the  field  the  two  walls  of  a  fault 
will  be  found  in  contact  at  the  surface;  in  fact  the  fault  line  may  be 
covered  by  surface  material  and  its  presence  is  determined  from  the 
structural  relationships  of  the  surrounding  outcrops,  on  opposite  sides 
of  the  fault  line. 

General  Classes  of  Faults 

Faults  in  stratified  rocks.  —  Among  stratified  rocks  the  character 
of  the  displacement  of  the  strata  due  to  a  fault  is  so  much  influenced 
by  the  relation  of  the  strike  of  the  fault  to  that  of  the  strata  that  special 
subclasses  may  generally  be  recognized  as  follows:  A  strike  fault  (Fig. 
98)  is  one  whose  strike  is  parallel  to  the  strike  of  the  strata.  A  dip 
fault  is  one  whose  strike  is  approximately  at  right  angles  to  the  strike 
of  the  strata,  or  in  other  words  parallel  to  the  dip.  An  oblique  fault  is 
one  whose  strike  is  oblique  to  the  strike  of  the  strata.  These  terms  are, 
of  course,  not  directly  applicable  in  regions  of  unstratified  rocks;  but 
they  might  be  used  in  such  regions  with  respect  to  the  strike  of  a  system 
of  parallel  dikes  if  this  were  distinctly  stated  in  the  description  of  the 
faults. 

Slip.  —  The  word  "  slip  "  indicates  the  displacement  as  measured 
on  the  fault's  surface;  the  qualifying  words  refer  to  the  strike  and  dip 
of  the  fault.  The  slip  or  net  slip  (Fig.  90)  is  the  distance  measured  on 
the  fault  surface,  between  two  formerly  adjacent  points  situated  re- 
spectively, on  opposite  walls  of  the  fault.  It  would  be  represented  by 
a  straight  line  in  the  fault  surface  connecting  those  two  parts  after  the 
displacement. 


172 


ENGINEERING  GEOLOGY 


The  strike-slip  (Fig.  90)  is  the  component  of  the  slip  parallel  with  the  fault  strike, 
or  the  projection  of  the  net  slip  on  a  horizontal  line  in  the  fault  surface.  The  dip- 
slip  (Fig.  90)  is  the  component  of  the  slip  parallel  with  the  fault  dip,  or  to  the  pro- 
jection of  the  slip  on  a  line  in  the  fault  surface  perpendicular  to  the  fault  strike. 


FIG.  90.  —  Faulted  block  with  parts  named,  ab  =  slip  or  net  slip;  cb  =  dip-slip; 
ac  =  strike-slip;  de  =  net  shift;  fe  =  dip-shift;  fd  =  strike-shift;  gb  =  heave; 
gc  =  throw.  The  fault  movement  is  oblique.  (After  Reid.) 

The  strike-slip  and  the  dip-slip  are  rectangular  components  of  the  net  slip.  The 
trace-slip  is  the  component  of  the  slip  parallel  with  the  trace  of  a  bed,  vein,  or  other 
surface  on  the  fault  plane. 

The  perpendicular  slip  is  the  component  of  the  slip  at  right  angles  to  the  trace  of 
a  bed,  vein,  or  other  surface  on  the  fault  plane.  The  trace-slip  and  the  perpendicu- 
lar slip  are  rectangular  components  of  the  net  slip. 

Shift.  —  It  frequently  happens  that  a  fault  has  not  a  single  surface 
of  shear,  but  consists  of  a  series  of  small  slips  on  closely  spaced  sur- 
faces, and  in  some  faults  the  strata  in  the  neighborhood  of  the  fault 
surface  are  bent,  so  that  the  relative  displacements  of  the  rock  masses 
on  opposite  sides  of  the  fault  may  be  quite  different  from  the  slip  and 
not  even  parallel  with  it.  i  The  word  shift  (Fig.  90)  is  used  to  denote 
the  relative  displacements  of  the  rock  masses  situated  outside  the  zone 
of  dislocation;  the  qualifying  words  relate  to  the  strike  and  dip  of  the 
fault  with  one  exception,  in  which  the  meaning  is  clear. 

The  shift  or  net  shift  (Fig.  90)  denotes  the  maximum  relative  dis- 
placement of  points  on  opposite  sides  of  the  fault  and  far  enough  from 
it  to  be  outside  the  dislocated  zone. 


STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS     173 

The  strike-shift  (Fig.  90)  is  the  component  of  the  shift  parallel  with  the  fault 
strike.  The  dip-shift  (Fig.  90)  is  the  component  of  the  shift  parallel  with  the  fault 
dip. 

The  bending  of  the  strata  near  the  fault  may  be  so  great  that  the  direction  of 
the  shift  is  no  longer  even  nearly  parallel  with  the  fault  surface;  it  is  better,  then, 
to  use  the  three  following  components  of  the  shift:  The  strike-shift  denotes  the 
horizontal  component  of  the  shift  parallel  with  the  fault  strike,  as  already  defined. 
The  normal  shift  denotes  the  horizontal  component  of  the  shift  at  right  angles  to  the 
fault  strike.  It  equals  the  horizontal  shortening  or  lengthening  of  the  earth's  sur- 
face at  right  angles  to  the  fault  strike,  due  to  the  fault.  The  vertical  shift  denotes 
the  vertical  component  of  the  shift.  These  components  of  the  shift  may  evidently 
be  used  when  the  shift  is  parallel  with  the  general  trend  of  the  fault  surface. 

Throw  and  heave.  —  Throw  (Figs.  90  and  91)  is  the  vertical  dis- 
tance between  corresponding  lines  in  the  two  fracture  surfaces  of  a 
disrupted  stratum,  etc.,  measured  in  a  vertical  plane  at  right  angles  to 
the  fault  strike. 

By  perpendicular  throw  is  meant  the  distance  between  the  two  parts  of  the  dis- 
rupted bed,  etc.,  measured  perpendicularly  to  the  bedding  plane  or  to  the  plane  of 
the  surface  in  question.  Special  terms  applied  to  perpendicular  throw  are:  Strati- 
graphic  throw  (Figs.  92  and  93),  the  distance  between  the  two  parts  of  a  disrupted 
bed  measured  at  right  angles  to  the  plane  of  the  bed;  and  dip  throw,  the  component 
of  the  slip  measured  parallel  with  the  dip  of  the  strata. 

Heave  (Figs.  90  and  91)  is  the  horizontal  distance  between  cor- 
responding lines  in  the  two  fracture  surfaces  of  a  disrupted  stratum, 
etc.,  measured  at  right  angles  to  the  fault  strike. 


d 


r 

FIG.  91.  —  Normal  fault  in  horizontal  beds,  ss,  surface;  ff',  fault  plane;  db,  upthrow 
side;  dc,  downthrow  side;  cba,  angle  of  hade  or  slope;  cbo,  angle  of  dip;  ab, 
throw  (also  stratigraphic  throw  in  this  case);  ac,  heave  (hprizontal  throw); 
left  side  of  ff',  foot  wall;  right  side  of  ff',  hanging  wall;  fbc,  fault  scarp;  /"/'", 
fault  plane  (vertical). 

The  words  throw  and  heave  are  essential  elements  of  a  fault.  For 
example,  if  a  fault  were  encountered  when  a  coal  seam  was  being 
worked,  it  would  be  important  to  know  how  far  a  drift  should  be  run 


174 


ENGINEERING  GEOLOGY 


horizontally,  and  how  far  a  shaft  should  be  opened  vertically  to  reach 
the  other  part  of  the  disrupted  seam. 

Offset.  —  This  is-  the  distance  between  the  two  parts  of  the  dis- 
rupted stratum  measured  at  right  angles  to  the  strike  of  the  stratum, 
and  on  a  horizontal  plane.  The  term  heave  has  been  used  by  some  for 
offset. 

Classification  of  faults  according  to  direction  of  movement. — 
Faults  may  be  classified,  according  to  the  direction  of  movement  on  the 
fault  plane,  into  the  following:  Dip-slip  faults,  where  the  net  slip  is 
practically  in  the  line  of  the  fault  dip.  Strike-slip  faults,  where  the  net 


FIG.  92.  —  Normal  fault  hading 
against  dip  of  beds,  ab,  throw 
(vertical);  be,  stratigraphic 
throw;  others  same  as  Fig.  91. 


FIG.  93.  —  Normal  fault  hading  with 
dip  of  beds,  ab,  throw  (vertical);  oc 
stratigraphic  throw;  others  same  as 
Fig.  91. 


slip  is  practically  in  the  direction  of  the  fault  strike.     Oblique-slip  faults, 
where  the  net  slip  lies  between  these  directions. 

Strike  faults.  —  Most  geological  textbooks  and  books  on  field 
methods  have  confined  themselves  almost  exclusively  to  the  discussion 
of  dip-slip  faults,  and  have  given  little  attention  to  the  other  two  classes. 


FIG.  94.  —  Section  showing  distributive  or 
step  faulting. 


FIG.  95.  — Section  showing 
reverse  fault. 


Normal  faults  (Figs.  91  to  94),  where  the  hanging  wall  has  been 
depressed  relatively  to  the  foot  wall. 

Reverse  faults  (Fig.  95),  where  the  hanging  wall  has  been  raised 
relatively  to  the  foot  wall. 


STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS     175 

Overthrusts  are  reverse  faults  with  low  dip  or  large  hade.  In  some 
cases  the  dip-slip  has  been  enormous,  amounting  to  tens  of  kilometers. 
Scott  calls  them  "thrusts"  and  separates  them  entirely  from  faults  of 


/  C 

FIG.  96.  —  Sections  showing  development  of    fault,  of    either  normal  or  reverse 
character.     A,  unfractured  beds;  B,  normal  fault;  C,  reverse  fault. 

high  dip;  but  the  word  "thrust"  has  been  used  for  ordinary  reverse 
faults  of  high  dip.  The  word  "overthrust"  has  been  very  generally 
used  for  this  kind  of  fault  and  is  very  descriptive.  It  should  be  adopted. 

Vertical  faults,  where  the  dip  is  90  degrees  (Fig.  91). 

The  relative  displacement  has  usually  been  determined  by  means  of 
a  dislocated  bed.  The  horizontal  distance  between  two  points  on 
opposite  sides  of  a  fault,  measured  on  a  line  at  right  angles  to  the  fault 
strike,  is  always  shortened  by  a  reverse  strike  fault,  lengthened  by  a 
normal  strike  fault,  and  unchanged  in  length  by  a  vertical  fault. 

The  expressions  "normal"  and  "reverse"  may  be  used  in  connection 
with  oblique  and  dip  faults,  even  when  they  are  strike-slip  or  oblique- 
slip  faults,  provided  they  are  applied  to  designate  the  apparent  relative 
displacement  of  the  two  parts  of  a  dislocated  stratum,  or  other  recog- 
nized surface,  in  a  vertical  plane  at  right  angles  to  the  fault  strike.  It 
very  frequently  happens  that  nothing  more  than  the  apparent  dis- 
placement of  the  strata  can  be  determined,  and  we  recommend  the 
terms  "normal"  and  "reverse"  faults  as  denned  be  used  purely  for 
purposes  of  description  and  not  for  the  purpose  of  indicating  extension 
or  contraction,  tension  or  compression,  vertical  or  horizontal  forces. 

Special  classes  of  faults.  —  There  are  two  classes  of  faults  which  have  played 
such  important  roles  in  altering  the  structure  of  some  regions  that  they  have  re- 
ceived special  names. 

Flaws.  —  A  term  applied  by  Suess  to  certain  faults  in  which  the  strike  is  trans- 
verse to  the  strike  of  the  rocks,  the  dip  high  and  varying  from  one  side  to  the  other, 
in  the  course  of  the  fault,  and  the  relative  movement  practically  horizontal  and 
parallel  with  the  strike  of  the  fault. 


PLATE  XXIV,  FIG.  1  —  Diagram  illustrating  trough  faults. 


FIG.  2.  —  Strike  fault  section,  hading  with  dip;  cuts  out  some  beds  at  surface. 


FIG.  3.  —  Fault  showing  change  of  dip. 


FIG.  4.  —  Faulting  of  an  unconformable  series  of  beds  showing  age  of  fault. 


(176) 


Flo.  5.  —  Strata  repeated  by  faulting. 


STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS     177 

Movements  on  faults.  —  These  may  be  classified,  according  to  the  character  of 
the  local  displacement,  into  translatory  and  rotatory  movements. 

Translatory  movements  are  those  in  which  all  straight  lines  on  opposite  sides  of 
the  fault,  and  outside  the  dislocated  zone,  which  were  parallel  before  the  displace- 
ment, are  parallel  afterwards.  If  at  a  later  date,  or  even  at  the  time  of  the  dis- 
placement, the  whole  region  were  tilted,  the  movement  would  still  be  considered  .a 
translatory  movement,  so  far  as  the  fault  is  concerned. 

Rotatory  movements  are  those  in  which  some  straight  lines  on  opposite  sides  of  the 
fault  and  outside  the  dislocated  zone,  parallel  before  the  displacements,  are  no 
longer  parallel  after  it  —  that  is,  where  one  side  of  the  part  of  the  fault  under 
consideration  has  suffered  a  rotation  relative  to  the  other  side. 

No  faults  of  any  magnitude  exhibit  merely  translatory  movements  over  their 
whole  lengths.  Faults  die  out  and  the  displacement  is  not  uniform  along  them,  so 
that  there  is  necessarily  some  slight  rotation,  varying  in  amount  in  different  parts 
of  the  fault's  course.  If  we  confine  our  attention  to  a  small  part  of  the  fault,  we  may 
describe  the  displacement  there  as  though  the  rock  were  rigid;  and  if  the  rotation 
is  very  small,  as  if  a  translatory  displacement  had  occurred,  and  for  conciseness  we 
may  use  the  terms  translatory  fault,  or  rotatory  fault,  to  describe  the  part  under 
consideration. 

Effect  of  Faults  on  the  Outcrop 

The  effect  of  faults  on  the  outcrop  (surface)  may  be  of  two  kinds: 
(1)  Topographic,  and  (2)  geologic. 

Topographic  effects.  —  The  expression  of  faults  at  the  surface  may 
be  shown  in  escarpments,  distribution  of  rocks  of  unequal  resistance, 
drainage  lines,  etc.  Faults  frequently  exhibit  no  surface  expression,  so 
that  their  existence  might  not  be  suspected.  This  is  apt  to  be  the  case 
in  faults  which  have  but  slight  displacement,  or  in  those  having  origi- 
nally moderate  or  great  displacement  resulting  in  the  formation  of  a 
well-defined  fault  scarp,  erosive  processes  having  reduced  the  scarp  side 
to  an  approximate  common  level  with  the  opposite  side.  In  many  cases, 
however,  a  scarp  that  is  of  gentle  or  steep  slope  and  of  moderate  or 
considerable  height,  dependent  upon  the  hade  and  amount  of  the  dis- 
placement, results  from  faulting.  The  Hurricane  fault  and  the  faults  of 
the  Basin  ranges  are  among  the  best  examples  of  faults  showing  escarp- 
ments. A  sequence  of  surface  forms  may  develop  during  the  progress 
of  erosional  work,  until  the  scarp  is  finally  obliterated  upon  completion 
of  the  cycle  of  erosion. 

Again  faults  may  bring  together  rocks  of  markedly  different  or  un- 
equal resistance,  so  that  the  more  resistant  rocks  will  rise  above  the 
softer,  forming  a  belt  of  higher  ground,  the  margin  of  which  is  marked 
by  the  line  of  dislocation.  The  juxtaposition  of  a  hard  and  soft  rock  is 
not  always  proof  of  faulting,  for  we  might  have  a  soft  limestone  inter- 
bedded  normally  between  two  hard  sandstone  formations.  If  these 
had  a  steep  dip,  a  depression  might  be  worn  in  the  limestone,  while  the 
resistant  sandstones  remained  as  bordering  ridges  on  either  side. 


178 


ENGINEERING  GEOLOGY 


The  courses  of  faults  are  sometimes  marked  by  lines  of  springs; 
also  they  may  become  lines  of  control  for  surface  drainage,  the  erosion 
along  them  developing  valleys. 


FIG.  97.  —  Plan,  illustrating  shifting  of 
beds  by  faulting. 


FIG.  98.—  (A)  Plan  of  strike  fault  show- 
ing repetition  of  beds  at  surface,  ff, 
fault.  (B)  Section  along  line  ab 
normal  to  strike  fault  showing  re- 
petition of  beds. 


Geologic  effects.  —  Faults  may  produce  various  complications  in 
the  outcrops  of  the  rock  at  the  surface. 

Strike  faults  may  repeat  a  given  layer  or  bed  at  the  surface  (Fig.  98) 
or  may  eliminate  or  cut  it  out  altogether  (Plate  XXV,  Fig.  2),  de- 
pendent upon  whether  the  downthrow  is  against  or  in  the  direction  of 
the  dip  of  the  beds.  Dip  faults  cause  horizontal  shift  of  the  outcrops, 
either  forward  or  backward,  according  to  the  direction  of  downthrow 
(Plate  XXV,  Fig.  3).  Oblique  faults  result  in  offset  with  overlap  if  the 
downthrow  is  to  the  left  (Plate  XXV,  Fig.  4) ,  or  offset  with  gap,  "if  the 
downthrow  is  to  the  right  (Plate  XXV,  Fig.  5) .  The  amount  of  over- 
lap and  gap  increases  with  increase  of  throw  and  hade,  and  decreases 
with  increase  of  dip. 

A  fault  which  crosses  a  fold  at  right  angles  to  its  axis  changes  the 
distance  between  the  outcrop  of  a  given  bed  on  opposite  sides  of  the 
fault;  the  distance  being  decreased  on  the  upthrow  side  of  a  syncline 
(Fig.  99),  and  increased  on  the  upthrow  side  of  an  anticline. 

Various  other  complications  arise  under  different  conditions,  but 
these  will  serve  to  indicate  the  effect  on  outcrop  which  may  result  from 
some  of  the  common  kinds  of  faulting. 


STRUCTURAL  FEATURES  AND  METAMORPHISM   OF  ROCKS     179 


FIG.  1 


FIG.  2 


FIG.  3 


FIG.  4 


FIG.  5 


PLATE  XXV.  —  Diagram  showing  effects  of  different  kinds  of  faults  on  block 
with  monoclinal  structure  and  one  coal  bed.  Fault  fissure,  /.  The  block  is 
supposed  to  have  been  worn  off  in  each  case  after  faulting.  FIG.  1.  —  Repe- 
tition of  beds  by  normal  strike  fault  hading  in  opposite  direction  from  dip. 
FIG.  2.  —  Cutting  out  of  bed  by  strike  fault  hading  in  same  direction  as  dip. 
FIG.  3.  —  Horizontal  separation  of  bed,  by  dip  fault  whose  downthrow  side  is 
on  farther  size  of  fault  plane.  FIG.  4.  —  Overlapping  of  bed  by  oblique  fault. 
FIG.  5.  —  Separation  of  bed  by  oblique  fault.  (Chamberlin  and  Salisbury.) 


180 


ENGINEERING  GEOLOGY 


FIG.  99.  —  Diagram  showing  effect  of  faulting  on  the  outcrops  of  a  syncline.    (From 
Chamberlin  and  Salisbury,  College  Geology.) 

Relation    between    faults    and    folds.  —  From   earth   movements 
which  result  in  over-intense  folding,  folds  may  pass  into  faults  both 

B 


FIG.  100.  —  (a)  Stepfold,  showing  break  in  the  massive  limestone  bed  which  de- 
termines the  plane  of  the  break-thrust,  (6)  along  which  the  displacement  re- 
sults from  further  compression.  (Willis.) 

vertically  and  horizontally.     Beds  involved  in  such  cases  often  show 
thickening  and  thinning,   stretching  and  shortening.     Frequently  in 


FIG.  101.  —  Fold  passing  into  a  fault.     (Van  Hise.) 

monoclinal  folds,  these  may  pass  into  a  fault  when  followed  along  the 
strike.     This  may  be  because  the  fold  is  so  strongly  compressed  or 


STRUCTURAL  FEATURES  AND  METAMORPHISM   OF  ROCKS     181 

drawn  out  that  the  flexure  disappears  and  a  fault  takes  its  place. 
Thus  in  the  Kaibab  fault  of  the  high  plateaus  of  Utah,  a  normal  fault 
grades  along  the  strike  into  a  monocline. 

In  the  southern  Appalachians  overthrust  faults  are  frequently  found 
associated  with  overthrust  folds;  also  there  may  be  found  in  the  same 
region  excellent  examples  of  distributive  faults  associated  with  minute 
overthrust  folds. 


Relation  of  Faulting  to  Engineering  Work 

Faulting  is  not  an  uncommon  phenomenon  in  many  regions  of  dis- 
turbed rocks.  It  causes  engineers  trouble  not  only  for  the  reason  that  it 
has  in  the  past  disturbed  the  rock  formations,  but  sometimes  because 
fault  movements  take  place  at  the  present  day.  Several  cases  may  be 
noted. 

Tunneling.  —  The  importance  of  having  firm  solid  rock  to  tunnel 
through  is  well  recognized,  not  only  as  a  matter  of  safety  and  con- 
venience in  working,  but  for  easy  maintenance  after  the  tunnel  is  com- 
pleted. If,  therefore,  a  rock  which  has  been  pierced  by  a  tunnel  is  much 
shattered  by  faulting,  it  becomes 
necessary  to  line  the  same,  at  least 
in  the  crushed  territory.  Further- 
more, if  the  fault  fissure  extends 
to  the  surface,  it  may  serve  as  a 
channel  way  for  rain  waters. 

A  most  interesting  case  was  that 
developed  on  the  line  of  the  Cana- 
dian Pacific  Railway  between  the 
summit  of  the  pass  at  Hector, 
B.  C.,  and  Field,  B.  C.,  in  the 
valley  of  the  Kicking  Horse  River. 
In  order  to  reduce  the  grade  be-  / 

tween  these  points,  the  road  was  ^  102.- Section  showing  relation  of 
lengthened  and  two  spiral  tunnels  tunnel  to  fault  zone, 

were  constructed.     The  upper  one 

of  these  was  in  the  quartzite  of  Cathedral  Mountain  on  the  south  side  of 
the  valley  (Plate  XXIII,  Fig.  2),  while  the  other  was  in  the  limestone  of 
Mt.  Ogden  on  the  north  side.  Now  it  happens  that  a  fault  of  nearly  3000 
feet  displacement  passes  between  Cathedral  Mountain  and  Mt.  Stephen 
to  the  west  of  it,  and  the  upper  tunnel  lies  partly  within  the  shear  zone  of 
this  fault.  This  has  given  much  trouble  first,  because  of  the  shattered 


182 


ENGINEERING  GEOLOGY 


PLATE  XXVI.  —  Sections  to  illustrate  development  of  overthrust  folding  and 
faulting,  accompanied  by  minor  drag  folds,  as  inferred  from  Alpine  structure. 
(After  Heim,  from  Leith's  Structural  Geology.) 


STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS     183 

character  of  the  rock,  which  necessitated  lining  the  tunnel,  and  second, 
because  of  the  surface  water  which  ran  down  along  this  fissured  zone. 
The  lower  tunnel  in  the  massive  limestone  of  Mt.  Ogden  is  free  from 
these  annoyances.1- 

Another  interesting  case2  is  that  of  a  tunnel  at  Franklin,  Cal.,  which 
follows  the  soft  clay  gouge  of  a  thrust  fault.  The  tunnel  is  timbered, 
but  the  swelling  of  the  wet  clay  dislodges  the  wooden  supports. 
A  geological  examination  of  the  ground  at  the  time  of  the  railroad 
survey  might  have  avoided  this. 

Aqueducts.  —  In  aqueduct  construction  engineers  have  had  to 
deal  not  only  with  past  but  present  faulting.  In  the  selection  of  a 
route  for  the  new  Catskill  aqueduct,  in  New  York  state,3  much  of  the 
construction  was  tunnel  work,  especially  where  it  became  necessary  to 
cross  under  river  valleys  with  inverted  syphons.  Consequently,  in  the 
selection  of  a  route  which  would  insure  solid  rock  for  as  great  a  distance 
as  possible,  much  attention  was  given  to  the  occurrence  of  faults,  which 
might  have  shear  zones  of  variable  width.  Such  lines  of  fracture  were 
encountered  at  several  points. 

At  times  the  existence  of  faults  can  be  inferred  or  even  definitely  de- 
termined from  drill  records.  If,  for  example,  hi  boring,  the  beds  en- 
countered are  in  an  order  which  is  known  not  to  be  the  normal  one  for 
the  deposits  of  that  region,  and  the  drill  also  strikes  crushed  or  brecciated 
zones,  faulting  may  be  inferred.  Occasionally  the  drill  on  meeting 
these  fault  fissures  is  deflected.4 

In  parts  of  California  where  fault  lines  are  known  to  exist,  as  from 
San  Francisco  southward,  it  is  well  known  that  movement  along  some 
of  these  is  recurring  at  not  widely  separated  intervals. 

The  movement  which  produced  the  San  Francisco  earthquake  took 
place  along  a  fissure  traceable  for  at  least  250  miles,  and  although  hav- 
ing a  small  horizontal  displacement  (8  to  20  feet)  did  considerable 
damage.  Pipe  lines  which  crossed  the  fracture,  and  in  one  case  a  water 
supply  tunnel  connecting  two  lakes,  were  broken. 

The  recently  completed  Los  Angeles  aqueduct,  which  is  to  bring  water 
from  Owens  Lake  to  Los  Angeles,  Cal.,  must  of  necessity  cross  some  of 
these  fault  lines,  and  provision  has  been  made  to  keep  repair  parts  near 
these  lines  of  fracture  for  ready  use  hi  case  any  movement  occurs  along 
them  in  the  future. 

1  Oral  communication  from  Prof.  J.  A.  Allan. 

a  Oral  communication  from  Prof.  A.  C.  Lawson. 

8  Berkey,  N.  Y.  State  Museum,  Bull.  146,  1911. 

«  Berkey,  N.  Y.  State  Museum,  Bull.  146,  p.  166,  1911. 


184  ENGINEERING  GEOLOGY 

Earthquakes.  —  Fault  movements  are  a  frequent  cause  of  earth- 
quakes, and  the  vibrations  set  up  in  the  rocks  by  faulting  cause  more  or 
less  damage,  sometimes  for  a  distance  of  several  miles  from  the  fault 
line.  Structures  standing  on  hard  rock  are  less  violently  shaken  (other 
things  being  equal)  than  those  on  unconsolidated  material. 

The  problem  which  confronts  the  engineer  in  countries  subject  to 
such  shocks  is  to  determine  what  type  of  structure  will  best  resist  the 
disturbance.  The  question  has  been  given  renewed  attention  in  this 
country  since  the  San  Francisco  earthquake,  and  while  there  exists  a 
difference  of  opinion,  it  seems  probable  that  structures  which  are  set 
firmly  on  their  foundations,  and  having  all  their  parts  well  bound  to- 
gether are  probably  the  most  resistant.1 

Coal  mines.  —  In  some  coal  fields  as  those  of  the  southern  Ap- 
palachian region,  the  beds  are  not  only  folded  but  are  also  at  times  dis- 
placed by  faults.  The  effect  of  this  is :  First,  that  the  two  parts  of  a 
fractured  bed  may  become  completely  separated  so  that  the  engineer, 
especially  if  he  lacks  geological  knowledge,  may  have  difficulty  in  dis- 
covering the  continuation  of  the  bed  on  the  other  side  of  the  fracture; 
and  second,  the  coal  along  the  fault  is  usually  badly  crushed,  and 
even  mixed  with  rock  and  dirt. 

Ore  deposits.  —  Mining  engineers  probably  have  more  trouble  with 
faults  than  any  other  class  of  engineers. 

Mineral  veins  are  frequently  formed  by  the  filling  of  fault  fissures 
(see  Chapter  XVII).  If  now,  there  is  more  than  one  set  of  fissures,  of 
different  ages  in  a  given  region,  and  those  of  one  series  are  mineralized, 
while  those  of  the  other  series  are  of  much  less  importance  (as  at  Butte, 
Mont.),  it  is  highly  essential  for  the  engineer  to  recognize  this  fact,  to 
avoid  following  barren  leads. 

But  aside  from  this,  ore  veins  and  other  types  of  ore  bodies  are  some- 
times displaced  by  one  or  more  later  faults,  and  then  the  engineer  or 
mining  geologist  must  determine  if  possible,  the  amount  and  direction 
of  the  fault  movement  in  order  to  find  the  continuation  of  the  ore  body. 

Abundant  and  complex  faulting  sometimes  makes  the  problem  an  ex- 
ceedingly difficult  one.2 

1  See  Gilbert  and  others,  U.  S.  Geol.  Survey.,  Bull.  324,  1907,  San  Francisco  Earth- 
quake and  Fire,  and  Effects  on  Structures  and  Structural  Materials;  Hobbs,  Construc- 
tion in  Earthquake  Countries,  Eng.  Mag.,  XXXVII,  p.  1,  1909;  Milne,  Construction 
in  Earthquake  Countries,  Trans.  Seismol.  Soc.,  Japan,  XIV,  p.  1,  1889-1890;  Hobbs, 
Study  of  Damage  to  Bridges  during  Earthquakes,  Jour.  Geol.,  XVI,  p.  636, 1908;  Hobbs, 
Earthquakes,  Appleton,  New  York,  1907;  Bulletin  Seismol.  Soc.  America. 

2  See  Lindgren,  Mineral  Deposits,  p.  114,  1913;  Spurr,  U.  S.  Geol.  Survey,  Prof. 
Pap.  42,  1905,  on  Tonopah,  Nev. 


STRUCTURAL  FEATURES  AND   METAMORPHISM   OF  ROCKS     185 


Curiously  enough  engineers  sometimes  on  coming  to  a  fault  plane 
think  that  the  ore  has  given  out,  although  the  evidence  of  displacement 
such  as  slickensides  and  breccia,  may  be  present.  A  simple,  and  almost 


-Slip  fractures 

Slips     /Drag  and  narrow  breccia  on 
-    -   ^  fault,  as  well  as  sUckensides 


i*— Test  bole 
i     found  ore 

FIG.  103.  —  Section  showing  case  of  bedded  ore  cut  off  by  fault. 

self-explanatory,  case  found  in  a  bedded  ore  deposit  in  the  eastern  states 
is  given  in  Fig.  103.  Many  others  more  or  less  complex  can  be  found 
in  the  literature. 

Submarine  cables.  —  In  one  case  at  least  faulting  appears  to  have 
been  responsible  for  the  breaking  of  a  submarine  cable.  This  was  the 
breaking  of  the  lines  near  Valdez,  Alaska,  during  the  earthquake  shock 
of  Feb.  14,  1908.1 

It  is  stated  that  both  the  Valdez-Sitka  and  Valdez-Seward  cables  were 
interrupted  close  to  the  city  of  Valdez,  and  well  inside  Valdez  Narrows. 
The  Valdez-Seward  cable  was  broken  in  four  places  three-eighths  to  one 
and  one-eighth  miles  apart,  while  the  Valdez-Sitka  cable  was  broken 
in  seven  places  five-eighths  to  seven-eighths  miles  apart. 

Landslides.  —  As  explained  in  Chapter  VII  fault  fissures  which  con- 
tain clay  gouge  and  become  wet  and  slippery  by  infiltrating  waters  may 
serve  as  gliding  surfaces  which  cause  landslides.  Slips  of  this  type 
were  among  those  encounted  in  the  construction  of  the  Panama  Canal. 

Determination  of  Faults.2 

"Where  exposures  are  sufficiently  abundant  the  facts  necessary  for  the  determi- 
nation of  the  direction  and  extent  of  a  displacement,  particularly  if  it  is  relatively 
small  in  amount,  may  be  observed  directly.  As  a  rule,  however,  the  dip  of  the  fault 

1  Tarr  and  Martin,  U.  S.  Geol.  Survey,  Prof.  Pap.  69,  p.  97,  1912. 

2  Quoted  from  Hayes,  Handbook  for  Field  Geologists.     In  this  connection  see 
also  Tolman,  Graphical  Solution  of  Fault  Problems,  Min.  &  Sci.  Press,  San  Fran- 
cisco, 1911. 


186  ENGINEERING  GEOLOGY 

plane  and  the  direction  and  amount  of  displacement  must  be  inferred  from  a  num- 
ber of  observations  at  different  localities.  Field  observations  should  be  made  with 
especial  care  and  completeness  in  the  vicinity  of  faults,  for  it  is  here  that  the  un- 
expected is  always  apt  to  occur. 

Dip  of  fault  plane.  —  It  often  happens  that  the  contact  of  rocks  on  opposite  sides 
of  a  fault  plane  cannot  be  seen  at  any  point,  although  the  fault  may  be  traced  for 
many  miles.  To  afford  data  for  determination  of  the  dip,  as  many  points  as  pos- 
sible on  the  fault  should  be  accurately  located  both  horizontally  and  vertically. 
The  points  should  be  selected  so  that  the  horizontal  distances  will  be  as  small,  and 
the  vertical  as  large,  as  possible.  Three  points  properly  selected  and  accurately 
located  will  give  better  results  than  a  larger  number  less  carefully  chosen  and  de- 
termined. The  best  locations  are  at  the  bottom  of  a  valley  transverse  to  the  fault 
and  on  the  hills  on  either  side.  The  three  points  fix  the  position  of  the  fault  plane, 
and  its  dip  or  the  angle  it  makes  with  the  horizontal  may  be  determined  by  con- 
struction or  trigonometric  methods.  The  trigonometric  method  involves  the  solu- 
tion of  a  number  of  triangles  and  the  extraction  of  square  roots.  Its  practical  ap- 
plication, therefore,  necessitates  the  use  of  logarithmic  tables,  which  are  not  gen- 
erally accessible  in  the  field.  The  method  by  construction  is  relatively  simple  and 


FIG.  104.  —  Diagram  illustrating  determination  of  dip  of  fault  plane. 
(After  Hayes.) 

requires  only  a  protractor,  dividers,  and  scale.  This  method  is  illustrated  in  Fig. 
104,  and  is  as  follows: 

Let  the  three  points  in  the  fault  plane  be  A,  B,  and  C.  Let  C  be  the  lowest  and 
B  the  highest,  the  difference  in  elevation  having  been  determined.  The  horizontal 
or  slope  distances  from  C  to  A  and  B,  and  the  azimuth  of  the  lines  connecting  them, 
have  also  been  determined.  Lay  off  with  the  protractor  the  lines  CA  and  CB,  in 
proper  azimuth  on  the .  scale  adopted.  If  these  lines  represent  slope  distances, 
project  the  points  A  and  B  upon  the  horizontal  plane  passing  through  C,  as  follows: 

Construct  a  right  triangle  (BCU)  with  CB  as  the  hypothenuse  and  the  difference 
in  elevation  between  C  and  B  as  the  perpendicular.  Lay  off  on  CB  a  distance  equal 
to  the  base  of  this  right  triangle  —  that  is,  Cb  =  Cb'.  Determine  the  point  a  on 
CA  in  like  manner.  Draw  a  line  through  a  and  6  and  extend  it  beyond  a.  The 
triangle  aCb  is  the  horizontal  projection  of  the  portion  of  the  inclined  plane  included 
by  the  lines  connecting  A,  B,  and  C.  If  the  distances  between  C  and  A  and  B  are 
horizontal  distances  this  projection  is  not  necessary,  since  the  triangle  can  be  drawn 
at  once  —  in  the  horizontal  plane  —  and  the  line  completing  the  triangle  will  be 
drawn  through  A  and  B. 


STRUCTURAL  FEATURES  AND  METAMORPHISM   OF  ROCKS     187 

At  a  and  6  erect  perpendiculars  equal  respectively  to  Aaf  and  Bbr,  and  draw  a 
line  through  their  extremities  to  its  intersection  with  the  line  ba  extended  at  O. 
This  point  of  intersection  will  be  in  the  horizontal  plane  and  also  in  the  inclined 
plane.  Since  C  also  is  in  the  same  horizontal  plane  and  in  the  inclined  plane,  a  line 
connecting  O  and  C  will  be  the  intersection  of  these  two  planes,  and  hence  the  strike 
line.  From  the  horizontal  projection  of  either  of  the  points,  as  6,  let  fall  a  per- 
pendicular to  D  on  this  strike  line  OC  extended.  From  b  draw  bd  perpendicular  to 
bD  and  equal  to  Bb',  the  difference  in  elevation  between  C  and  B.  Connect  its  ex- 
tremity with  D  and  the  angle  bDd  will  be  the  angle  sought,  the  inclination  of  the 
fault  plane  to  the  horizontal. 

Unless  the  field  measurements  have  been  made  with  exceptional  accuracy  the 
error  in  the  above  solution  will  come  well  within  the  limit  of  error  of  observation. 

This  method  is  of  course  applicable  in  the  determination  of  strike  and  dip  of 
any  inclined  plane  in  which  the  relative  position  of  three  points  is  known.  Thus  it 
will  be  found  useful  in  determining  the  strike  and  dip  of  a  bed  which  is  intersected 
by  drill  holes,  or  which,  from  the  nature  of  its  exposures,  does  not  admit  of  direct 
measurement. 

Angle  of  intersection  with  oblique  vertical  plane.  —  It  frequently  becomes  neces- 
sary to  determine  the  angle  of  intersection  of  a  fault  (or  other  inclined  plane)  with 
a  vertical  plane  oblique  to  the  strike  of  the  fault. 

The  trigonometric  solution  may  be  used  when  tables  of  natural  or  logarithmic 
functions  are  at  hand.  Let  m  be  the  angle  of  dip  of  the  inclined  plane  and  n  the 
angle  between  the  strike  of  the  inclined  plane  and  the  vertical  plane.  To  find  x, 
the  angle  which  the  line  of  intersection  of  the  two  planes  makes  with  the  horizontal 

tan  x  =  tan  m  sin  n. 

The  problem  may  be  solved  by  construction  as  follows:  Let  AK,  Fig.  105,  be  the 
azimuth  of  the  vertical  plane;  draw  AL  so  that  the  angle  KAL  =  n  =  the  angle 
made  by  the  strike  of  the  inclined  plane  and  the  azimuth  of  the  vertical  plane. 


FIG.  105.  —  Diagram  illustrating  determination  of  angle  of  intersection 
of  fault  plane  with  vertical  plane  oblique  to  strike  of  fault. 

Take  any  point  C  on  AL  and  erect  a  perpendicular  CB.  With  CB  as  a  base  con- 
struct a  right  triangle  with  the  angle  BCD  =  m  =  the  angle  of  dip  of  the  inclined 
plane.  Draw  BD'  =  BD  and  at  right  angles  to  AB.  Connect  A  and  D'.  The 
angle  BAD'  =  x  will  be  the  angle  sought. 


188 


ENGINEERING  GEOLOGY 


Per  cent  and  angular  inclination.  —  The  attitude  of  slightly  inclined  bedding 
planes  or  other  surfaces  is  generally  expressed  by  engineers  in  percentages,  and  it 
is  therefore  frequently  necessary  to  convert  such  percentages  into  their  equivalent 
angles.  It  is  also  at  times  desired  to  convert  angular  inclination  into  the  equivalent 
percentage.  This  conversion  involves  the  use  of  a  table  of  natural  tangents  more 
extended  than  that  given  on  page  160,  and  the  table  of  equivalents  is  therefore  in- 
serted below.  The  angles  are  given  only  to  the  nearest  five  minutes,  which  is  suf- 
ficient for  geologic  purposes,  and  is  nearer  than  the  angles  can  be  plotted  with  an 
ordinary  protractor. 

CONVERSION  OP  PER  CENT  GRADE  TO  ANGULAR  INCLINATION 


Per  cent  grade 

Angular  incli- 
nation 

Per  cent  grade 

Angular  incli- 
nation 

Per  cent  grade 

Angular  inclina- 
tion 

1 

Deg.     Min. 
35 

7.00 

Deg.    Min. 
4 

13.00 

Deg.     Min. 
7        25 

1.50 

52 

7.50 

4      15 

14.00 

8 

1.75 

1 

8.00 

4      35 

15.00 

8      30 

2.00 

1      10 

8.50 

4      50 

15.85 

9 

2  50 

1      25 

8.75 

5 

16.00 

9        5 

3.00 

1      45 

9.00 

5      10 

17.00 

9      40 

3.50 

2 

9.50 

5      25 

17.65 

10 

4.00 

2      15 

10.00 

5      45 

18.00 

10      15 

4.50 

2      35 

10.50 

6 

19.00 

10      45 

5.00 

2      50' 

11.00 

6      15 

19.45 

11 

5.25 

3 

11.50 

6      35 

20.00 

11      20. 

5.50 

3      10 

12.00 

6      50 

21.00 

11      50 

6.00 

3      25 

12.25 

7 

21.35 

12 

6.50 

3      45 

12.50 

7      10 

Form  of  Outcrop 

The  line  drawn  on  the  map  to  represent  a  formation  boundary  is  the  trace  of 
two  intersecting  surfaces  —  the  land  surface  and  the  surface  separating  the  over- 
lying and  underlying  formations.  Since  both  are  irregularly  warped  surfaces  their 
intersection  will  be  a  complicated  trace,  and  unless  careful  consideration  is  given  to 
the  geometric  relations  involved,  the  location  of  the  line  is  apt  to  be  inconsistent 
with  the  geologic  structure.  If  it  were  possible  or  practicable  to  actually  trace  on 
the  ground  all  lines  which  will  be  shown  on  the  map,  their  location  would  be  a  simple 
matter,  but  the  nature  of  exposures  generally  prevents  such  continuous  tracing, 
and  even  where  this  is  not  the  case  the  expenditure  involved  would  be  excessive  and 
prohibitory.  In  practice,  therefore,  the  location  is  determined  of  as  many  points 
as  possible  under  the  limitations  of  time  and  expense,  and  the  line  is  drawn  upon 
the  map  between  these  determined  points  so  as  to  be  consistent  with  the  form  of 
the  two  intersecting  surfaces. 

It  is  assumed  that  the  land  surface  will  be  accurately  represented  by  contours; 
the  form  of  a  line  marking  the  intersection  of  a  land  surface  so  represented  and 
any  geological  surface,  as  a  bedding  plane,  fault  plane,  unconformity,  eruptive  con- 
tact, etc.,  may  be  considered  under  three  cases:  (1)  in  which  the  geologic  plane  is 
approximately  horizontal;  (2)  in  which  it  is  approximately  vertical,  and  (3)  in 
which  its  inclination  varies  anywhere  between  0°  and  90°. 

(1)  It  is  evident  that  the  intersection  of  a  horizontal  geologic  plane  with  any 
land  surface  will  coincide  with  an  interpolated  land  surface  contour,  since  by  defi- 
nition contours  are  simply  the  traces  of  intersections  of  the  land  surface  with  equidis- 
tant imaginary  horizontal  planes. 


STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS     189 

A  boundary  between  horizontal  formations  will  therefore  be  drawn  between 
located  points  in  such  a  manner  as  not  to  cross  a  contour  line.  The  drawing  of 
such  lines,  particularly  if  a  large  number  of  points  are  located,  is  a  rigid  check  on 
the  accuracy  of  the  contouring  and  will  generally  necessitate  more  or  less  revision 
of  the  latter. 

(2)  It  is  equally  evident  that  the  intersection  of  a  vertical  geologic  plane  with  a 
land  surface  is  not  influenced  by  the  inequalities  of  the  latter,  and  therefore  has  no 


FIG.  106.  —  Diagram  illustrating  form  of  outcrop.     (After  Hayes.) 

definite  relation  to  the  surface  contours.  Hence  a  boundary  between  vertical 
formations  will  be  drawn  between  located  points  by  straight  lines  or  confluent  curves, 
regardless  of  contours. 

(3)  Between  the  two  extremes,  horizontal  and  vertical  dips,  an  infinite  variety 
of  relations  occur  between  the  intersection  and  the  contour  lines.  Two  general 
cases  may  be  discriminated;  (a)  where  the  geologic  plane  dips  into  a  sloping  land 
surface,  and  (6)  where  it  dips  with  the  land  surface.  The  two  cases  are  illustrated 
by  the  formation  boundaries;  (a)  on  the  face,  and  (6)  on  the  back  of  a  monoclinal 
ridge,  as  shown  in  Fig.  106,  in  which  the  contour  interval  is  100  feet  and  the  dis- 
tance from  A  to  B  is  one  mile. 


190  ENGINEERING  GEOLOGY 

Let  it  be  assumed  that  a  section  has  been  made  across  this  ridge  from  A  to  B 
and  the  points  M,  N,  0,  and  P  on  the  formation  boundaries  accurately  located; 
also  that  the  strike  and  dip  of  the  beds  have  been  determined.  The  problem  is  to 
determine  the  location  of  the  boundaries  on  the  map  with  reference  to  the  contours 
when  continued  on  either  side  of  the  section. 

Points  on  these  lines  may  be  determined  in  the  following  manner.  Construct  the 
profile  AB  to  scale.  The  distance  between  the  horizontal  ruled  lines  is  equal  to 
the  contour  interval,  100  feet,  and  the  profile  is  constructed  by  projecting  the  points 
of  intersection  of  the  profile  AB  and  the  various  contours.  Project  upon  this  pro- 
file the  points  M,  N,  O,  and  P,  and  draw  the  lines  M'm',  N'n',  etc.,  the  angles  cor- 
responding to  the  determined  dip  of  the  beds.  In  the  same  manner  construct  the 
profiles  A'B',  and  A"B".  The  point  m',  at  which  the  dip  line  M'm'  intersects  the 
profile  A'B'  is  projected  upon  the  section  line  A'B',  and  fixes  the  point  on  the  map 
at  which  the  boundary  crosses  the  bottom  of  the  ravine.  Between  M'  and  m'  the 
dip  line  crosses  the  horizontal  ruled  line  corresponding  to  the  700-foot  contour. 
This  point  projected  upon  the  map  gives  the  several  points  at  which  the  boundary 
mm  crosses  this  contour,  and  in  a  similar  manner  the  points  at  which  it  crosses  the 
600-  and  500-foot  contours  are  obtained.  Connecting  the  points  thus  located  on  the 
map  the  correct  position  of  the  boundary  is  fixed. 

The  dip  line  N'n'  does  not  cross  a  horizontal  line  between  N'  and  n',  hence  the 
boundary  nn  remains  between  the  900-  and  1000-foot  contours  in  crossing  the  ravine 
A'B'. 

In  the  same  way  points  are  located  on  oo  and  pp.  The  dip  line  O'o'  crosses  the 
two  horizontal  lines  between  0'  and  o',  hence  the  boundary  crosses  two  contours 
between  the  point  0  and  the  bottom  of  the  ravine  in  which  the  section  A  'B'  is  located. 
The  points  at  which  it  crosses  the  contours  are  determined  as  above,  by  projecting 
the  intersections  of  the  dip  line  and  the  horizontal  ruled  lines  upon  the  correspond- 
ing contours. 

From  an  inspection  of  the  diagram  it  will  be  observed  (1)  that  wherever  the 
boundary  lines  cross  surface  depressions  they  bend  in  the  direction  of  the  dip;  (2)  that 
where  the  bedding  planes  dip  into  the  slope  (mm  and  nn,  case  (a)  above),  the  bound- 
ary lines  bend  in  the  same  direction  as  the  contours,  but  less  acutely;  (3)  that 
where  the  bedding  planes  dip  with  the  slope  (oo  and  pp.,  case  (6)  above),  the 
boundary  lines  bend  in  the  opposite  direction  from  the  contours,  and  the  deviation 
from  a  straight  line  increases  as  the  dip  decreases,  (4)  that  the  width  of  outcrop 
of  a  formation  which  occurs  on  a  slope  is  less  than  the  outcrop  of  the  same  for- 
mation on  a  level  surface  if  the  beds  dip  into  the  slope,  and  greater  if  they  dip  with 
the  slope." 

ROCK  CLEAVAGE 

Definition.  —  The  term  rock  cleavage  as  used  in  its  broadest  sense, 
and  only  in  a  structural  one,  includes  the  property  which]  many  rocks 
possess  of  splitting  along  parallel  surfaces  in  certain  directions  more 
readily  than  in  others. 

Original  and  secondary.  —  We  can  then  recognize  two  types  of 
rock  cleavage,  viz.,  original  and  secondary.  The  former  would  include 
such  structures  as  bedding  and  lamination  in  sedimentary  rocks,  flow 
structure  in  lavas,  etc.;  in  other  words,  all  original  planes  of  low  co- 
hesion along  which  the  rock  may  split.  The  latter  includes  parallel 
structures  which  have  been  induced  in  rocks  by  metamorphism  subse- 


STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS    191 

quent  to  their  formation,  and  includes  such  structures  as  true  cleavage, 
slatiness,  schistosity,  foliation,  fissility,  etc. 

We  can,  however,  subdivide  secondary  cleavage  into  (1)  fracture  cleav- 
age and  (2)  flow  cleavage. 

Fracture  cleavage. — Fracture  cleavage1  is  the  structure  that  causes 
a  rock  under  stress  to  break  along  closely-spaced,  incipient,  parallel 
joints,  and  the  term  fissility  applies  to  such  partings. 

In  many  quarries  the  rock  appears  massive,  but  when  struck  with  a 
hammer,  the  stone  breaks  along  definite  planes.  Such  structural 
weaknesses  are  known  to  the  quarryman  as  blind  joints.  They  are 
common  in  the  older  crystalline  rocks,  constitute  lines  of  weakness, 
and  prevent  the  use  of  the  rock  for  dimension  blocks. 

Fracture  cleavage  is  independent  of  any  parallel  arrangement  of  the 
minerals,  and  there  may  be  two  or  more  intersecting  sets  of  planes. 

Flow  cleavage.  —  In  flow  cleavage  the  capacity  of  the  rocks  to 
part  along  parallel  surfaces,  not  necessarily  planes,  is  dependent  on  a 


FIG.  107.  —  Sliced  feldspars  in  micaceous  and  chloritic 
schist  from  southern  Appalachians.  Shows  granula- 
tion of  feldspars  due  to  flow  cleavage.  (After  Leith, 
Structural  Geology.) 

parallel  arrangement  of  the  mineral  constituents.  It  is  the  "cleavage 
proper"  and  includes  the  cleavage  of  most  writers.  Slatiness  or  slaty 
cleavage  of  slates,  schistosity  or  foliation  of  schists,  and  banding  or  gneissic 
structure  of  gneisses  are  all  phases  of  it. 

Slaty  cleavage.  —  The  development  of  flow  cleavage  in  the  group  of  fine-grained 
metamorphic  argillaceous  rocks  known  as  slates  is  called  slatiness  or  slaty  cleavage, 
It  is  a  secondary  structure  developed  independently  of  the  original  bedding  planes, 
and  the  two  structures  (slatiness  and  bedding)  may  or  may  not  coincide.  Cleavage 
is  best  developed  in  the  fine-grained,  homogeneous  clay  rocks  like  slate.  It  is  more 
or  less  perfect  in  slates,  separating  them  into  very  thin  layers  or  laminae  with  rela- 
tively smooth  surfaces,  thus  adapting  them  to  the  various  commercial  uses.  (See 
further  under  Slates,  p.  137,  and  in  Chapter  XI.) 

Schistosity.  —  The  foliation  or  cleavage  in  schists  is  referred  to  as  schistosity,  a 
characteristic  feature  of  this  group  of  rocks  as  described  in  Chapter  II.  In  schists 

1  Other  names  for  fracture  cleavage  are  close-joint  cleavage,  false  cleavage, 
strain-slip  cleavage,  slip  cleavage. 


192  ENGINEERING  GEOLOGY 

the  mineral  particles  are  larger  than  in  slates,  and  the  rocks  cleave  into  layers  (schist- 
ose structure)  with  more  or  less  rough  or  wavy  surfaces,  the  degree  of  smoothness 
being  conditioned  in  large  measure  by  the  abundance  of  good  cleavage-producing 
minerals,  such  as  mica,  hornblende,  etc.  Schistosity,  a  similar  structure  to  slati- 
ness,  indicates  more  severe  metamorphism  than  slatiness,  although  there  occur  all 
gradations  between  schists  and  slates. 

Gneissic  (gneissoid)  structure.  —  All  gneisses,  regardless  of  origin,  whether 
igneous  or  sedimentary,  show  a  banded  or  gneissic  structure,  but  they  may  or  may 
not  show  a  parallel  arrangement  of  the  mineral  particles.  Parallel  or  banded  struc- 
ture is  original  in  some  gneisses  and  has  not  developed  subsequently  from  second- 
ary causes.  (See  further  p.  127,  and  under  Building  Stone,  Chapter  XI.) 

ORIGIN  OF  FOLDS,  FAULTS,  JOINTS  AND  CLEAVAGE 

Introduction.  —  When  subjected  to  stresses  of  sufficient  intensity, 
rocks  are  deformed  either  by  fracturing  or  by  flowing.  Among  the 
chief  factors  involved  in  the  deformation  are  the  character  of  the  rock, 
and  depth  of  burial,  because  pressure  increases  with  depth.  Based 
upon  the  character  of  the  deformation  of  rocks  when  subjected  to 
stresses,  Van  Hise  has  divided  the  outer  crust  of  the  earth  into  three 
zones : 

I.  An  upper  zone  of  fracture  in  which  the  rocks  are  deformed  mainly 
by    fracture.     The   ruptures   are  those    of    jointing,    faulting,    differ- 
ential movement  between  the  layers,  fissility,  and  brecciation.     The 
maximum  thickness  of  this  zone  is  placed  by  Van  Hise  at  from  10,000 
to  12,000  meters,  but  these  limits  have  recently  been  extended  by  Adams 
to  a  depth  of  11  miles  or  17,600  meters  for  granite.1 

II.  A  middle  zone  of  combined  rock  fracture  and  flowage,  in  which 
the  pressure  is  only  sufficiently  great  to  cause  some  rocks  to  fracture, 
but  enough  to  make  others  flow.     This  zone  is  estimated  to  have  a 
thickness  of  possibly  as  much  as  5000  meters. 

III.  A  lower  zone  of  flowage  in  which  deformation  is  by  granulation 
or  recrystallization,  no  openings  being  produced,  or  at  least  none  ex- 
cept those  of  microscopic  size  because  larger  ones  would  be  closed  by 
pressure.     Rock  flowage  results  in  a  parallel  arrangement  of  the  rock 
constituents   producing   foliation,    which    is   variously    designated    as 
cleavage  (flow  or  true  cleavage),  banded  structure,  etc. 

Gradational  structures  occur  between  fracture  and  flow.  Folds,  for 
example,  may  be  developed  by  both  flowage  and  fracture. 

With  this  explanation  we  may  consider  a  little  further  the  origin  of 
some  of  the  structures  discussed  in  the  preceding  pages. 

Cause  of  folds.  —  Folds  are  the  result  of  compressive  forces  in- 
cident to  the  shrivelling  of  the  earth's  crust  in  its  effects  to  accomodate 
1  Jour.  Geol.,  XX,  p.  97,  1912. 


STRUCTURAL  FEATURES  AND  METAMORPHISM   OF  ROCKS     193 

itself  to  the  cooling  and  shrinking  interior.1  Where  folding  takes  place 
in  the  zone  of  fracture  it  is  probably  caused  by  the  rock  slipping  along 
fissures  of  jointing,  faulting,  or  cleavage.  But  where  it  occurs  in  the 
zone  of  flowage  the  rocks  become  practically  plastic  and  the  folding 
may  be  due  either  to  the  mineral  particles  gliding  one  upon  the  other 
or  actually  changing  their  individual  shapes,  or  in  part  dissolving  in 
points  of  higher  pressure  and  recrystallizing"  at  points  of  less  pressure. 
Factors  affecting  the  result  are  the  degree  of  rigidity  of  the  beds, 
rate  of  application  of  the  pressure,  duration  of  same,  and  depth  below 
surface. 

Leith  contrasts  folds  in  the  zone  of  fracture  with  those  in  the  zone  of  flow  as 
follows: 

ZONE  OF  FRACTURE  ZONE  OF  FLOW 

Beds  of  uniform  thickness.  Beds  thickened  and  thinned. 

No  interior  deformation.  Interior  deformation  of  all  parts. 

Relative  competence.  Relative  incompetence. 

Simple  outlines  of  competent  structure.  Crenulated   and   complex    outlines   of 

incompetent  structure. 

Much  slipping  between  beds;   dying  out  Little  slipping  between  beds;    persist- 

of  folds  vertically.  ence  of  folds  vertically. 

Folds  of  the  above  characteristics  are  Folds    of    above    characteristics    are 

"parallel."  "similar." 

All  folds  result  from  yielding  to  pressure,  and  field  studies  show  that  rocks  have 
varying  degrees  of  competence,  so  that  in  areas  of  folding  the  weaker  beds  have 
been  controlled  by  that  of  the  stronger  or  more  competent  beds.  In  a  series  of  in- 
terbedded  quartzite  and  shale  the  folding  of  the  rigid,  competent  beds  of  quartzite 
might  very  well  show  the  characteristics  of  the  zone  of  fracture,  while  the  asso- 
ciated weaker  and  incompetent  beds  of  shale  would,  from  development  of  cleavage, 
characterize  the  zone  of  flow.  In  such  a  series  the  rigid,  competent  beds  of  quartz- 
ite sometimes  exhibit  little  or  no  folding,  while  the  weaker,  incompetent  beds  of 
shale  are  folded,  the  quartzite  being  hi  the  zone  of  fracture  and  the  shale  in  the 
zone  of  flow,  as  indicated  by  the  development  of  cleavage  in  the  latter.  This  prin- 
ciple is  well  exemplified  in  parts  of  the  Valley  region  of  Virginia,  in  which  beds  of 
limestone  have  been  deformed  (folded)  by  fracturing,  while  the  associated  beds  of 
shale  have  been  deformed  by  flow. 

A  concrete  case  quoted  from  Leith  illustrates  the  desirability  of  the  knowledge 
of  such  characteristics  of  folds.2 

"The  attempt  to  analyze  a  fold  in  the  field  and  determine  what  combination  of 
fracture  and  flowage  conditions  it  represents  will  lead  to  a  better  understanding  of 
the  structure  than  will  the  mere  naming  of  the  fold  according  to  form.  For  in- 
stance, explorations  for  iron  ore  have  been  going  on  extensively  in  the  great  slate 
area,  completely  covered  by  glacial  drift,  in  central  Minnesota.  Drilling  soon 
demonstrated  the  fact  that  the  slate  was  folded  in  the  zone  of  flowage.  The  ob- 
server was  therefore  justified  in  concluding  that  the  folding  was  probably  close  and 
complex,  that  there  was  much  thickening  and  thinning  of  the  beds,  that  the  folds 
were  largely  of  a  similar  type,  not  dying  out  above  or  below.  The  application  of 

1  For  other  views  see  Ref.  la. 

2  Structural  Geology,  p.  108,  1913. 


194  ENGINEERING   GEOLOGY 

these  principles,  therefore,  has  been  of  great  aid  in  interpreting  fragmentary  records 
brought  up  from  the  drill  holes,  has  made  it  possible,  for  instance,  to  correlate  a 
thirty-foot  bed  of  ore  on  the  limb  of  a  fold  with  a  fifty-foot  bed  near  the  crest.  In 
the  Marquette  district  of  Michigan,  where  there  are  beds  of  quartzite  interbedded 
with  softer  slates  and  iron  formation,  it  has  been  possible  by  the  application  of 
these  principles  to  correlate  some  of  the  simpler  and  broader  structures  of  the  quartz- 
ites  with  the  closer,  much  more  complex,  and  quite  different  folds  of  the  softer 
beds.  In  making  any  satisfactory  estimate  of  the  thickness  of  folded  beds  the 
first  question  to  be  settled  is  the  degree  in  which  the  folds  are  characteristically 
those  of  the  zone  of  flowage  and  therefore  to  what  extent  they  are  likely  to  be 
thickened  or  thinned." 

Cause  of  joints.  —  Joints  are  limited  to  the  zone  of  fracture,  and 
may  occur  in  rocks  of  both  disturbed  and  undisturbed  regions,  but 
while  their  horizontal  distribution  may  be  great,  they  are  limited  ver- 
tically by  the  depth  of  the  zone  of  fracture,  though  some  believe  they 
may  not  extend  to  the  full  depth  of  that  zone. 

Among  the  causes  to  which  joints  are  referred,  are  tension,  com- 
pression, torsion,  shearing,  earthquakes,  etc.,  but  a  discussion  of  all 
these  is  not  contemplated  here. 

Two  important  classes  of  joints  are  those  formed  by  stretching  of  the 
rocks  or  tension  joints,  and  those  formed  by  pressure  or  compression 
joints.  For  certain  lines  of  structural  geologic  work,  their  discrimi- 
nation is  of  importance. 

Tension  joints,  whose  formation  indicates  extension  of  surface.  They  may  in- 
clude: (1)  Open  joints,  not  modified  by  solution  or  weathering;  (2)  joints  de- 
veloped along  crests  of  anticlines;  (3)  those  formed  during  cooling  of  igneous 
rocks,  a  special  case  being  that  of  the  prismatic  jointing  in  basalt;  and  (4)  joints 
developed  in  drying  and  shrinkage  of  sediments,  as  for  example  mud  cracks. 

Compression  joints,  whose  formation  indicates  compression.  They  are  more  diffi- 
cult to  recognize.  They  may  include:  (1)  Some  joints  showing  slip  surfaces;  (2) 
joints  which  pass  into  overthrust  faults  or  folds;  (3)  certain  ones  observed  on  limbs 
of  folds,  as  shown  by  way  in  which  they  follow  directions  of  compressive  shear;  and 
(4)  horizontal  sheeted  structure  developed  in  granite,  due  probably  in  part  at  least 
to  compression. 

Cause  of  cleavage.  —  The  term  cleavage  has  been  defined  on 
page  190,  and  it  was  stated  there  that  secondary  cleavage  was  of  two 
kinds,  fracture  and  flow  cleavage.  Both,  however,  are  due  to  com- 
pressive forces,  but  fracture  cleavage,  including  fissility,  is  probably 
more  characteristic  of  the  harder  rocks,  and  slaty  cleavage  of  the 
softer  ones.  Composition,  and  fineness  of  division  of  the  mineral,  par- 
ticles also  affect  the  result. 

Cause  of  fracture  cleavage.  —  Fracture  cleavage  is  developed  in  the  zone  of 
fracture,  and  is  independent  of  the  parallel  arrangement  of  the  minerals,  but  such 
parallel  arrangement  as  is  sometimes  seen  in  chlorite  and  mica,  may  result  from  rub- 
bing on  fracture  planes.  If  at  the  same  time  the  cleavage  planes  are  closely  spaced  it 


STRUCTURAL  FEATURES  AND   METAMORPHISM   OF   ROCKS     195 

may  be  difficult  to  distinguish  from  flow  cleavage.  Normally,  fracture  cleavage 
planes  are  more  widely  spaced  than  flow  cleavage  planes  (described  below),  and 
moreover,  may  be  developed  in  two  or  more  intersecting  sets. 

Cause  of  flow  cleavage.  —  Flow  cleavage  means  parallel  dimensional  arrange- 
ment of  the  mineral  particles,  and  results  from  differential  pressure  in  the  zone  of 
flow,  causing  the  rock  to  deform  by  flowage  and  not  by  fracture;  it  therefore  in- 
volves a  combination  of  physical  and  chemical  changes.  The  processes  which 
bring  about  the  parallel  arrangement  of  the  mineral  particles  are:  (1)  Crystalliza- 
tion and  recrystallization  resulting  in  the  flattening  of  old  minerals  and  development 
of  new  ones  in  the  planes  of  easiest  relief,  and  (2)  rotation  and  granulation  of  the 
original  minerals,  such  as  quartz  and  feldspar.  Gliding  along  definite  planes  of 
some  minerals,  especially  calcite,  will  also  result  in  flattening  of  the  mineral  particles, 
and  consequently  in  parallel  arrangement. 

STRUCTURES  DUE  TO  EROSION 

Under  this  head  are  discussed  several  structures,  (1)  unconformity 
and  overlap,  and  (2)  inliers  and  outliers,  which  owe  their  origin  in  most 
cases  to  erosion,  although  they  may  be  the  result  in  part  at  tunes  of 
other  causes,  such  as  faulting  and  folding. 


FIG.  108.  —  Section  showing  erosion  unconformity  aa,  with  concordant  dips. 

Unconformity  and  Overlap 

Unconformity.  —  Strata  that  have  been  deposited  in  orderly  sequence,  so  as  to 
form  a  continuous  succession  of  beds,  and  when  disturbed  have  been  similarly 
affected  by  movements,  are  said  to  be  conformable,  and  the  structure  is  known  as 
conformity  (Fig.  71).  In  such  a  succession  of  beds,  each  one  has  been  regularly  laid 
down  upon  the  next  preceding  one. 

In  many  places,  however,  this  orderly  succession  of  beds  has  been  interrupted  by 
cessation  in  deposition  for  a  period  of  time,  represented  by  a  hiatus  or  break  in  the 
geological  record,  and  marked  by  an  erosion  interval  of  more  or  less  magnitude. 
There  has  been  a  loss  of  a  part  of  the  geological  record.  The  formations  are  dis- 
cordant and  are  said  to  be  unconformable,  the  structure  being  called  unconformity 
(Figs.  108  and  109). 

Unconformities  are  of  great  importance  in  the  interpretation  of  geological  history. 
Thus  in  Fig.  109,  the  structure  indicates  that  the  conformable  series  of  lower  in- 
clined beds  was  first  deposited  under  water  and  afterwards  raised,  tilted,  or  folded 
into  a  land  surface.  After  elevation  above  water  into  a  land  surface,  the  beds  were 
subjected  to  a  long  period  of  erosion  whereby  they  were  reduced  to  a  nearly  com- 
mon level,  and  again  depressed  beneath  the  water,  when  the  second  set  of  beds  was 
deposited  on  them,  and  the  whole  finally  elevated  to  form  a  land  surface.  The  two 
sets  of  beds  are  discordant  as  shown  in  (1)  dissimilarity  of  dip,  (2)  an  erosion  inter- 
val and  therefore  a  hiatus  or  time  break,  and  (3)  hi  a  coarse  conglomerate  bed  form- 
ing the  basal  member  of  the  upper  conformable  series  of  horizontal  beds. 


196 


ENGINEERING  GEOLOGY 


Discordance  of  dip  is  not  to  be  interpreted  in  every  case  as  indicating  uncon- 
formity, for  it  results  from  various  causes,  such  as  faulting,  folding,  etc.  Moreover, 
unconformities  occur  in  horizontal  beds  in  which  the  two  series  of  bedded  rocks 
exhibit  similarity  of  dip,  as  shown  in  Fig.  108. 


FIG.  109. — Section  showing  unconf orm-        FIG.  1 10. — Igneous  unconformity  between 
ity  aa,  with  discordant  dips.  (a)  granite,  and  (6)  sedimentary  rocks. 


FIG.  111.  —  Igneous  unconformity,  between  extrusive  lava  sheets.  Upper  surface 
of  sheet  (a)  marked  by  scoria,  amygdules  and  gas  cavities;  upper  surface  of 
sheet  (6)  shows  amygdaloidal  texture. 


FIG.  112.  —  Section  along  contact  of  Piedmont  crystalline  rocks  (A),  and  Coastal 
Plain  sediments  (B\  showing  overlap. 

Unconformities  are  not  limited  to  groups  of  stratified  rocks,  but  are  sometimes 
observed  between  stratified  and  igneous  rocks  (Fig.  110),  and  between  stratified  and 
metamorphic  rocks.1  The  line  of  contact  between  two  unconformable  series  of  beds 
is  sometimes  a  line  of  weakness  and  decay  that  causes  trouble  in  underground  work. 

Overlap.  —  Overlap  defines  the  relation  between  members  of  a  conformable 
series  of  rocks,  and  is  dependent  on  the  existence  of  an  unconformity.  In  a  con- 

1  For  a  detailed  discussion  of  the  criteria  of  unconformity  see  Van  Hise,  U.  S. 
Geol.  Survey,  16th  Ann.  Kept.,  p.  1,  1896. 


STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS     197 

formable  series  overlap  is  shown  when  an  upper  bed  extends  beyond  the  limits  of 
the  one  or  ones  below,  so  that  the  edges  of  the  lower  bed  or  beds  are  concealed. 
The  structure  indicates  sudsidence  accompanied  by  deposition,  and  is  well  illus- 
trated in  the  eastern  United  States  in  the  overlapping  of  the  beds  of  the  Coastal 
Plain  formations  onto  the  older  crystalline  rocks  of  the  Piedmont  region  (Fig.  112). 
Overlap  is  of  much  practical  importance  in  mining  operations  as  well  as  in  ques- 
tions of  water  supply,  and  failure  to  recognize  this  structure  has  led  in  some  places 
to  disappointment  and  loss.  Well-known  cases  of  this  kind,  especially  those  re- 
lating to  the  exploitation  of  coal  beds,  are  reported  both  from  this  country  and 
abroad. 

Inliers  and  Outliers 

Inliers.  —  An  inlier  represents  outcrops  of  rocks  that  are  surrounded  on  all  sides 
by  geologically  younger  rocks.  It  is  usually  the  result  of  erosion,  and  the  structure 
is  often  observed  in  valleys  or  similar  depressions.  Thus  in  North  Carolina  and 
other  southern  states,  isolated  outcrops  of  granite  belonging  to  the  Piedmont  crystal- 


FIG.  113.  —  Section  showing  an  inlier  (a)  formed  at  summit  of  an  anticline 

by  erosion. 

line  rocks  are  observed  some  distance  east  of  the  fall-line,  chiefly  along  or  near  stream 
courses,  lying  well  within  the  limits  of  the  Coastal  Plain,  and  surrounded  by  the 
younger  rocks  of  this  province. 

Sometimes  an  inlier  is  observed  on  the  crest  of  an  eroded  anticline  (Fig.  113), 
and  again  as  the  result  of  faulting,  as  shown  in  Fig.  114. 

Outliers.  —  An  outlier  is  the  converse  of  an  inlier,  and  as  the  name  implies  rep- 
resents an  isolated  portion  of  rock  separated  from  the  main  mass  and  surrounded 


FIG.  114.  —  Section  showing  an  inlier 
(a)  formed  by  faulting,    ff,  faults. 


FIG.    115.  —  Section  of   outlier   (a) 
formed  by  erosion. 


by  rocks  that  are  geologically  older  (Figs.  115  and  116).  Outliers  are  usually 
the  result  of  denudation  and  are  of  frequent  occurrence  in  areas  of  strong  erosion. 
They  frequently  occur  capping  hills  and  ridges,  and  may  owe  then*  existence  to 
either  the  resistant  character  of  the  rock  composing  them  or  to  their  geological 


198 


ENGINEERING  GEOLOGY 


structure,  or  both.     They  may  be  separated  from  the  parent  mass  by  a  long  or 
short  distance. 

Outliers  may  sometimes  be  the  direct  result  of  faulting,  as  shown  in  Fig.  117. 
According  to  their  mode  of  formation  two  principal  classes  of  outliers  may  be  recog* 
nized,  (1)  erosion  outliers,  the  most  common;  and  (2)  faulted  outliers. 


.."";<  SZ"^--^. 


Section  along  A  A 
FIG.  116.  —  Plan  and  section  of  outlier  (?/)  and  inlier  (x).     Section  along  line  A  A. 

w  Richmond 


Coal 


Richmond.  Faults. 

Cogy|a8to  "^  ^\^^— Granite  Richmond     CW.^  p 


FIG.  117.  —  Outliers  formed  by  faulting. 


CONCEETIONS 

Form  and  occurrence.  —  Concretions  or  nodules  are  bodies  of 
foreign  material,  of  usually  more  or  less  rounded  shape,  found  mostly 
in  sedimentary  rocks,  and  of  later  origin  than  the  material  containing 
them.  They  are  often  nearly  perfect  spheres,  or  again  flattish  with 
elliptical  outline,  and  in  still  other  cases  assume  grotesque  forms, 
causing  many  people  to  mistake  them  for  fossilized  animal  remains. 
They  often  contain  a  nucleus  which  may  be  a  fossil,  piece  of  rock  or 
other  object.  Their  size  varies  from  a  fraction  of  an  inch  in  diameter 
to  several  feet,  but  some  contain  a  central  cavity,  and  one  form  which 
is  divided  by  radial  cracks,  filled  with  mineral  matter,  is  known  as  a 


STRUCTURAL  FEATURES  AND   METAMORPHISM   OF  ROCKS     199 

septarium.  Concretions  are  usually  harder  and  more  resistant  than 
the  inclosing  rock,  and  so  they  often  stand  out  in  more  or  less  strong 
relief  on  the  weathered  surface  (Plate  XIII,  Fig.  2). 

Origin.  —  While  it  is  known  that  concretions  are  of  later  age  than 
the  inclosing  rock,  still  their  exact  mode  of  formation  is  not  always 
clearly  understood,  although  many  of  them  represent  a  segregation  of 
foreign  matter  around  some  nucleus. 

Material  forming  concretions.  —  The  materials  forming  concre- 
tions, and  the  kinds  of  rock  they  often  occur  in  are:  (1)  Flint  or  chert 
in  limestone  (Plate  XIII,  Fig.  2)  and  chalk;  (2)  pyrite,  in  coal,  shale 
and  slate;  (3)  iron  carbonate  in  clays  (Plate  XXVII,  Fig.  1)  and  shales; 
(4)  clay  and  lime  carbonate  in  clays  (Plate  XXVII,  Fig.  2) ;  (5)  cemented 
sand  grains  in  sands;  (6)  gypsum  in  clays  and  shales;  and  (7)  barite  in 
some  sands  and  clays;  etc. 

Practical  considerations.  —  Concretions  are  rarely  of  economic 
value,  but  on  the  contrary  are  usually  a  source  of  trouble. 

Iron  carbonate  concretions,  when  found  in  clays  or  shales,  are  some- 
times used  as  a  source  of  iron  ore,  if  sufficiently  abundant,  and  not  too 
high  in  impurities. 

Flint  nodules  occurring  in  some  of  the  European  chalk  deposits, 
which  have  become  rounded  when  washed  out  of  the  cliffs  and  rolled 
by  wave  action,  are  used  in  ball  mills. 

Flint  concretions  are  undesirable  in  limestone  that  is  to  be  used  for 
structural  work  (Chapter  XI)  or  in  cement  manufacture  (Chapter 
XII).  They  interfere  with  the  dressing  and  grinding  of  the  stone. 

In  clays  or  shales  which  are  to  be  used  for  brick  or  sewer-pipe  manu- 
facture, concretions  will,  unless  removed  or  crushed,  be  the  cause  of 
various  troubles,  such  as  cracking,  pimples,  fused  spots,  etc.  Gypsum 
nodules  found  in  shales  are  never  a  commercial  source  of  that  material. 
Pyrite  nodules  in  coal  lower  its  market  value,  because  they  raise  its 
sulphur  content,  while  in  slate  they  injure  its  durability  and  appearance. 


METAMORPHISM   OF  ROCKS 

Introduction.  —  Broadly  speaking,  metamorphism  applies  to  any 
change  in  the  constitution  of  any  kind  of  rock.  Under  a  given  set  of 
conditions,  minerals  tend  to  form  in  rocks  under  those  conditions 
which  remain  permanent,  that  is,  they  tend  to  adapt  themselves  to  their 
new  environment.  The  adjustment,  however,  of  a  -rock  to  new  con- 
ditions takes  place  slowly,  so  that  it  may  remain  essentially  under  the 
same  conditions  for  a  long  period  of  time. 


200  ENGINEERING  GEOLOGY 

The  conditions  under  which  rocks  alter  are  numerous  and  varied, 
and  may  be  those  of  ordinary  pressure  and  temperature  at  or  near 
the  surface,  or  they  may  be  those  of  very  high  temperature  and  pressure 
which  exist  at  some  depth  below  the  surface.  A  rock  mass  may  be 
subjected  alternately  to  each  of  these  conditions.  Most  changes  in 
rocks  take  place  under  conditions  that  cannot  be  directly  observed, 
but  can  only  be  inferred,  such  as  all  changes  below  a  mile  in  depth. 
Rocks  may  undergo  change  near  the  surface  and  later,  as  the  result  of 
burial,  be  changed  at  greater  depth;  or  they  may  undergo  changes  at 
great  depth  and  subsequently  be  brought  near  the  surface  by  erosion 
of  the  overlying  rocks  and  there  be  changed  again.  Such  modifica- 
tion means  that  one  set  of  changes  in  a  rock  may  be  superimposed 
on  another. 

Definition.  —  In  view  of  the  above  statements  then  metamorphism 
might  be  defined  as  any  change  in  any  rock,  regardless  of  origin,  and 
may  be  the  result  of  chemical  or  physical  agencies,  or  both.  If  such 
changes  take  place  at  or  near  the  surface  we  call  them  weathering,  but 
if  they  go  on  at  some  depth  and  involve  densification,  recrystallization, 
or  change  in  mineral  composition  we  call  them  alteration  or  metamor- 
phism proper.  The  subject  of  weathering  is  treated  in  Chapter  IV,  so 
that  the  discussion  here  is  restricted  to  the  deep-seated  changes  in 
rocks  —  alteration  - —  or  metamorphism  proper. 

There  are  many  degrees  of  metamorphism  and  they  vary  greatly  in 
intensity.  In  some  cases  the  rock  has  been  so  slightly  changed  that 
the  original  characters  are  still  evident,  whether  of  sedimentary  or 
igneous  origin,  but  in  others  the  metamorphism  has  been  so  complete 
as  to  obscure  all  trace  of  the  original  character  of  the  rock,  so  that  it 
becomes  conjectural  as  to  what  its  original  nature  was.  Such  metamor- 
phism in  a  rock  may  result  in  partial  or  complete  change  of  texture, 
structure,  or  mineral  composition.  Thus,  a  sandstone  may  be  changed 
to  a  quartzite,  in  which  only  a  change  of  texture  has  been  involved, 
while  that  of  structure  and  mineral  composition  remain  unaffected. 
It  frequently  happens,  however,  that  a  rock,  after  metamorphism, 
especially  under  conditions  of  deep  burial,  shows  no  change  in  chemical 
composition,  but  a  profound  one  as  to  mineral  composition  and  struc- 
ture. Thus,  a  pyroxene-bearing  rock,  such  as  dolerite,  might  be  trans- 
formed into  hornblende  schist,  which  would  be  both  a  structural  and 
mineralogical  change.  Igneous  rocks,  such  as  granite,  diorite,  gabbro, 
etc.,  may  be  rendered  gneissic  without  essential  change  either  in 
chemical  or  mineral  composition.  A  change  of  structure  (foliation), 
however,  in  igneous  rocks  may  not  be  the  only  one  involved. 


PLATE  XXVII,  FIG.  1.  —  Siderite  concretions  in  clay,  Anne  Arundel  County, 
Md.  (Md.  Geol.  Survey,  IV). 


FIG  2  —  Lime  carbonate  concretions  at  Hopyard,  Rappahannock  River,  Va. 

(201) 


PLATE  XXVIII,  FIG.  1.  —  Much  contorted  and  metamorphosed  argillaceous  and 
calcareous  beds,  filled  with  contact  silicates,  due  to  granite  intrusion,  Cirque 
d'  Arbison,  Pyrenees.  (H.  Hies,  photo.) 


FIG.  2,  —Fractures  in  limestone  produced  by  folding  and  filled  with  calcite,  Chris- 

tiansburg,  Va.     (H.  Ries,  photo.) 
(202) 


STRUCTURAL  FEATURES  AND  METAMORPHISM  OF  ROCKS    203 

Agents  of  metamorphism.  —  The  principal  agents  of  metamorphism 
are  (1)  mechanical  movements  of  the  earth's  crust  and  pressure;  (2) 
liquids  and  gases;  and  (3)  heat.  All  of  these  are  considered  necessary 
to  the  complete  metamorphism  of  a  rock,  but  not  necessarily  to  the 
same  degree,  since  one  of  them  may  be  predominant  in  producing  the 
change  in  one  case,  and  some  other  in  another.  We  may  consider  these 
chief  agencies  of  metamorphism  separately  below,  in  the  order  named. 

Mechanical  movements  and  pressure.  —  Downward  pressure  alone 
exerts  little  or  no  metamorphic  effect  because  many  sediments  which 
have  been  deeply  buried  and  subsequently  uncovered  by  erosion  show 
little  evident  change  except  consolidation.  Earth  movements,  on  the 
other  hand,  are  very  effective  in  producing  changes  in  rock  masses,  as 
shown  in  the  production  of  folds  and  the  accompanying  structures, 
such  as  joints,  faults,  and  of  foliation  in  some  or  all  of  the  involved 
rocks.  Shearing  stresses  are  set  up  as  a  result  of  pressure,  which  result 
in  differential  movement  of  the  rock  constituents,  as  shown  in  the 
broken  fragments  that  are  often  flattened  and  elongated  in  the  direction 
of  shear.  The  degree  of  change  will  depend  upon  the  intensity  of  com- 
pression and  the  depth  at  which  it  operates.  Earth  movements  when 
accompanied  by  heat  and  water  effect  important  chemical  changes, 
and  frequently  the  production  of  new  minerals. 

Solutions  (liquids  and  gases).  —  Of  these  water  is  the  most 
abundant  and  therefore  the  most  important.  Whatever  its  source 
(whether  meteoric  or  magmatic,  Chapter  XVII)  may  be,  water  is  an 
effective  agent  of  metamorphism,  the  role  which  it  plays  in  producing 
rock  changes  being  a  chemical  one,  which  becomes  more  effective  when 
accompanied  by  heat  and  pressure. 

Water  acts  as  a  solvent  upon  nearly  all  rock-forming  minerals,  slowly  trans- 
ferring mineral  matter  from  one  point  to  another,  which  promotes  recrystallization. 
It  is  partly  taken  up  into  the  molecules  of  new  compounds  (minerals),  such  as  stauro- 
lite,  epidote,  mica,  etc.,  and  it  is  necessary  to  their  formation.  It  is  further  aided 
by  the  substances  which  it  may  carry  in  solution,  such  as  the  emanations  (fluorine, 
boric  acid,  etc.)  given  off  from  intrusive  magmas,  and  which  can  only  account  for 
the  formation  of  such  minerals  as  tourmaline,  vesuvianite,  etc. 

Heat.  —  The  heat  involved  in  metamorphism  may  come  from 
several  different  sources:  (1)  Interior  of  the  earth,  which  increases  with 
depth,  (2)  developed  from  earth  movements,  and  (3)  from  the  in- 
trusion of  molten  magmas.  Whatever  the  source,  heat  is  a  most  potent 
agent  of  metamorphism,  as  shown  by  the  pre-eminence  of  contact  or 
local  metamorphism  discussed  on  page  206.  Heat  greatly  augments 
the  solvent  action  of  solutions,  and  it  promotes  the  formation  of  new 
chemical  compounds. 


204  ENGINEERING  GEOLOGY 

Zones  of  metamorphism.  —  As  already  explained,  the  processes  of 
change  in  rocks  near  the  surface  are  quite  different  from  those  taking 
place  at  depth.  Consequently,  depth  is  regarded  as  an  important 
geological  factor  determining  the  character  of  the  alteration.  Based  on 
depth  we  recognize  two  zones,  viz. :  (1)  An  upper  one  in  which  mineral 
compounds  are  broken  down  (katamorphic  zone),  and  a  lower  one  in 
which  simple  compounds  are  built  over  into  more  complex  ones  (ana- 
morphic  zone).  The  upper  part  of  the  first  zone,  which  extends  to  the 
groundwater  level  (see  Chapter  IV)  is  the  belt  of  weathering,  and  the 
lower  part  has  been  called  by  Van  Hise  the  belt  of  cementation. 

Katamorphic  zone.  —  The  limits  of  the  zone  of  katamorphism  are  essentially 
those  of  the  zone  of  fracture  (p.  192),  and  extend  from  the  surface  to  a  depth  ordi- 
narily of  10,000  to  12,000  meters  for  the  strongest  rocks  under  quiescent  conditions. 
The  alterations  that  take  place  in  the  two  belts  (weathering  and  cementation)  of 
this  zone  are  strongly  contrasted.  The  characteristic  reactions  of  the  belt  of  weather- 
ing are  discussed  in  Chapter  IV,  and  it  suffices  here  to  state  in  summary  that  it  is 
especially  characterized  by  solution,  decrease  of  volume,  and  softening  of  the  ma- 
terials; the  processes  are  destructive,  resulting  in  degeneration.  On  the  other  hand, 
the  belt  of  cementation  is  especially  characterized  by  deposition,  increase  of  volume, 
and  induration  of  the  materials;  the  processes  are  constructive  and  result  in  physical 
coherence.  The  materials  dissolved  from  the  belt  of  weathering  are  transferred  in 
solution  to  the  lower  belt  of  cementation  and  there  deposited.  It  must  not  be  mis- 
understood that  solution  may  and  does  go  forward  in  the  belt  of  cementation,  but 
that  solution  and  deposition  are  more  nearly  balanced  in  this  belt. 

Anamorphic  zone.  —  The  zone  of  anamorphism  corresponds  to  the  zone  of  flow- 
age,  in  which  there  is  great  pressure  in  all  directions.  It  is  a  zone  of  reconstruction, 
and  is  especially  characterized  by  silication  involving  decarbonation,  dehydration, 
and  deoxidation;  the  minerals  formed  are  numerous,  of  high  specific  gravity,  and 
probably  of  complex  structure.  The  reactions  take  place  with  decrease  of  volume, 
and  usually  little  absorption  of  heat. 

Kinds  of  metamorphism.  —  The  various  kinds  of  metamorphism 
that  have  been  recognized,  based  on  the  dominant  force,  agent,  or 
process  involved  in  the  alteration  of  a  particular  kind  of  rock  are: 
(1)  Thermo-metamorphism,  which  refers  to  heat;  (2)  .hydro-metamor- 
phism,  which  refers  to  the  presence  of  water;  (3)  chemical  metamor- 
phism, which  refers  to  the  action  of  chemical  forces;  (4)  static  and 
pressure  metamorphism,  which  refer  to  quiescent  conditions;  (5)  dyna- 
mo-metamorphism,  which  refers  to  conditions  of  motion;  (6)  regional 
metamorphism,  which  refers  to  the  extent  of  alteration;  and  (7)  con- 
tact or  local  metamorphism,  which  refers  to  the  proximity  of  an  igneous 
rock. 

It  is  probable  that  no  one  of  these  agents  acting  alone  is  of  great 
importance,  but  that  they  are  all  involved,  though  in  varying  degrees 
in  different  cases.  For  convenience  of  discussion,  we  may  divide 
metamorphism  into:  (1)  Contact  or  local  metamorphism,  and  (2)  general 


PLATE  XXIX,  FIG.  1.  — Granite  quarry,  near  Woodstock,  Md.,  showing  horizontal 
joints.     (T.  L.  Watson,  photo.) 


.  2. Slate  quarry,  Penrhyn,  Pa.,  showing  folded  beds  and  cleavage.      (H.  Ries, 

photo.) 

(205) 


206 


ENGINEERING  GEOLOGY 


or  regional  metamorphism.  These  two  kinds  of  metamorphism  are 
generally  recognized  by  most  geologists,  and  especially  the  economic 
geologist,  as  having  an  important  bearing  on  the  formation  of  ore 
deposits. 

Contact  or  Local  Metamorphism 

Introduction.  —  By  contact  metamorphism  is  meant  the  changes 
produced  by  intruding  igneous  masses  in  contact  with  other  rocks 
which  they  invade.  The  invaded  rock  may  be  either  sedimentary, 
igneous,  or  metamorphic,  but  the  most  pronounced  changes  are  shown 


K "7  «>\'  7  I  x\ •?  - V^-^-A 

^v^'^crv^^ 

tg^^M^S 

'     /         I   /  \ '  "~   x/  \ '  *^s  ~~ I  s — I      x  "S 

\u^lj>i-r1\s/-;-f/Ar^\N//-!^Tv 
^>/vfj>  (5Vi<-^  ^^rl'^N  -^\- 


r^J<*'l',^\ '^  \*  x\-  x  i  \  ', TA 


FIG.  118.  —  Section  through  a  contact  metamorphic  zone;  showing  (a)  intrusive 
rock;  (6)  quartzite;  (c)  limestone;  (d)  shale.  Contact  metamorphic  zone 
shown  in  stippled  area,  including  ore  in  black. 

in  sedimentary  rocks,  especially  limestones.  This  is  because  the  siliceous 
crystalline  character  and  dense  texture  of  the  igneous  and  metamorphic 
rocks  makes  them  resist  alteration. 

Igneous  rocks  of  volcanic  character  rarely  cause  pronounced  metamor- 
phism, save  that  of  hardening  and  baking  of  the  rock  surface  over 
which  they  flow,  and  even  here  the  changes  are  best  developed  in  sedi- 
mentary rocks. 

The  changes  which  result  from  contact  metamorphism  may  affect 
both  the  intrusive  rock  and  the  intruded  ones  at  or  near  their  contact. 
Those  developed  in  the  intrusive  body  may  be  termed  endomorphic, 
and  those  affecting  the  intruded  rocks  are  called  exomorphic. 

Endomorphic  changes.  —  The  commonest  endomorphic  effects  ob- 
served are:  (1)  Change  in  mineral  composition,  and  (2)  change  in 
texture,  the  latter  being  the  more  common.  The  border  changes  in 


STRUCTURAL  FEATURES  AND  METAMORPHISM   OF  ROCKS     207 

chemical  composition  may  be  due  to  magmatic  differentiation  (p.  69), 
or  to  the  presence  of  mineralizers  (p.  66),  which  tend  to  be  squeezed 
out  towards  the  margin  as  the  interior  solidifies,  and  collect  there.  As 
a  result  we  sometimes  find  tourmaline,  as  around  the  borders  of  granites, 
or  the  development  of  pegmatite.  The  textural  change  may  be  shown 
by  finer  grain  due  to  chilling  of  the  outer  portion  of  the  intrusive  mass, 
or  in  other  cases  a  porphyritic  texture  is  developed. 

Exomorphic  changes.  — These  depend  on:  (1)  Character  of  country 
or  invaded  rock;.  (2)  size  of  intrusive;  (3)  character  of  vapors  expelled 
by  intrusive  during  solidification;  and  (4)  structural  features  and  posi- 
tion of  beds  of  country  rock.  The  area  in  which  the  exomorphic  changes 
occur  is  known  as  the  contact  zone,  and  is  of  variable  width,  being  one  or 
even  two  miles  in  some  cases,  but  usually  much  smaller  as  well  as 
irregular.  Shales  and  slates  are  baked  to  a  hard  siliceous  rock  called 
"hornfels,"  while  limestones  are  converted  into  marble.  New  minerals 
are  developed,  and  these  are  especially  abundant  in  limestone,  where 
they  are  of  both  metallic  and  non-metallic  character.  Indeed  the 
former  are  often  in  sufficient  abundance  to  form  ore-deposits  (Chapter 
XVII). 

One  theory  formerly  advanced  was,  that  the  minerals  developed  in 
the  contact  zone,  represented  re-arrangement  of  the  materials  already 
present  in  the  country  rock,  but  since  the  latter  in  its  original  form  may 
be  quite  pure,  as  in  the  case  of  some  limestones,  it  seems  clear  that 
many  of  the  minerals  found  in  the  contact  zone  are  made  up  of  materials 
given  off  by  the  intrusive,  and  this  view  is  now  quite  generally  held. 
The  contact  silicates  formed  in  limestone  are  quite  characteristic  and 
include  such  minerals  as  garnet,  epidote,  wollastonite,  and  pyroxene. 

Contact-metamorphic  effects  on  different  kinds  of  rocks.  —  As  previously  re- 
marked, the  intensity  and  extent  of  contact  metamorphism  depends  in  part  on  the 
size  and  character  of  the  intrusive  rock,  but,  other  things  being  equal,  it  is  usually 
most  pronounced  in  sedimentary  rocks,  such  as  limestones,  clay  shales,  and  slates, 
and  to  a  less  extent  in  sandstones.  The  effect  produced  on  each  of  these  is  briefly 
stated  below. 

Limestones  are  especially  susceptible  to  alteration  along  contacts  with  igneous 
intrusions.  The  pure  limestones  are  changed  into  crystalline  marbles  through 
crystallization.  The  impure  varieties  containing  such  impurities  as  siliceous  and 
argillaceous  materials  usually  exhibit  the  most  marked  effects  in  development  of 
contact  metamorphic  minerals,  such  as  garnet,  epidote,  diopside,  tremolite,  vesu- 
vianite,  tourmaline,  etc.,  among  the  silicate  forms,  and  in  many  cases  ore  minerals, 
—  a  mixture  of  sulphides  and  oxides,  —  which  latter  are  often  present  in  sufficient 
quantity  to  yield  valuable  ore-deposits  of  the  contact  metamorphic  type  (see  Chapter 
on  Ore-Deposits).  The  limestone  may  be  entirely  changed  into  the  lime-bearing 
silicate  minerals,  or  it  may  be  changed  into  a  highly  crystalline  marble  containing 
the  silicate  minerals  irregularly  grouped  or  distributed  in  bunches. 

Shales  and  slates,  especially  of  the  clayey  or  argillaceous  type,  may  be  changed 


208  ENGINEERING  GEOLOGY 

to  hard  and  dense  rock  having  a  conchoidal  fracture,  and  of  dark  or  black  color, 
called  hornstone;  or,  if  of  a  light  gray  to  greenish  color,  it  is  termed  adinole.  Fre- 
quently, however,  the  change  is  to  a  hard  and  compact,  fine-grained  rock,  called 
hornfels,  containing  andalusite,  staurolite,  biotite,  etc.,  in  which  all  visible  evidence 
of  bedding  may  be  lost.  This  metamorphism  gradually  diminishes  with  distance 
from  the  contact,  the  only  evidence  of  change  shown  being  the  development  of  a 
knotty  structure  in  the  rock,  which  gradually  fades  into  the  unaltered  rock  beyond. 

Sandstones,  especially  the  pure  quartzose  varieties,  are  usually  changed  to  quartz- 
ites  at  the  contact. 

Igneous  and  metamorphic  rocks  are,  as  a  rule,  less  altered  by  contact  metamor- 
phism, but  there  are  many  exceptions  to  this  statement  and,  in  some  cases,  rather 
notable  effects  are  produced. 


General  or  Regional  Metamorphism 

Over  many  parts  of  the  earth  are  extensive  regions  of  rocks  that 
have  been  more  or  less  profoundly  altered  and  which,  because  of 
great  area!  extent  involved  and  the  character  in  part  of  changes 
produced,  cannot  be  ascribed  to  local  or  contact  metamorphism.  A 
typical  region  of  widespread  and  profound  alteration  of  rocks  is  the 
crystalline  province  of  the  eastern  United  States,  but  local  or  contact 
metamorphism  within  this  province  is  by  no  means  lacking.  An- 
other area  is  found  in  the  Lake  Superior  region,  in  which  occurs  im- 
portant iron-ore  deposits  (Chapter  XVII).  The  principal  metamor- 
phic rocks  composing  such  extensive  regions  are  gneisses,  crystalline 
schists,  slates,  etc.  Alteration  on  such  an  extensive  scale  is  known 

as  general  or  regional  metamorphism, 
and  applies  to  the  reconstruction  of 
rocks  over  extensive  areas. 

The  principal  effects  of  regional  met- 
amorphism    are     crystallization     and 
recrystallization  involving  the  forma- 
tion   of    new    minerals    and    the    pro- 
duction of  foliated  structure,  such   as 
schistosity,   gneissoid   structure,    slaty 
cleavage,  etc.  (discussed  on  pages  125, 
FIG.  119.  —  Slate  showing    fine     190,  194),  which  result  in  the  develop- 
cleavage    lines,    and    layer    of     ment   of  gneisses     schists     slates     etc> 
calcareous    quartzite,     showing      /a  ,         ,,    '  ,  .        ^      , 

crumpling   of   bedding   planes.     (See    under    Metamorphic     Rocks    in 
(After  Dale).  Chapter  II.)     Ordinarily  the  chemical 

composition  of  the  rock  is  not  much 

affected  by  metamorphism,  although  the  changes  may  result  in  the 
loss  of  some  substances,  especially  the  volatile  ones,  and  the  addition 
of  others. 


STRUCTURAL  FEATURES  AND  METAMORPHISM   OF  ROCKS    209 

Under  the  conditions  of  regional  metamorphism  the  original  char- 
acters of  the  rocks  are  frequently  completely  obscured  or  destroyed, 
so  that  it  becomes  difficult,  if  not  in  some  cases  almost  impossible, 
to  state  with  certainty  whether  the  original  rock  mass  was  a  sedi- 
mentary or  igneous  one.  These  are  changes  which  take  place  at 
depth  below  the  surface  under  conditions  of  deep  burial  in  the  ana- 
morphic  zone,  from  long  and  continued  action  of  earth  movements 
and  pressure;  liquids  and  gases,  especially  water;  and  heat,  which 
are  discussed  above  (pages  203,  204).  The  rocks  are  subsequently  ex- 
posed at  the  surface  through  erosion,  and  most  of  those  now  exposed 
over  many  parts  of  the  earth  are  among  the  older  rocks  of  the  earth's 
crust. 

TOPOGRAPHIC  AND  GEOLOGIC  MAP  AND  SECTION 

No  attempt  is  made  in  this  book  to  go  into  the  methods  of  topographic  and 
geologic  surveying,  since  these  are  available  in  any  one  of  several  excellent  texts. 

Topographic  (base)  map.  —  Some  form  of  an  accurate  base  is  highly  desirable 
and  in  most  cases  necessary  for  the  mapping  of  the  geology  of  any  area  that  is  be- 
ing studied.  The  best  basis  for  geological  work  is  a  topographic  map  of  the  type 
published  by  the  U.  S.  Geological  Survey.  If  the  topography  of  the  base  map  is 
inaccurate,  the  geological  lines  must  necessarily  be  distorted. 

The  features  represented  on  the  topographic  map  by  the  U.  S.  Geological  Survey 
are  of  three  distinct  kinds:  (1)  Inequalities  of  surface,  called  relief,  as  plains,  pla- 
teaus, valleys,  hills,  and  mountains;  (2)  distribution  of  water,  called  drainage,  as 
streams,  lakes,  and  swamps;  and  (3)  the  works  of  man,  called  culture,  as  roads, 
Railroads,  boundaries,  villages,  and  cities. 

The  relief  is  indicated  by  contour  lines,  that  is,  lines  of  equal  elevation.  The 
interval  between  contour  lines  will  vary  according  to  the  purposes  for  which  the 
map  is  designed  and  the  surface  character  of  the  area  mapped,  whether  the  relief  is 
slight,  moderate,  or  very  strong.  In  general,  with  areal  mapping  on  a  scale  of  one 
inch  equals  a  mile,  convenient  contour  intervals  may  be  50  or  100  feet.  Frequently, 
however,  in  the  case  of  property  and  mine  maps,  the  requirements  necessitate  a 
larger  scale  in  order  to  include  more  detail  when  the  contour  interval  may  be  made 
smaller,  10  or  20  feet. 

Geologic  map.  —  A  good  geologic  map,  to  be  of  practical  use  and  value,  should 
show  the  following  features:  (1)  The  boundaries  and  therefore  areal  distribution  of 
the  rock  masses  (formations)  on  the  surface;  (2)  structural  data,  such  as  dip  and 
strike  of  beds  and  of  schistosity  in  schists  and  gneisses,  structure  axes,  faults,  zones 
of  crushing,  brecciation,  etc.;  (3)  economic  data,  such  as  outcrops  of  ore  bodies 
and  ore-bearing  formations,  locations  of  mines,  quarries,  gravel  pits,  prospect  pits, 
etc. ;  also  mills,  breakers,  etc.  Dip  and  strike  of  the  ore  bodies  should  be  indicated 
and  the  outline  and  type  of  mineralization  should  be  represented  as  far  as  possible. 
If  the  area  mapped  contains  coal  or  oil,  it  is  important  to  determine  accur  tely  the 
underground  structure  so  that  the  depth  to  a  particular  bed  may  be  shown  and  the 
underground  structure  so  indicated  on  the  map.  (4)  Accompanying  structure  sec- 
tions which  show  the  distribution  of  geologic  formations  at  the  surface,  and  their 
attitude  and  position  below  the  surface. 

The  geologic  maps  prepared  by  the  U.  S.  Geological  Survey  represent  the 
geology  shown,  by  colors  and  conventional  signs  printed  on  the  topographic  base 
map,  the  distribution  of  rock  masses  on  the  surface  of  the  land  and,  by  means  of 


210  ENGINEERING  GEOLOGY 

structure  sections,  their  underground  relations,  so  far  as  known,  and  in  such  detail 
as  the  scale  permits.  The  kinds  of  rock  distinguished  on  the  map  are  igneous  (ex- 
trusive from  intrusive),  sedimentary,  and  metamorphic. 

Conventions  and  symbols.  —  There  should  be  uniformity  as  far  as  possible  in 
the  usage  of  conventional  signs  in  the  making  of  both  topographic  and  geologic 
maps.  For  the  preparation  of  geographic  maps  it  is  well  to  use  the  conventional 
signs  adopted  by  the  U.  S.  Geographic  Board,  which  are  published  and  for  sale  by 
the  U.  S.  Geological  Survey  at  Washington.  In  the  preparation  of  geologic  maps 
it  is  desirable  that  the  usage  of  symbols  and  abbreviations  by  the  U.  S.  Geological 
Survey  be  followed.  Reference  to  the  folios  and  other  publications  of  the  Federal 
Survey  will  make  plain  this  usage. 

Boundaries  between  rock  formations,  mapable  units,  should  be  indicated  on  the 
map  by  solid  (unbroken)  lines  when  accurately  observed  and  located;  and  by 
broken  lines  or  lines  of  fine  dots  when  not  accurately  located.  Faults  may  be  rep- 
resented by  heavy  solid  lines  when  their  exact  position  has  been  determined,  and  by 
a  series  of  heavy  broken  lines  and  dots  when  not  accurately  determined.  Veins, 
when  accurately  located,  may  be  shown  by  a  system  of  arrows;  thus,  f  ,  so  oriented 
as  to  indicate  the  direction  of  strike.  In  case  veins  of  different  ill  kinds  occur 
in  the  area  being  mapped,  it  is  desirable  to  distinguish  between  |[|  them, which 
can  conveniently  be  done  by  arrows  of  different  colors.  Dikes  may  '  be  shown  by 
very  heavy  solid  or  broken  lines  in  color  if  preferred,  according  to  whether  they  are 
accurately  or  doubtfully  located. 

Each  formation  is  shown  on  the  maps  of  the  U.  S.  Geological  Survey  by  a  dis- 
tinctive combination  of  color  and  pattern  and  is  labeled  by  a  special  letter  symbol. 

"  Patterns  composed  of  parallel  straight  lines  are  used  to  represent  sedimentary 
formations  deposited  in  the  sea,  in  lakes,  or  in  other  bodies  of  standing  water.  Pat- 
terns of  dots  and  circles  represent  alluvial,  glacial,  and  eolian  formations.  Patterns 
of  triangles  and  rhombs  are  used  for  igneous  formations.  Metamorphic  rocks  of 
unknown  origin  are  represented  by  short  dashes  irregularly  placed;  if  the  rock  ia 
schist  the  dashes  may  be  arranged  in  wavy  lines  parallel  to  the  structure  planes. 
Suitable  combination  patterns  are  used  for  metamorphic  formations  known  to  be 
of  sedimentary  or  of  igneous  origin.  The  patterns  of  each  class  are  printed  in 
various  colors.  With  the  patterns  of  parallel  lines,  colors  are  used  to  indicate  age, 
a  particular  color  being  assigned  to  each  system. 

The  symbols  consist  each  of  two  or  more  letters.  If  the  age  of  a  formation  is 
known  the  symbol  includes  the  system  symbol,  which  is  a  capital  letter  or  mono- 
gram; otherwise  the  symbols  are  composed  of  small  letters."1 

Strike  and  dip  are  conveniently  represented  in  a  single  symbol,  thus,  — r~ "  , 
in  which  the  horizontal  line  open  at  both  ends  indicates  direction  of  strike      * 
and  the  arrow  that  of  dip.     Measurements  of  dip  and  strike  may  be  recorded  on  the 

N.10  E 

map  in  the  following  way:  I 

so 

Sections.  —  In  geologic  mapping  it  is  not  enough  to  show  cartographically  the 
areal  distribution  of  rock  formations,  but  it  is  important,  and  in  most  cases  necessary, 
to  represent  by  sections  along  a  particular  direction  or  directions  on  the  map  the 
arrangement  or  structural  relations  of  the  rocks  below  the  surface.  A  section  ex- 
hibiting this  arrangement  of  the  rocks  in  the  earth  is  called  a  structure  section.  The 
structural  relations  of  rock  masses  may  be  observed  in  natural  and  artificial  cuts, 
but  the  geologist  is  not  entirely  dependent  upon  these  for  his  information  concern- 
ing structure.  If  the  manner  of  formation  of  the  rocks  and  their  relations  on  the 
surface  are  known,  their  relative  positions  beneath  the  surface  can  be  inferred  and 

1  See  Geologic  Atlas  Folios  issued  by  U.  S.  Geol.  Survey. 


STRUCTURAL  FEATURES  AND   METAMORPHISM   OF  ROCKS     211 

sections  can  be  drawn  representing  the  structure.     The  patterns  used  for  sections 
are  those  shown  hi  Fig.  120. 

Geologic  maps  are  frequently  accompanied  by  columnar  (vertical)  sections,  which 
describe  very  briefly  the  formations  that  occur  hi  the  area;  such  as  character  of  the 


SCHISTS  MASSIVE  AND  BEDDED  IGNEOUS  ROCKS 

Fig.  120.  —  Symbols  used  in  sections  to  represent  different  kinds  of  rocks. 

rocks,  thickness  of  the  formations  expressed  in  feet;  and  order  or  age  of  accumulation, 
the  oldest  being  at  the  bottom  and  the  youngest  at  the  top. 

Method  of  constructing  geologic  maps  and  sections.  —  In  the  construction  of  a 
geologic  map,  the  geologist  rarely  finds  a  large  number  of  closely-placed,  outcrops; 
on  the  contrary  they  are  scattered  over  the  country,  sometimes  near  together, 
sometimes  far  apart.  Each  of  these  must  be  carefully  located,  the  kind  of  rock 
noted,  and  the  strike  and  dip  measured  wherever  possible. 

From  the  plotted  outcrops  the  boundaries  of  the  several  formations  are  to  be 
determined  as  accurately  as  possible.  Where  the  geologic  structure  is  very  com- 
plex, and  where  solid  rock  outcrops  are  few,  due  to  a  widespread  mantle  of  uncon- 
solidated  surface  deposits,  accurate  mapping  is  sometimes  difficult  even  to  the 
expert. 

In  the  location  of  such  boundaries  where  the  formations  are  not  in  actual  con- 
tact, the  geologist  often  makes  use  of  topographic  features.  The  contact  of  two 
formations  may  be  a  line  of  weakness,  and  its  position  indicated  by  a  valley  or  other 
depression.  Again,  hi  some  regions  the  nature  of  the  vegetation  covering  different 
formations  is  quite  characteristic,  or  occasionally  residual  soils  may  serve  as  a  guide. 
On  hillsides  the  difference  in  resistance  to  weathering  may  also  yield  characteristic 
results,  a  series  of  firm,  durable  beds  giving  steep  slopes,  while  a  less  resistant  series 
yields  gentle  ones. 

In  addition  to  natural  exposures  the  geologist  also  makes  use  of  all  additional 
data,  such  as  those  obtainable  from  railway  cuts,  drilled  wells,  tunnels,  mines,  etc. 

In  constructing  a  geologic  section,  for  the  purpose  of  ascertaining  the  structural 
features  of  a  region,  it  is  preferable  to  draw  this  normal  to  the  line  of  strike  if  the 
rocks  are  sedimentary. 


212  ENGINEERING  GEOLOGY 

Such  a  structure  section  is  often  desired  by  an  engineer  who  is  engaged  in  ex- 
cavating or  tunneling.  He  may  have  at  his  disposal  a  geologic  map,  already  pre- 
pared, from  which  he  can  construct  a  geologic  section  with  fair  accuracy. 

The  first  step  in  constructing  a  section  showing  underground  structure  along 
any  given  line  is  to  draw  a  surface  profile,  and  lay  off  upon  it  the  intercepts  of  the 
different  formations.  The  next  step  is  to  interpret  the  position  and  relationships 
of  these  rocks.  To  do  this  it  is  necessary  to  consider:  (1)  The  position  of  the  sedi- 
mentary strata,  whether  flat  or  folded;  (2)  faults;  (3)  igneous  rock  masses;  (4)  un- 
conformities; and  (5)  basal  metamorphic  and  igneous  rocks. 

1.  If  the  sedimentary  beds  are  horizontal,  they  are  drawn  simply  as  horizontal 
layers,  one  upon  the  other,  and  if  the  surface  is  perfectly  level,  a  geologic  map  of 
such  a  region  would  show  but  one  formation  (erosion  unconformity  and  faulting 
excepted).     If  additional  formations  are  shown  on  the  geologic  map  of  an  area  of 
flat-lying  strata,  it  is  because  they  have  been  exposed  by  erosion.     In  such  cases, 
then,  the  deeper  the  valley  the  older  the  beds  exposed  (if  the  beds  are  in  their  normal 
order  of  deposition),  and  on  a  map  of  such  a  region  the  different  formations  will  often 
show  a  peculiar  sinuous  outline,  with  tongues  extending  up  the  valleys  of  the  tri- 
butary streams.     A  somewhat  similar  disposition  of  formation  boundaries  would 
be  noticed  in  an  eroded  region  of  slightly-folded  rocks. 

Many  strata  are  more  or  less  folded,  and  in  order  to  represent  the  various  anti- 
clines (p.  148),  synclines  (p.  149),  and  monoclines  (p.  149),  into  which  the  strata  are 
folded,  it  is  necessary  to  draw  them  so  that  the  older  beds  dip  under  the  younger 
ones.  This  is  because  of  the  fact  that  in  sedimentary  rocks  the  older  ones  were 
laid  down  first  and  the  younger  ones  on  top  of  them.  An  exception  to  the  rule  that 
in  tilted  beds  the  older  dip  under  the  younger,  is  found  in  the  case  of  an  overturned 
fold  where,  by  reference  to  Fig.  76,  it  can  be  seen  that  in  one  limb  of  the  fold  the 
strata  are  inverted.  This  exception  together  with  other  irregularities  in  strike  and 
dip  are  often  shown  by  symbols  (p.  210).  The  steepness  of  the  dip,  if  not  shown  by 
these  symbols,  may  be  judged  by  the  comparative  widths  of  the  different  out- 
cropping belts  of  a  bed  or  formation,  the  wide  areas  of  outcrop  indicating  low  dips, 
and  the  narrow  ones  steep  dips  because  a  formation  often  maintains  a  uniform 
thickness  throughout  a  small  area. 

Having  determined  the  direction  in  which  the  beds  dip  it  is  necessary  only  to  con- 
nect different  parts  of  the  same  formation  in  order  to  determine  the  characte'r  of 
the  folding.  If,  for  example,  a  given  bed  is  bordered  on  either  side  by  beds  of  younger, 
but  themselves  of  similar  age,  it  signifies  that  the  older  bed  dips  in  both  directions 
under  the  younger  ones  and  hence  we  have  an  anticline. 

2.  Faults,  as  mentioned  on  page  168,  are  indicated  on  the  map  by  a  solid  or  short 
dashed  line.     In  drawing  the  section,  it  is  carried  along  until  the  fault  line  is  reached 
where  it  terminates  abruptly  and  the  construction  is  begun  independently  on  the 
other  side  of  the  fault.     We  cannot  tell  from  the  geologic  map  what  the  dip  of  the 
fault  plane  is,  or  whether  it  represents  a  normal  or  reverse  displacement.     These 
can  only  be  determined  from  the  field  evidence.1 

3.  Of  the  various  igneous  rock  masses,  flows  and  sills  show  the  same  general  re- 
lations on  maps  and  sections  as  sedimentary  rocks,  provided  they  occur  as  mem- 
bers of  a  normal  succession.     Dikes,  however,  appear  on  the  map  as  bands  of  color, 
cutting  haphazardly  across  the  other  formations,  and  are  drawn  in  the  section  as 
bands  of  uniform  width  with  nearly  vertical  dips.     The  outcrops  of  a  laccolith 
(p.  70),  appear  more  rounded  in  outline,  and  evidence  of  its  existence  is  afforded 
chiefly  by  the  upturning  of  neighboring  formations.     It  can  be  drawn  as  shown  in 
Fig.  52.     The  larger  intrusive  masses,  such  as  stocks,  bosses  (p.  55),  and  batholiths 

1  The  Geologic  Atlas  Folios  issued  by  the  U.  S.  Geological  Survey  contain  not 
only  geologic  maps,  but  also  structure  sections  across  the  map,  and  should  be  referred 
to  by  the  student  or  reader. 


FIG.  121.  —  Geologic  map  with  strikes  and  dips  indicated  along  £he  line  AB,  and 
structure  section  along  the  same.     (Adapted  from  U.  S.  Geol.  Survey,  Bull.  470.) 

(213) 


214  ENGINEERING  GEOLOGY 

(p.  56),  appear  on  the  map  as  more  or  less  irregular  outcrops  of  igneous  rock,  the 
position  and  outline  of  which  seem  to  have  been  wholly  unaffected  by  the  adjac- 
ent formations,  and  they  are  indicated  on  the  section  (Fig.  121)  as  having  irregular 
boundary  lines,  with  gradually  downward  increasing  size. 

4.  Unconformities  are  usually  indicated  by  the  absence  at  some  point  on  the 
map  of  one  or  more  formations  which  should  be  present  in  the  normal  order  of  suc- 
cession.    Corroborative  evidence  may  be  change  in  dip  and  strike.     Fig.  109  indi- 
cates an  unconformity. 

5.  Mention  might  be  made  lastly,  of  those  metamorphic  and  igneous  rocks, 
which  form  the  floor  upon  which  all  sediments  were  laid.     These,  when  known  to  be 
present,  are  represented  in  the  section  as  masses  underlying  the  oldest  sediments 
indicated  on  the  map.     Their  presence  is  known  by  their  appearing  at  the  surface 
where  the  younger  rocks  have  been  removed  by  erosion,  or  their  becoming  known 
through  underground  workings  or  borings. 


List  of  References  on  Structural  Features  and  Metamorphism 

of  Rocks 

1.  Chamberlin  and  Salisbury,  A  College  Text-book  of  Geology, 
Henry  Holt  and  Co.,  N.  Y.,  1909,  978  pages,  la.  Chamberlin  and 
Salisbury,  Geology,  Vols.  I  and  II,  Henry  Holt  &  Co.,  N.  Y.,  1904. 
2.  Dana,  James  D.,  Manual  of  Geology,  4th  edition,  American 
Book  Co.,  N.  Y.,  1895,  1087  pages.  3.  Farrejl  and  Moses,  Prac- 
tical Field  Geology,  McGraw-Hill  Book  Co.,  N.  Y.,  1912,  273  pages. 
4.  Geikie,  James,  Structural  and  Field  Geology,  D.  Van  Nostrand 
Co.,  N.  Y.,  1905,  435  pages.  5.  Geikie,  A.,  Text-book  of  Geology, 
4th  edition,  Vols.  I  and  II,  Macmillan  &  Co.,  Ltd.,  London,  1903, 
1472  pages.  6.  Hayes,  C.  W.,  Handbook  for  Field  Geologists,  John 
Wiley  &  Sons,  N.  Y.,  1909,  159  pages.  7.  Leith,  C.  K.,  Rock  Cleav- 
age, Bull.  239,  U.  S.  Geological  Survey,  1905,  216  pages.  8.  Leith, 
C.  K.,  Structural  Geology,  Henry  Holt  &  Co.,  N.  Y.,  1913,  169  pages. 
9.  Pirsson,  L.  V.,  Rocks  and  Rock  Minerals,  John  Wiley  &  Sons, 
N.  Y.,  1908,  414  pages.  10.  Reid,  Davis,  Lawson,  and  Ransome, 
Report  of  the  Committee  on  the  Nomenclature  of  Faults,  Bull.  Geol. 
Soc.  Amer.,  Vol.  24,  1913,  pp.  163-186.  11.  Scott,  W.  B.,  An  In- 
troduction to  Geology,  2d  edition,  The  Macmillan  Co.,  1907,  816 
pages.  12.  Tolman,  C.  F.,  Graphical  Solution  of  Fault  Problems, 
Min.  and  Sci.  Press,  San  Francisco,  1911.  13.  Van  Hise,  C.  R.,  A 
Treatise  on  Metamorphism,  U.  S.  Geol.  Survey,  Monograph  XLVII, 
1904,  1286  pages.  14.  Van  Hise,  C.  R.,  Principles  of  North  American 
Pre-Cambrian  Geology,  16th  Ann.  Rept.,  U.  S.  Geol.  Survey,  Pt.  I, 
1894-1895,  pp.  571-843;  see  also  Hoskins,  L.  M.,  Flow  and  Fracture 
of  Rocks  as  Related  to  Structure,  pp.  845-874.  15.  Willis,  Bailey, 
The  Mechanics  of  Appalachian  Structure,  13th  Ann.  Rept.,  U.  S. 
Geol.  Survey,  Pt.  II,  1891-1892,  pp.  211-282. 


CHAPTER  IV 
ROCK-WEATHERING  AND    SOILS 

Introduction.  —  When  exposed  for  a  sufficient  length  of  time  to  the 
atmosphere,  all  rocks  undergo  decay  from  disintegration  and  decom- 
position, and  are  ultimately  converted  superficially  into  a  loose, 
incoherent  mixture  of  sand,  gravel,  and  clay,  the  upper  few  feet  of 
which  is  called  the  sail.  If  the  erosive  action  of  water,  wind  or  ice 
is  not  too  excessive,  a  mantle  of  variable  thickness  of  decayed  material 
overlies  the  hard  and  fresh  rock  into  which  it  usually  passes  gradually. 
The  southern  Appalachians  furnish  an  excellent  illustration  of  this, 
the  rocks  being  very  generally  covered  with  a  mantle  of  loose  residual 
materials  of  variable  thickness  (frequently  exceeding  100  feet).  Fre- 
quently this  loose,  decayed-rock  mantle  must  be  removed  before 
quarrying  can  be  commenced. 

The  changes  involved  in  the  weathering  of  rocks  are  in  part  physical 
and  in  part  chemical  in  nature,  the  latter  representing  a  readjustment 
from  unstable  to  stable  compounds  under  prevailing  surface  conditions. 
The  processes  involved  may  be  simple  or  complex,  and  are  confined 
almost  entirely  to  the  belt  of  weathering  (p.  204),  or  surface  zone, 
which  extends  from  the  surface  to  the  level  of  groundwater  (Chapter 
VI).  They  are  wholly  atmospheric  and  are  operative  on  all  land 
surfaces  above  sea-level,  becoming  usually  quite,  if  not  entirely,  in- 
active, at  comparatively  slight  depths. 

Importance  of  rock  weathering.  —  Rock  weathering  is  of  funda- 
mental importance  from  the  purely  scientific  as  well  as  from  the 
economic  standpoint.  In  the  study  of  soils,  building  stone,  and  the 
superficial  portions  of  ore-deposits,  a  knowledge  of  the  principles  of 
weathering  is  indispensable. 

In  all  engineering  surface  projects,  in  the  selection  of  stone  for 
structural  and  decorative  purposes,  in  mining  and  quarrying  opera- 
tions, and  in  problems  of  water  supply,  as  well  as  in  excavations  of  all 
kinds,  the  engineer  is  concerned  either  with  the  direct  results  of  rock 
weathering  or  else  its  probabilities  as  affecting  any  stone  used  in 
constructional  work. 

215 


216  ENGINEERING  GEOLOGY 

WEATHERING  PROCESSES 

Definition  of  weathering.  —  All  physical  and  chemical  changes 
produced  in  rocks,  at  or  near  the  surface,  by  atmospheric  agents,  and 
which  result  in  more  or  less  complete  disintegration  and  decomposi- 
tion, are  commonly  grouped  under  the  general  term  of  weathering. 
The  action  of  physical  agents  alone  is  called  disintegration,  which 
results  in  the  rock  breaking  down  into  smaller  particles  without  de- 
stroying its  identity.  On  the  other  hand,  the  action  of  chemical 
agents  destroys  the  identity  of  the  mineral  particles  by  breaking  them 
up  into  new  compounds,  and  is  known  as  decomposition. 

In  most  cases  disintegration  and  decomposition  are  concurrent,  but 
for  a  given  locality  one  may  predominate  over  the  other.  Thus,  in 
the  arctic  regions,  disintegration  is  the  dominant  process  by  which  rock 
masses  are  broken  down,  while  in  tropical  regions,  decomposition 
becomes  the  important  process.  Again,  the  former  predominates  in 
the  arid  climate  of  the  west,  while  the  latter  is  the  dominant  factor 
in  the  east. 

Mechanical  Agents 

The  changes  produced  in  rock  masses  by  physical  agents  result  in 
disintegration,  and  ultimately  the  rock  crumbles  into  fine  particles  of 
the  consistency  of  sand  and  powder,  which  may  consist  of  fresh 
mineral  grains.  The  principal  mechanical  agents  involved  in  the 
disintegration  of  rocks  are  (1)  temperature  changes,  (2)  mechanical 
abrasion,  and  (3)  growing  organisms. 

Temperature  changes.  —  The  disintegration  of  rocks  through  tem- 
perature changes  may  result  from  (1)  unequal  expansion  and  contrac- 
tion of  the  component  minerals,  and  (2)  expansion  caused  by  alternate 
freezing  and  thawing  of  interstitial  water. 

Expansion  and  contraction.  —  Most  rocks  are  composed  of  an  aggre- 
gate of  minerals  each  one  of  which  has  a  different  rate  of  expansion. 
Unequal  expansion  and  contraction  of  the  individual  minerals  result 
both  from  diurnal  and  seasonal  changes  of  temperature.  In  the 
crystalline  rocks  the  mineral  particles  are  crowded  together  closely 
and  many  of  them  expand  unequally  in  different  directions.  When, 
therefore,  the  temperature  rises  the  minerals  crowd  against  each  other 
with  almost  irresistible  force,  and  when  the  temperature  lowers  they 
contract  and  draw  farther  apart  from  one  another.  The  result  of 
these  alternating  temperatures  producing  expansion  and  contraction 
is  to  weaken  the  rock,  and  the  formation  of  small  cracks  into  which 


PLATE  XXX,   FIG.  1.  —  Quartzite  broken  by  temperature  changes,   frost  and 
plant  roots.     Monroe,  N.  Y.     (H.  Ries,  photo.) 


FIG.  2.  —  Concretionary  sandstone,  weathered  by  solution  and  wind  action.     Snake 
Island,  near  Nanaimo,  B.  C.     (H.  Ries,  photo.) 


218  ENGINEERING  GEOLOGY 

water  may  percolate  and  chemical  action  set  up,  or  into  which  roots 
may  penetrate  and  further  aid  in  disintegration. 

The  coefficient  of  cubical  expansion  for  some  of  the  common  rock-forming  minerals 
is  given  by  Clarke  as  follows: 

Quartz 0000360  Calcite 0000200 

Orthoclase 0000170  Garnet 0000250 

Hornblende 0000284  Tourmaline 0000220 

* 

Bartlett  has  determined  experimentally  the  actual  expansion  of  granite,  marble, 
and  sandstone  to  be  as  follows: 

Granite 000004825  inch  per  foot  for  each  degree  F. 

Marble 000005668  inch  per  foot  for  each  degree  F. 

Sandstone 000009532  inch  per  foot  for  each  degree  F. 

While  these  figures  indicate  only  a  very  small  rate  of  expansion,  if 
continued  from  season  to  season  through  a  long  period  of  time,  the 
weakening  effect  produced  will  have  an  appreciable  bearing  upon  the 
economic  importance  of  the  stone.  Such  action  will  finally  result  in 
opening  invisible  cracks  and  crevices  in  the  rock,  or,  it  may  be,  in 
pulling  the  stone  away  from  the  mortar,  which  will  afford  ready 
entrance  for  water  and  thereby  pave  the  way  for  decay,  and  final  dis- 
integration of  the  stone  must  result. 

The  expansion  of  stone  when  heated  is  sometimes  recognized  by 
engineers,  in  placing  elastic  joints  in  long  walls  of  masonry. 

Expansion  due  to  alternate  freezing  and  thawing.  —  All  rocks  are 
more  or  less  porous  and  are  capable  of  absorbing  varying  amounts  of 
water.  In  passing  from  the  liquid  to  the  solid  state,  water  expands 
with  a  force  equal  to  about  150  tons  to  the  square  foot.  One  cubic 
centimeter  of  water  at  0°  C.  occupies  1.0908  cm.  in  the  form  of  ice  at 
0°  C.,  which  is  equivalent  in  expansion  to  about  one-tenth  of  the 
original  volume:  The  effects  produced  on  rocks  from  the  action  of 
continued  freezing  and  thawing  when  the  stone  is  saturated  with  water 
are  much  greater  than  from  expansion  and  contraction  through  di- 
urnal temperature  changes  described  above. 

Water  gains  access  into  rocks  through  the  openings  and  spaces  of 
various  kinds,  namely,  pores,  bedding  and  foliation  planes,  and  joint- 
ing and  other  fissile  planes.  The  latter  form  of  openings  (structures) 
permits  a  freer  circulation  of  water  than  the  pore  spaces  in  the  rock, 
and  at  times  the  water  may  collect  in  these  passages  more  rapidly 
than  it  can  be. carried  away,  so  that  if  the  temperature  lowers  to  the 
freezing  point  it  congeals  into  ice,  which  acts  as  a  wedge  to  force  the 
walls  farther  apart.  The  freezing  of  water,  however,  in  these  struc- 


ROCK-WEATHERING  AND  SOILS  219 

tures  in  building  stone,  except  in  some  stratified  and  foliated  rocks,  is 
usually  attended  with  less  danger  than  from  freezing  of  water  in  the 
pores. 

In  rocks  whose  pores  are  large  in  size  as  well  as  straight,  the  water 
of  saturation  may  be  expelled  with  comparative  readiness,  but  when 
the  cavities  are  of  subcapillary  size  the  water  is  retained  with  greater 
tenacity;  hence,  the  danger  from  freezing  in  the  latter  becomes  in- 
creasingly great.  Ordinarily,  then,  the  danger  from  freezing  of  water 
in  rocks  used  for  constructional  purposes  becomes  increasingly  great 
as  the  pores  approach  those  of  subcapillary  size  (see  further  in  Chap- 
ter XI). 

The  amount  of  water  contained  in  the  pores  at  a  given  time  depends 
upon  the  quantity  of  water  initially  absorbed,  the  time  that  has 
elapsed  since  absorption,  the  condition  of  the  atmosphere,  the  size  and 
shape  of  the  pores,  and  the  position  of  the  stone.  It  is  only  in  excep- 
tional cases  that  the  stone  in  the  wall  of  a  building  is  saturated 
(Buckley). 

Named  in  their  order  of  importance,  then,  it  is  possible  that  the 
factors  in  estimating  the  danger  from  freezing  and  thawing  are:  (1) 
size  of  pore  spaces,  which  controls  the  rate  at  which  the  interstitial 
water  is  expelled,  (2)  the  amount  of  water  contained  in  the  pores  at 
the  time  of  freezing,  and  (3)  the  total  amount  of  pore  space. 

Effects  of  frost  and  temperature  changes.  —  As  already  explained, 
small  cracks  may  be  started  by  temperature  changes,  and  into  these 
as  well  as  other  fissures  the  frost  works  its  way,  breaking  down  the 
stone  into  a  number  of  large  and  small  angular  fragments.  If  the  rock 
surface  is  flat  or  gently  sloping,  the  angular  debris  lies  where  it  was 
formed  (Plate  XXX,  Fig.  1),  but  if  the  disintegration  takes  place  on  a 
steep  hillside,  or  the  face  of  a  cliff,  the  material  falls  to  the  bottom  of 
the  slope  or  cliff  and  builds  up  a  talus  pile  (Plate  IX,  Fig.  2  and  Fig.  63), 
which  in  time  may  assume  large  size  (Plate  XXXI,  Fig.  1),  and  even 
eventually  break  down  to  a  fertile  soil. 

The  much-jointed  character  of  the  rocks  in  some  mountain  regions 
causes  frequent  and  dangerous  rock  falls,  as  the  water  freezing  in  them 
pries  off  large  and  small  pieces  of  rock. 

Foliation  planes  in  schist,  and  bedding  planes  in  sandstone,  are  good 
examples  of  lines  of  weakness  sought  out  by  frost,  so  that  stones  of 
this  sort  frequently  scale  off  when  set  in  a  building  on  edge,  instead  of 
on  the  natural  bed. 

Mechanical  abrasion.  —  Mechanical  abrasion  is  one  of  the  most 
important  agents  in  the  disintegration  of  rock  masses.  It  is  accom- 


PLATE  XXXI,  FIG.  1.  —  Talus  of  weathered  schist,  French  Pyrenees.  The  rock 
has  broken  down  to  a  soil  which  can  be  tilled,  but  has  to  be  terraced  to  prevent 
erosion.  (H.  Ries,  photo.) 


FIG.  2.  —  Diabase  dike,  Virginia.    The  weathering  has  broken  the  rock  down  to  a 

mass  of  boulders.    (T.  L.  Watson,  photo.) 
(220) 


ROCK-WEATHERING  AND  SOILS  221 

plished  mainly  by  wind  and  running  water  working  concurrently  with 
the  other  agents  of  disintegration. 

In  many  parts  of  the  world,  the  wind  does  considerable  work  in 
removing  the  fine-grained  products  of  rock  decay  or  other  sandy 
deposits.  Not  only  does  it  remove  this  loose  material,  but  often  drives 
it  with  such  force  against  rock  surfaces  as  to  wear  them  down  by 
mechanical  abrasion.  The  etching  and  engraving  of  glass  by  artificial 
sand  blasts  well  illustrates  the  nature  and  potency  of  this  agent. 
Many  authors  have  put  on  record  the  work  wrought  by  this  agent. 
J.  Walther  has  described  the  polishing  effect  of  the  wind-blown  sand 
on  the  Egyptian  monuments;  M.  Choisy  no,ted  similar  action  on  the 
rocks  by  the  blown  sand  of  the  Sahara;  Gilbert  has  observed  the 
peculiar  wearing  away  from  the  erosive  action  of  the  wind  of  the  blocks 
of  sandstone  and  limestone  in  the  western  United  States;  Endlich  has 
noted  some  interesting  results  wrought  by  wind  action  on  rocks  in 
Colorado;  and  Egleston  observed  that  the  gravestones  in  many  of 
the  churchyards  of  New  York  City  are  worn  nearly  smooth  and  the 
inscriptions  rendered  almost  illegible  by  this  agent.  Finally,  as  illus- 
trative of  the  work  done  by  wind-blown  sand,  mention  may  be  made 
of  telegraph  poles  that  have  been  worn  nearly  through  by  this  agent. 

The  work  accomplished  by  this  agent  is  naturally  most  effective 
in  arid  regions,  which  are  generally  characterized  by  an  almost  total 
absence  of  vegetation.  Its  effects,  however,  are  oftentimes  present 
in  our  humid  Atlantic  coast  climate,  where  the  beach  sands  are  caught 
up  and  driven  with  much  violence  before  the  wind.  In  the  case  of  one 
of  the  light-houses  on  Cape  Cod,  the  impact  of  the  wind-driven  sand 
was  so  great  on  the  heavy  glass  in  the  windows  as  to  render  some  of 
them  no  longer  transparent,  and  necessitating  their  removal  in  a  few 
instances. 

Naturally  the  action  resulting  from  wind  abrasion  is  a  very  slow 
one,  but,  after  long  lapses  of  time,  and  under  constant  blast,  the  effects 
are  manifest. 

Growing  organisms.  —  Both  plants  and  animals  aid  to  some  extent 
in  the  breaking  down  of  rock  masses,  through  action  that  is  partly 
physical  and  partly  chemical.  While  they  are  not  usually  the  princi- 
pal agents  involved  in  the  processes  of  rock  decay,  yet  they  become  at 
times  important  factors  in  such  destruction.  The  chemical  action 
resulting  from  these  organisms  is  mainly  that  of  deoxidation  and 
solution. 

An  important  function  of  plant  growth  is  the  retention  of  moisture, 
whereby  the  rock-surfaces  are  kept  constantly  damp,  and  thus  solvent 


PLATE  XXXII,  FIG.  1.  —  Granite  boulders  produced  by  disintegration  and  de- 
composition, Faith,  N.  C.     (T.  L.  Watson;  photo.) 


FIG.  2.  —  Granite  boulders  produced  mainly  by  disintegration  in  an  arid  climate. 

Winchester,  Cal.     (H.  Ries,  photo.) 
(222) 


ROCK-WEATHERING  AND  SOILS  223 

action  by  the  water  is  promoted.  Similarly  chemical  decay  among 
rocks  is  promoted  by  the  -  formation  of  vegetable  mould  (humus) 
derived  from  the  decay  of  plants,  by  the  retention  of  moisture,  by 
furnishing  carbon  dioxide  to  the  water,  and  by  a  leaching  process  which 
is  reducing  in  action. 

The  physical  action  exercised  by  plants  results  principally  from 
the  force  exerted  by  the  penetration  of  their  roots  into  cracks  and 
crevices,  which  tend  to  wedge  apart  the  rock,  and,  it  may  be,  in  the 
total  dislodgment  of  varying  size  fragments  from  the  parent  ledges. 
This  action  sometimes  results  in  partial  detachment  of  parts  of  the 
masonry  from  the  walls  of  buildings  and  other  structures,  where 
creeping  vines  are  allowed  to  cover  the  structure.  The  small  rootlets 
of  a  tree  penetrating  a  crevice  may  produce  little  effect,  but  as  these 
grow  and  expand  they  often  exert  a  powerful  force  (Plate  XXXIII, 
Fig.  2). 

While  plant  growth  may  promote  rock  decay,  it  may  also  exercise  a 
protective  action.  Where  vegetation  is  abundant,  the  erosive  action 
of  wind  and  rain  is  retarded.  Such  protective  influence  is  well  shown 
in  reclaiming  lands  over  parts  of  France  by  planting  trees  on  the 
extensive  sand  hills,  in  order  to  prevent  further  encroachment.  Similar 
protection  is  afforded  by  the  sage  brush  and  other  forms  of  plant 
growth  over  the  sandy  tracts  of  the  western  United  States.  In 
mountain  districts,  avalanches  are  sometimes  prevented  by  plant 
growth.  On  buildings  the  main  function  of  plant  growth  is  to  attract 
moisture  to  the  wall. 

Chemical  Agents 

Normal  atmospheric  air  consists  chiefly  of  a  mechanical  mixture  of 
nitrogen  and  oxygen  in  the  proportion  of  four  volumes  of  nitrogen  to 
one  of  oxygen.  In  addition  to  these,  there  are  usually  present  small 
quantities  of  other  substances,  chief  among  which  are  water  vapor 
and  carbon  dioxide.  Of  these  oxygen  and  carbon  dioxide  are  much 
the  most  important  chemically  active  compounds,  the  most  abundant 
constituent  nitrogen  being  chemically  inactive  under  normal  atmos- 
pheric conditions. 

Besides  the  gaseous  solutions  of  oxygen  and  carbon  dioxide,  the 
water  solutions  usually  contain  variable  amounts  of  different  sub- 
stances, especially  the  carbonates  of  the  alkalies  and  the  alkali  earths, 
and  acids,  such  as  hydrochloric,  sulphuric,  nitric,  etc.,  which  are  active 
agents  in  decomposing  rocks. 

The  most  important  chemical  reactions  that  take  place  in  the  belt 


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ROCK-WEATHERING  AND  SOILS  225 

of  weathering  as  the  result  of  the  action  of  various  agents  are:  (1) 
Hydration,  (2)  oxidation,  (3)  carbonation,  and  (4)  solution..  These 
are  discussed  below  in  the  order  named. 

Hydration.  —  By  hydration  is  meant  the  assumption  of  water  which 
results  in  the  production  of  hydrous  minerals.  Among  the  important 
hydrous  minerals  formed  are  many  silicates,  such  as  kaolin,  serpentine, 
talc,  chlorite,  zeolites,  etc.;  oxides,  especially  those  of  iron  and  al- 
umina, such  as  goethite,  turgite,  and  limonite,  diaspore  and  gibbsite; 
and  of  the  sulphates,  gypsum.  The  water  for  hydration  is  derived 
chiefly  from  the  atmosphere  and  the  reaction  is  one  of  the  most  exten- 
sive and  important  that  takes  place  hi  the  belt  of  weathering. 

By  comparing  analyses  of  fresh  and  decayed  rock,  it  will  be  found 
that  an  increase  in  water  invariably  occurs,  the  amount  of  water 
increasing  with  the  stage  of  decay.  In  rock  decomposition,  therefore, 
hydration  is  one  of  the  main  factors,  and,  when  not  accompanied  by  a 
loss  of  constituents  through  solution,  it  involves  expansion  of  volume 
and  great  liberation  of  heat,  becoming  thereby  a  physical  agent  of 
decay.  In  simple  hydration  the  volume  increase  ranges  from  a  very 
small  per  cent  to  as  high  as  160  per  cent,  but  commonly  the  increase  in 
volume  is  less  than  50  per  cent  (Van  Hise). 

Although  hydration  involves  increase  of  volume,  the  rocks  so 
affected  do  not  always  have  room  to  expand.  Engineers  engaged  in 
tunneling  have  sometimes  noticed  that  apparently  fresh  rock  when 
brought  to  the  surface  crumbles  rapidly.  This  is  because  the  rock, 
whose  minerals  are  partly  or  wholly  hydrated,  was  under  strain  while 
in  the  ground,  and  therefore  disintegrates  rapidly  when  released. 
This  slaking  has  been  observed  by  Merrill  in  the  granites  of  the  District 
of  Columbia  and  by  Derby  in  sedimentary  rocks  in  the  railway  cuttings 
of  Brazil.  Hydration  caused  by  percolating  water  may  at  times  cause 
swelling  and  heaving  ground  in  tunnels  or  mines. 

Brainier  quotes  the  Compte  de  la  Hure  who  gives  it  as  his  opinion  that  some  of  the 
hills  of  Brazil  have  actually  increased  in  height  through  hydration.  Merrill  "has 
calculated  that  the  transition  of  a  granitic  rock  into  arable  soil,  provided  the  same 
took  place  without  loss  of  material,  must  be  attended  by  an  increase  in  bulk  amount- 
ing to  88  per  cent."  Concerning  the  disintegration  of  the  District  of  Columbia  rocks, 
Merrill  says:  " Granitic  rocks  in  the  District  of  Columbia  have  been  shown  by  the 
author  to  have  become  disintegrated  for  a  depth  of  many  feet  with  a  loss  of  but  com- 
paratively small  quantities  of  their  chemical  constituents  and  with  apparently  but 
little  change  in  their  form  of  combination.  .  .  .  Aside  from  its  state  of  disintegration, 
the  newly  formed  soil  differs  from  the  massive  rock  mainly  in  that  a  part  of  its  feld- 
spathic  and  other  silicate  constituents  have  undergone  a  certain  amount  of  hydration.', 

Dehydration,  the  opposite  reaction  of  hydration,  while  not  recog- 
nized as  an  important  process  in  weathering  may  take  place  in  regions 


PLATE   XXXIV,   FIG.   1.  — Granite    quarry,    Manchester,  Va.     Shows    sheeted 
structure  of  granite  and  covering  of  residual  clay.     (H.  Ries,  photo.) 


FIG.    2.  —  Stone    Mountain,  Wilkes    County,  N.  C.      A  granite  dome,  which  has 
resisted  the  weather  better  than  the  surrounding  rocks.     (T.  L.  Watson,  photo.) 
(226) 


ROCK-WEATHERING  AND  SOILS  227 

of  high  temperature,  such  as  in  some  of  the  surface  hydrous  iron  com- 
pounds of  the  southern  Appalachian  soils. 

Oxidation.  —  The  process  of  oxidation  is  promoted  by  the  presence 
of  moisture  and  is  usually  accompanied  by  hydration.  All  rocks  which 
carry  iron  in  the  form  of  sulphide  (pyrite,  marcasite,  and  pyrrhotite) 
and  as  ferrous  oxide  in  many  silicates  (pyroxene,  amphibole,  micas, 
and  olivine)  and  carbonates,  are  oxidized  in  the  belt  of  weathering. 
The  process  is  also  of  great  importance  in  the  surface  alteration  of  ore- 
deposits  (see  Chapter  on  Ore-Deposits). 

The  principal  cause  of  weathering  in  these  cases  is  largely  the  affinity 
of  iron  in  the  ferrous  state  for  oxygen,  which  finally  results  in  a  chemi- 
cal combination  of  the  two,  forming  hydrated  ferric  oxide.  The  bright 
red  and  yellow  colors  of  the  residual  products  of  rocks  containing  these 
minerals  are  due  to  the  formation  of  iron  oxides  by  oxidation.  The 
red  and  yellow  soils  derived  from  the  deeply-weathered  crystalline 
rocks  of  the  Piedmont  province  in  the  southern  Appalachians  furnish 
an  excellent  illustration  of  the  oxidation  of  iron  compounds  to  ferric 
oxide.  The  early  stages  of  oxidation  accompanied  by  hydration  may 
frequently  be  observed  in  the  "sap"  portions  of  granite  and  other 
siliceous  crystalline  rocks  used  for  building  stone  containing  biotite  or 
other  ferromagnesian  minerals,  in  the  slight  discoloration  from  liberated 
iron  oxide  of  the  iron-bearing  minerals. 

Another  frequent  and  familiar  example  of  oxidation  is  that  of  the 
iron  sulphides  (pyrite,  marcasite,  and  pyrrhotite),  which  are  common 
constituents  of  many  rocks.  The  iron  becomes  oxidized  to  the  hydrated 
sesquioxide  form  (limonite,  turgite,  or  goethite)  with  the  liberation  of 
sulph-acids  which,  eventually,  through  oxidation,  form  sulphuric  acid, 
and  which  if  in  sufficient  amount  immediately  becomes  a  free  destroyer 
of  the  rock  in  which  the  mineral  liberating  it  occurs.  The  first  stage 
in  the  oxidation  of  the  sulphides  is  the  formation  of  the  corresponding 
sulphates.  When  formed  in  building  stones,  these  sulphates  sometimes 
cause  an  unsightly  scum  on  the  surface  of  the  building  (see  Chapter  XI). 

Oxidation  may  be  accompanied  by  either  decrease  or  increase  in 
volume.  Probably  decrease  in  volume  usually  attends  the  oxidation  of 
carbonates  and  sulphides,  but  oxidation  of  silicates  not  involving  a  loss 
from  solution  may  be  accompanied  by  increase  in  volume. 

The  oxidation  of  sulphides  is  a  most  important  process  in  the  weather- 
ing of  many  ore-deposits  (Chapter  XVII),  for  the  reason  that  the  sul- 
phates of  the  metals  thus  formed  are  carried  in  solution  to  lower  levels, 
where,  under  favorable  conditions,  they  may  be  reprecipitated  as  sul- 
phides. 


228  ENGINEERING  GEOLOGY 

Deoxidation.  —  Deoxidation,  the  reverse  of  oxidation,  is  a  less 
frequent  reaction  in  the  belt  of  weathering,  than  oxidation.  When 
carrying  organic  matter  in  solution  water  becomes  a  reducing  agent, 
and  ferric  iron  is  reduced  to  the  ferrous  condition,  which  in  the  presence 
of  carbon  dioxide  unites  to  form  ferrous  carbonate  (siderite).  From 
this  source  and  by  this  process  ferrous  carbonate  may  be  derived  for 
the  material  of  chalybeate  (iron)  springs,  and  the  iron-carbonate  de- 
posits (black-band  ore  and  clay  ironstone)  so  often  associated  with 
coal  beds.  Frequent  illustration  of  the  reaction  is  found  in  the  bleach- 
ing of  red  soils  to  gray  or  white  ones,  and  in  the  local  bleaching  of  some 
ferruginous  sands  and  sandstones.  By  a  similar  process  ferrous  sul- 
phates may  be  converted  into  sulphides. 

Carbonation.  —  Carbonation,  the  union  of  carbonic  acid  with  bases 
to  form  carbonates,  takes  place  on  a  vast  scale  in  the  belt  of  weather- 
ing, and  is  one  of  the  dominant  reactions.  It  consists  chiefly  in  the 
substitution  of  carbonic  for  silicic  acid  in  the  silicates.  It  has  been 
demonstrated  experimentally  that  carbon  dioxide  in  aqueous  solution 
attacks  many  minerals,  such  as  the  feldspars,  hornblende,  olivine, 
serpentine,  muscovite,  biotite,1  etc.,  among  the  silicates,  under  ordinary 
conditions  of  temperature  and  pressure.  The  carbonates  of  the  alka- 
lies and  the  alkali-earth  metals  formed  are  removed  in  solution.  They 
have  the  power  of  decomposing  many  silicates  and  hence  may  become 
important  agents  in  the  further  breaking  down  of  these  minerals. 

The  source  of  carbon  dioxide  for  the  process  of  Carbonation  in  the 
belt  of  weathering  is  derived  partly  from  the  atmosphere  in  which  it  is 
present  in  amount  equal  to  about  0.045  per  cent  by  weight,  and  partly 
by  oxidation  of  organic  materials  (plants  and  animals)  on  the  surface 
by  bacteria  and  oxygen.  Other  less  important  sources  are  known. 

The  process  of  Carbonation  in  silicates,  the  negative  side  of  which  is 
desilication,  is  accompanied  by  the  liberation  of  silica,  which  may  re- 
main as  quartz,  or  be  removed  in  solution  as  colloidal  silica.  It  has 
been  observed  that  when  plant  growth  is  abundant,  as  in  the  tropical 
regions,  the  amount  of  dissolved  silica  in  the  underground  waters  is 
larger  than  in  regions  where  vegetation  is  scant  or  lacking. 

Carbonation  may  take  place  without  other  reactions,  but  it  is  usually 
accompanied  either  by  hydration  or  by  hydration  and  oxidation.  In 
either  case  the  process  is  accompanied  by  an  increase  in  volume,  which 
rarely  falls  below  5  per  cent  and  may  be  as  high  as  75  per  cent,  with 
the  usual  range  between  15  and  50  per  cent  (Van  Hise). 

1  The  presence  of  carbon  dioxide  in  water  is  not  always  necessary  to  cause  the 
decay  of  these  minerals. 


PLATE  XXXV,  FIG.  1.  —  Elongated  boulders  of  granite,  produced  by  weathering 
along  the  joints,  Woodstock,  Md.     (T.  L.  Watson,  photo.) 


2.  —  Granite  boulder  showing  concentric  weathering,  Oglesby,  Ga.     (T.  L. 
Watson,  photo.) 

(229) 


230  ENGINEERING  GEOLOGY 

Solution.  —  Concurrent  with  and  promoted  by  the  chemical  proc- 
esses of  oxidation,  carbonation,  and  hydration,  described  above,  much 
mineral  matter  is  taken  into  solution  by  the  underground  waters  in  the 
belt  of  weathering.  The  dominant  processes,  carbonation  and  hydra- 
tion, render  the  compounds  more  soluble,  while  the  change  from  ferrous 
to  ferric  iron  by  oxidation  has  the  opposite  effect. 

The  rocks  most  readily  affected  by  solution  are  the  carbonates,  as 
limestones  and  dolomites,  and  in  the  former,  especially,  solution  some- 
times goes  on  actively  along  joint  and  stratification  planes  (Chapter 
VI).  Gypsum  is  also  attacked,  but  not  as  readily  as  limestone. 

This  dissolved  mineral  matter  in  the  belt  of  weathering  may  be  dis- 
posed of  in  one  of  several  ways:  (1)  It  may  be  delivered  in  part  to  the 
oceans  by  means  of  surface  streams,  (2)  much  of  it  may  be  carried 
lower  down  by  the  downward  percolating  waters  into  the  belt  of  cemen- 
tation and  there  precipitated  and  deposited,  and  (3)  it  may  be  partly 
precipitated  in  the  belt  of  weathering  as  in  the  formation  of  cave  de- 
posits, and  those  of  the  oxides  of  iron,  alumina,  and  manganese.  The 
process  of  secondary  enrichment  of  such  importance  in  many  ore-de- 
posits (Chapter  XVII)  is  a  phase  of  this  process. 

Only  a  very  few  minerals  are  readily  soluble  in  pure  water,  but 
chemically-pure  water  does  not  exist  in  nature,  and  when  carrying  in 
solution  certain  materials,  such  as  carbon  dioxide,  organic  matter,  etc., 
the  solvent  power  of  water  is  greatly  increased. 

As  early  as  1848  the  Rogers  brothers  showed  the  power  of  pure  water  to  ap- 
preciably dissolve  many  of  the  commonly  occurring  silicates,  and  that  within  less 
than  ten  minutes  the  action  of  carbonated  water  on  the  same  minerals  was  recog- 
nizable. 

T.  Mellard  Reade  estimates  that  the  amount  of  salts  annually  removed  in  solu- 
tion from  a  square  mile  of  the  earth's  surface  is  $6  tons,  divided  as  follows :  Calcium 
carbonate  50  tons,  calcium  sulphate  20  tons,  sodium  chloride  8  tons,  silica  7  tons, 
alkaline  carbonates  and  sulphates  6  tons,  magnesium  carbonate  4  tons,  and  oxide 
of  iron  1  ton. 

In  order  that  some  idea  may  be  had  of  the  total  amount  of  solids  removed  in 
solution  by  some  of  the  larger  rivers  the  following  table  taken  from  Russell  may  be 
cited  (see  further  under  Chapter  V) : 

Tons  per  year. 

Rhine 5,816.805 

Rhone 8,290^464 

Danube 22,521,434 

Thames 613,930 

Nile 16,950,000 

Croton 66,795 

Hudson 438,000 

Mississippi 112,832, 171 

Of  all  stone  ordinarily  used  for  building  purposes,  limestone  suffers 
most  from  solution,  its  solubility  being  given  in  the  ratio  of  1  to  1000 


ROCK-WEATHERING  AND  SOILS  231 

parts  in  water  charged  with  carbonic  acid.  This  becomes  the  more 
apparent  when  it  has  been  shown  that  the  total  solids  calculated  for 
European  and  American  river  waters  is  0.1888  of  which  0.088765  part 
per  thousand  is  calcium  carbonate.  These  figures  show  that  for  normal 
rivers  calcium  carbonate  is  approximately  one-half  of  the  total  solids. 
The  cementing  material  of  some  sandstones  (calcareous)  is  dissolved 
by  atmospheric  water,  causing  the  rock  to  crumble  into  loose  sand. 
On  the  other  hand,  the  calcium  carbonate  of  some  impure  limestones 
becomes  so  completely  removed  by  solution,  that  only  a  porous  skeleton 
of  clayey  and  siliceous  impurities  is  left.  The  rottenstone  used  for 
polishing  purposes  is  an  example  of  this. 

The  weathering  of  rocks  by  solution  begins  at  the  surface  and  also 
penetrates  the  rock  along  joint  planes  (Plate  XXXVII,  Fig.  1,  and 
Plate  LXIX,  Fig.  1). 

Summary  of  chemical  decay.  —  A  study  of  the  chemical  changes 
involved  in  the  weathering  of  siliceous  crystalline  rocks,  by  comparing 
analyses  in  the  usual  way  of  the  fresh  and  correspondingly  decayed 
rock,  as  shown  by  Merrill  from  his  own  work  and  that  of  others,  may  be 
summarized  as  follows: 

1.  Hydration  is  an  important  factor,  the  quantity  of  water  increas- 
ing as  the  stage  of  decomposition  advances,  and  in  the  early  stages  of 
weathering  it  may  be  the  most  important  factor. 

2.  The  formation  of  ferric  oxide  retained  as  a  pigment  in  the  insol- 
uble residual  decay  through  oxidation  of  ferrous  compounds. 

3.  There  is  in  every  case  a  loss  in  silica,  a  greater  proportional  loss 
in  lime,  magnesia,  and  the  alkalies  (soda  and  potash),  and  a  propor- 
tional increase  in  alumina  and  sometimes  iron  oxide,  resulting  on  the 
whole  in  a  decided  loss  of  materials  through  solution. 

4.  So  far  as  is  indicated  by  available  analyses  the  total  loss  of  con- 
stituents in  siliceous  crystalline  rocks  seldom  exceeds  60  per  cent  for 
the  entire  rock.     In  calcareous  rocks  the  loss  through  solution  may 
amount  to  99  per  cent  in  extreme  cases. 

Chemical  study  of  the  various  kinds  of  igneous  rocks  shows  the  total 
losses  produced  by  decomposition,  calculated  from  chemical  analyses 
of  the  fresh  and  correspondingly  decayed  rock,  to  be  as  follows: 


232  ENGINEERING  GEOLOGY 

TOTAL  PERCENTAGE' LOSSES  ACCOMPANYING  THE  DECAY  OF  IGNEOUS  ROCKS 


Rock. 

Locality. 

Per 
cent 
loss. 

Authority. 

Biotite  granite 

District  of  Columbia 

13  79 

Merrill   G   P 

Elberton   Ga 

7  92 

Watson   Thos  L 

Biotite  granite  

Oglesby,  Ga.   . 

7  71 

Watson   Thos   L 

Biotite  granite 

Lexington,  Ga. 

14  56 

Watson  Thos  L 

Biotite  granite  

Appling,  Ga  

15  84 

Watson   Thos  L 

Biotite  granite  ....  ;.     .......    . 

Newman,  Ga. 

38  45 

Watson  Thos  L 

Biotite  granite 

Oglesby,  Ga 

44  72 

Watson  Thos  L 

Biotite  granite  

Greenville,  Ga  

71  81 

Wat&on   Thos  L 

Porphyritic  biotite  granite  . 

Camak,  Ga 

34  04 

Watson   Thos  L 

Porphy  ritic  biotite  granite-gneiss  ...    . 
Biotite  granite-gneiss  
Biotite  granite-gneiss  

Coweta,  Ga  
Lithonia,  Ga  
North  Garden,  Va. 

35.07 
26.69 
44  67 

Watson,  Thos.  L. 
Watson,  Thos.  L. 
Merrill  G   P 

Nepheline  syenite  
Phonolite  

Fourche  Mtn.  region,  Ark  
Assig,  Bohemia 

55.28 
10  26 

Merrill,  G.  P. 

Diabase 

Medford,  Mass. 

14  93 

Merrill   G   P 

Diabase  

Chatham,  Va  

70  31 

Watson  Thos   L 

Basalt  

Bohemia 

43  96 

Basalt 

Haute  Loire  district  France 

60  12 

Diorite  

North  Garden,  Va. 

37  51 

Merrill 

Andesite  
Alnoite 

Grenada,  Windward  Islands.. 
Herkimer  County   N    Y 

63.09 
26  89 

Harrison  and  Merrill. 
Smyth   C   H    Jr 

Soapstone  
Soapstone 

Albermarle  County,  Va  
Fairfax  County,  Va. 

52.46 
77  95 

Merrill,'  G'.  P." 
Merrill   G  P 

Residual  clay  and  sand.  —  As  a  result  of  the  rock  being  broken 
down  by  weathering  there  forms,  as  already  stated,  a  mantle  of  in- 
coherent material,  which  if  clayey  in  its  nature  is  termed  a  residual 
clay;  if  sandy  a  residual  sand.  If  decomposition  has  been  active  the 
product  is  usually  clayey,  but  if  disintegration  has  been  the  dominant 
weathering  agent,  a  sandy  material  is  more  likely  to  result.  The 
character  and  extent  of  residual  clays  are  discussed  in  Chapter  XIII. 

The  following  analyses  give  the  composition  of  three  fresh  rocks  and 
the  residual  clays  derived  from  them,  but  it  should  be  pointed  out  that 
one  cannot  tell  from  the  composition  of  the  clay,  what  the  parent  rock  was. 
ANALYSES  OF  FRESH  ROCKS  AND  RESIDUAL  CLAYS 


Constituents. 

Gneiss.1 

Diabase.2 

Limestone.3 

Fresh. 

Decom- 
posed. 

Fresh. 

Decom- 
posed. 

Fresh. 

Decom- 
posed. 

SiO... 

60.69 
16.89 
9.06 

4.44 
1.06 
4*25 

2.82 

"0.25 
0.62 

45.31 
26.55 
12.18 

"6!i6 
1.10 
0.22 

47.90 
15.60 
3.69 
FeO    8.41 
9.99 
8.11 
0.23 
2.05. 

41.60 
37.20 
3.21 
0.30 
0.23 
0.02 

7.41 
1.91 
0.98 

28.29 
18.17 
1.08 
0.09 
41.57 
0.03 
H2O    0.57 

57.57 
20.44 
7.93 

0.51 
1.21 
4.91 
0.23 
0.38 
0.10 
6.69 

Al*Qi 

Fe"O3 

CaO.. 

MgO.  . 

K2O.. 

Na2O.. 

0.07 

CO2. 

P2O6  

0.47 
13.75 

Loss  on  ignition  

H2O    2.34 

13.54 

i  From  Virginia,  G.  P.  Merrill.    *  Penokee  district,  Mich.,  Irving  and  Van  Hise.    3  J.  S.  Diller,  authority. 


as 


^>tf- 
^«- 


PLATE  XXXVI,  FIG.  1.  —  Diabase,  showing  boulders  produced  by  weathering, 
surrounded  by  concentric  shells  of  decayed  rock.     (T.  L.  Watson,  photo.) 


FIG.  2.  —  Same  as  Fig.  1,  but  showing  one  of  the  boulders  in  more  detail. 

(233) 


234  ENGINEERING  GEOLOGY 

Mineral  resistance.  —  All  minerals  do  not  show  the  same  degree 
of  resistance  to  the  weathering  agents;  therefore,  other  things  being 
equal,  that  rock  will  yield  most  readily  which  contains  the  greater 
quantity  of  less  resistant  minerals.  Sulphides  yield  more  readily  than 
carbonates,  and  these  in  turn  weather  more  easily  than  silicates. 

From  a  general  study  of  weathering,  Buckman  (Ref.  2)  concludes  that  the  order 
of  probable  solubility  of  the  silicates  when  exposed  under  similar  conditions  is  as 
follows:  Nepheline,  leucite,  olivine,  apatite,  augite,  hornblende,  talc,  serpentine, 
epidote,  plagioclase,  orthoclase,  biotite,1  muscovite,  quartz  (least  soluble). 

As  a  further  result  of  his  studies,  Buckman  formulated  what  he  called  the  Laws 
of  Rock  Resistance,  which  are  as  follows: 

1.  The  more  basic  a  rock  becomes  the  more  rapid  is  decomposition;  and  the 
more  acid,  the  less  rapid. 

2.  An  increase  of  sodium  and  potash  accelerates  chemical  decomposition :    (a)  in- 
crease of  sodium  over  potash  decreases  relative  resistance;    (6)  increase  of  potash 
over  sodium  increases  relative  resistance. 

3.  The  more  magnesium  and  calcium  present  in  a  rock,  the  more  rapid  is  chemical 
decomposition:     (a)    increase   of   calcium   over   magnesium   decreases   resistance; 
(6)  increase  of  magnesium  over  calcium  increases  resistance. 

4.  Increase  of  iron  in  a  rock  lessens  resistance. 

5.  Increase  of  aluminium  checks  decomposition. 

6.  Silica  causes  less  rapid  chemical  decay. 

Relation  of  Structure  to  Weathering 

A  dense,  massive  rock,  even  though  made  up  of  minerals  of  com- 
paratively low  resistance,  will  withstand  the  attack  of  weathering 
agents  better  than  the  same  kind  of  rock  traversed  by  fractures  of 
different  kinds.  A  small  fissure  will  be  easily  discovered  by  the  agents 
of  decay,  and  persistent  fissures  may  sometimes  open  up  a  pathway 
for  the  weathering  agents  to  a  considerable  depth.  In  some  mines,  for 
example,  the  ore  may  be  weathered  to  a  depth  of  500  or  600  feet  at 
one  point,  and  only  100  feet  at  another,  the  greater  depth  in  the  former 
case  being  due  to  the  fact  that  joint  planes  or  fault  fissures  formed 
channels  of  access  for  the  surface  waters. 

Any  weak  spots  in  a  rock  weather  back  more  readily  (Plate  XXX, 
Fig.  2).  In  many  limestones,  we  find  layers  of  siliceous  or  clayey  im- 
purities interbedded  with  the  more  strongly  calcareous  ones.  When 
the  surface  waters  find  their  way  down  joints,  or  over  the  surface,  the 
more  soluble  layers  are  eaten  away,  while  the  impure  ones,  being  less 
soluble,  remain  in  relief  (Plate  LXIX). 

In  tunneling  and  mining,  streaks  of  soft,  weathered  rock  are  some- 
times met.  These,  in  some  cases,  represent  weathering  of  the  rock 

1  There  are  cases  where  biotite  seems  to  weather  more  readily  than  orthoclase,  but 
on  the  whole  it  seems  better  to  place  it  higher  up  in  the  series. 


PLATE  XXXVII,  FIG.  1.  —  Limestone  "chimneys,"  separated  by  hollows  caused 
by  solution  along  vertical  joint  planes.     (T.  L.  Watson,  photo.) 


FIG.  2.  —  Pinnacled  surface  of  limestone  bed  rock,  after*  the  residual  clay  has  been 
removed.    Limonite  pits,  Ivanhoe,  Va.     (H.  Ries,  photo.) 

(235) 


236 


ENGINEERING  GEOLOGY 


along  some  fissure,  or  in  other  instances  they  may  be  weathered  dikes 
which  have  been  less  resistant  than  the  wall  rock  (Chapter  II). 

On  the  surface  the  position  of  an  ore  vein,  or  dike,  may  be  repre- 
sented by  a  trench  or  a  ridge,  depending  on  whether  it  is  more  or  less 
resistant  than  the  country  rock. 

WEATHERING  OF  DIFFERENT  ROCKS 
Siliceous  Crystalline  (Igneous)  Rocks 

Igneous  rocks  like  granite  suffer  most  in  the  early  stages  of  weather- 
ing from  disintegration  caused  chiefly  by  changes  of  temperature,  al- 
though granulation  from  disintegration  is  accompanied  by  some  chemical 
action,  especially  hydration.  This  has  ample  verification  in  the  com- 
parison of  analyses  of  the  fresh  and  partially  decayed  rock,  in  the  usually 
very  small  percentage  of  silt  and  clay  in  the  partially  decayed  product 
as  shown  in  mechanical  analyses,  and  in  slight  discoloration  of  the  de- 
cayed rock  by  iron  oxides  set  free  from  the  iron-bearing  minerals 
through  oxidation.  The  incipient  stage  of  weathering  of  feldspathic 
rocks  may  usually  be  observed  in  exposed  ledges  by  the  whiteness  and 


FIG.  122.  —  Section  showing  formation  of  residual  clay  from  granite.  (A)  residual 
clay;  (B)  zone  of  clay  and  partly  decayed  rock  fragments;  (C)  un weathered 
granite. 

opacity  of  the  feldspars,  the  rock  having  undergone  kaolinization  from 
hydration.  The  amount  of  water  increases  rapidly  as  decomposition 
advances. 

In  the  more  advanced  stages  of  weathering,  carbonation,  oxidation, 
and  solution  promoted  through  atmospheric  waters  become  dominant 
factors  in  the  process.  As  a  result  of  these  changes,  disintegration  and 
decomposition,  the  rock  is  finally  reduced  to  sand  and  clay,  more  or 


PLATE  XXXVIII,  FIG.  1.  —  Residual  clay  derived  from  schist,  but  showing  no 
traces  of  the  structure  of  the  parent  rock.     (T.  L.  Watson,  photo.) 


FIG.  2.  —  Residual  clay  derived  from  gneiss.  The  banded  structure  of  parent 
rock  is  preserved,  and  dips  to  right.  The  vertical  grooves  are  pick  marks. 
(T.  L.  TVatson,  photo.) 

(237) 


238  ENGINEERING  GEOLOGY 

less  discolored  by  iron  oxides  set  free  through  decomposition  of  iron- 
bearing  minerals,  such  as  biotite,  hornblende,  etc. 

Unaltered,  massive  igneous  rocks  are  generally  traversed  by  joints, 
which  are  easy  lines  for  the  percolation  of  surface  waters,  that  move 
downward  along  the  vertical  joints  and  laterally  along  the  horizontal 
joints  (Plate  XXXV) ,  producing  decay  of  the  rock,  extending  inward 
from  the  joint  surfaces. 

An  interesting  form  of  weathering  frequently  observed  in  igneous 
rocks  is  illustrated  in  plates  XXXV  and  XXXVI,  which  show  granite 
and  diabase  boulders  consisting  peripherally  of  concentric  shells,  which 
break  off  one  after  another  in  passing  from  the  surface  towards  the 
center.  This  form  of  weathering  has  resulted  from  the  more  rapid 
decay  on  the  edges  and  corners  than  on  the  flat  sides  of  the  jointed 
blocks,  the  blocks  being  gradually  rounded  and  formed  into  boulder- 
like  masses  of  varying  size.  These  boulder-like  blocks  are  sometimes 
found  on  the  surface,  occurring  singly  or  in  groups  (Plates  XXXI,  Fig.  2, 
and  XXXII). 

In  the  advanced  stage  of  weathering  (decomposition)  of  metamor- 
phic  foliated  rocks,  such  as  gneiss  and  schist,  the  original  structure  of 
the  fresh  rock  is  frequently  perfectly  preserved  in  the  decayed  product 
(Plate  XXXVIII,  Fig.  2). 

Sedimentary  Rocks 

The  sedimentary  rocks,  as  explained  in  Chapter  II,  are  derived  from 
pre-existing  rocks  regardless  of  origin,  and  are  composed  therefore  of 
their  disintegrated  and  decomposed  products  which  have  become  con- 
solidated. It  is  natural  therefore  that  such  rocks  should  weather  as  a 
rule  through  changes  that  are  more  physical  than  chemical  in  nature 
than  the  igneous  rocks.  An  exception  to  this  statement  is  noted  in 
the  purely  calcareous  rocks.  Generally  speaking  then  we  may  say  that 
with  the  exception  of  the  purely  calcareous  rocks,  sedimentary  rocks, 
such  as  sandstones,  shales,  and  argillites,  weather  through  processes 
that  are  largely  mechanical. 

Sandstones.  —  Sandstones  vary  greatly  not  only  in  texture  and 
degree  of  compactness,  but  in  composition  and  cementing  material  as 
well.  Most  sandstones,  however,  are  composed  chiefly  of  quartz 
grains,  one  of  the  most  resistant  of  minerals  to  chemical  agents,  and  it 
suffers  chiefly  from  mechanical  breaking  up.  Those  sandstones  con- 
taining calcareous  and  ferruginous  cements  usually  crumble  and  fall 
away  to  sand  through  solution  of  the  cement  (Plate  XXX,  Fig.  2)  by 
atmospheric  waters.  On  the  other  hand,  those  sandstones  whose  bond 


ROCK-WEATHERING  AND  SOILS 


239 


is  silica  are  exceedingly  refractory  to  chemical  agents  and  suffer  through 
the  effect  of  physical  agents  (disintegration,  Plate  XXX,  Fig.  1).  On 
account  of  their  porosity  which  is  sometimes  appreciable  sandstones  often 
are  capable  of  absorbing  considerable,  but  variable,  amounts  of  water, 
and  in  climates  where  freezing  temperatures  are  reached,  they  may  suffer 
greatly  from  frost  action.  "It  is  to  their  great  absorption  power  that 
is  due  the  large  amount  of  disintegration  and  foliation  seen  in  the  softer 
sandstones,  as  the  Triassic  of  the  eastern  United  States  and  the  sub- 
Carboniferous  of  Ohio'"  (Merrill.)  (See  further  under  sandstones  in 
Chapter  on  Building  Stone.) 

Argillaceous  rocks  (shales  and  slates) .  —  These  are  indurated 
aluminous  or  clay  rocks,  the  individual  particles  of  which  are  extremely 
small  in  size  and  have  been  derived  from  weathering  of  pre-existing 
rocks.  They  are,  as  a  rule,  therefore,  refractory  rocks  to  purely  chemi- 
cal agents,  which,  except  in  the  calcareous  varieties,  break  down  from 
weathering  largely  through  physical  processes  (Fig.  123).  They  yield 
clays  which  differ  from  the  original 
rock  chiefly  in  the  degree  of  hydration 
and  the  state  of  oxidation  of  the  iron. 
The  deep  blue-black  argillites  of  Har- 
ford  County,  Maryland,  which,  accord- 
ing to  Merrill,  contain  considerable 
quantities  of  undecomposed  silicates, 
weather,  however,  largely  through 
chemical  change,  as  shown  in  a  per- 


Surface 


Shale 


FIG.  123.  —  Section  showing  residual 
clay  derived  from  shale. 


centage  loss  of  the  entire  rock  of  40.83 

per  cent  through  solution.     He  says: 

"The  first  physical  indication  of  decay  is  shown  by  a  softening  of  the 

slate,  so  that  it  may  be  readily  scratched  by  the  thumb  nail,  and  an 

assumption  of  a  soapy  or  greasy  feeling,  the  entire  mass  finally  passing 

over  to  the  deep  red-brown  unctuous  clay,  sufficiently  rich  in  iron  to 

serve  as  a  low-grade  ochre,  for  paints." 

Calcareous  rocks  (limestones  and  dolomites).  —  The  calcareous 
rocks,  especially  the  fine  crystalline  or  non-crystalline  compact  and 
homogeneous  limestones,  weather  almost  entirely  through  solution  effect, 
for  they  possess  a  minimum  capacity  for  absorbing  water,  and  are, 
therefore,  liable  to  little  or  no  injury  from  freezing.  This  has  ample 
verification  in  analyses  of  the  fresh  and  decayed  limestone,  in  which  a 
total  loss  for  the  entire  rock  by  removal  of  constituents  in  solution 
amounts  to  as  much  as  99  per  cent.  Further  confirmation  is  shown  in 
field  study,  where  vertical  sections  of  limestone  and  its  overlying 


240  ENGINEERING  GEOLOGY 

mantle  of  residual  decay  are  sharply  defined  from  each  other  (Fig. 
124).  In  some  districts  this  clay  contains  limonite  nodules,  and  when 
it  is  removed  to  obtain  these,  the  underlying  limestone  exhibits  a  curious 
pinnacled  surface  (Plate  XXXVII,  Fig.  2).  This  is  readily  manifested 
again  in  the  gravestones  made  of  limestone,  in  which  the  inscriptions 


Surface 


FIG.  124.  —  Section  showing  residual  clay  derived  from  limestone.     Note  the  sharp 
line  of  contact  between  clay  and  parent  rock,  as  well  as  irregular  surface  of  latter. 

are  rendered  illegible  in  many  cases  within  a  short  period  of  years  (see 
limestones  and  marbles  in  Chapter  on  Building  Stone). 

Calcic  limestones  yield  more  readily  to  the  solvent  effect  of  atmos- 
pheric waters  than  magnesian  or  dolomitic  limestones,  and  when  of 
coarsely  crystalline  texture,  as  in  southeastern  New  York,  the  crystal- 
line dolomite  weathers  through  granulation,  producing  at  the  base  of 
slopes  a  heap  of  dolomitic  sand. 

Gypsum.  —  Since  gypsum  forms  rock  masses  of  large  size,  often 
outcropping  at  the  surface,  its  weathering  qualities  should  be  referred 
to.  Like  limestone,  gypsum  is  soluble  in  surface  waters,  but  not  as 
readily.  The  weathered  surface  of  a  gypsum  deposit  may  therefore 
show  the  same  pinnacled  structure  and  underground  solution  channels 
as  are  found  in  limestone  areas.1  If  anhydrite  is  present,  this  will, 
under  the  influence  of  surface  waters,  alter  to  gypsum,  the  change  being 
accompanied  by  an  increase  in  volume.  In  some  gypsum  quarries, 
solution  channels,  filled  with  residual  clay  and  surface  dirt,  have  occa- 
sionally been  mistaken  for  fault  zones  by  the  quarrymen. 

Soils 

Having  discussed  the  weathering  processes  by  which  rocks  are  disintegrated  and 
decomposed  into  a  loose  mantle  of  varying  thickness  of  unconsolidated  rock  material, 
a  few  words  may  be  said  regarding  the  more  important  characteristics  of  the  super- 
ficial portion  of  the  mantle  to  which  the  name  soil  is  ordinarily  applied. 

1  For  caves  in  gypsum  see,  for  example,  Okla.  Geol.  Survey,  Bull.  11,  1913. 


ROCK-WEATHERING  AND  SOILS 


241 


Definition.  —  The  soil  may  be  considered  as  the  superficial,  unconsolidated 
mantle  of  disintegrated  and  decomposed  rock  material,  which,  when  acted  upon  by 
organic  agencies,  and  mixed  with  varying  amounts  of  organic  matter,  may  furnish 
conditions  necessary  for  the  growth  of  plants  (Coffey).  In  its  broadest  sense,  the 
term  soil  has  been  used  by  geologists  to  include  all  of  the  mantle  of  rock  decay,  but 
by  agriculturists  the  term  is  used  to  include  the  first  few  inches  of  the  surface  portion 
of  the  rock  mantle,  or  the  depth  to  which  most  of  the  small  forms  of  vegetation  have 
penetrated,  and  which  passes  by  insensible  gradation  into  the  subsoil,  which  in  turn 
merges  with  depth  into  the  underlying  decayed  rock  (in  the  case  of  residual  materials). 

Formation  of  soils.  —  Since  soils  have  been  formed  by  weathering  processes  the 
agents  involved  are  wholly  atmospheric,  and,  as  already  shown,  the  more  soluble 
portions  of  the  rock  may  be  removed  under  conditions  of  chemical  decay  (decompo- 
sition) either  partly  or  entirely  in  solution.  The  less  soluble  or  more  indestructible 
portions  of  the  rock  remain  to  form  the  mantle  of  unconsolidated  rock  material, 
the  superficial  portions  of  which  may  support  plant  life  and  is  ordinarily  mixed 
with  a  small  amount  of  organic  matter  (humus).  Some  soils  may  be  formed  largely 
through  the  action  of  physical  agents  when  little  or  no  loss  through  solution  may  be 
shown. 

While  all  soils  have  been  derived  from  the  disintegration  and  decomposition  of 
rocks,  it  must  be  understood  that  not  all  of  them  have  been  formed  from  weathering 
of  the  rocks  which  they  overlie.  On  the  contrary,  there  are  large  areas  of  soils  which 
owe  their  origin  not  to  the  decay  of  the  underlying  rocks,  but  represent  the  trans- 
ported and  deposited  products  of  rock  decay  by  either  (1)  water,  (2)  wind,  or  (3)  gla- 
cial ice. 

In  other  words  the  weathered  rock  material  formed  in  one  locality  and  from  a 
given  kind  of  rock  may  be  removed  from  the  place  of  formation  and  deposited  in 
another  locality  of  wholly  different  rock.  As  a  result  of  transportation,  the  ma- 
terials from  many  different  kinds  of  rocks  become  mixed  and  the  soils  are  both 
heterogeneous  and  complex  as  to  mineral  composition. 

Classification  of  soils.  —  According  to  whether  soils  have  been  formed  in  place, 
or  have  been  removed  from  their  original  place  of  formation,  we  may  group  them 
into  (1)  sedentary  or  residual  soils,  and  (2)  transported  soils.  These  two  primary 
groups  may  be  further  subdivided  on  the  basis  of  the  agencies  involved  in  trans- 
portation or  original  formation.  On  this  basis  Merrill  makes  the  following  classi- 
fication: 


Subdivisions  of  the  Regolith 


The 
regolith 


Sedentary 


Transported 


f  Residual  deposits 

I  Cumulose  deposits 

Colluvial  deposits 

Alluvial  deposits 
(including  aqueo- 
glacial) 

Aeolian  deposits 

Glacial  deposits 


Residuary  gravels,  sands  and  clays, 

wacke,  laterite,  terra  rossa,  etc. 
Peat,    muck,    and    swamp    soils,  in 

part. 
Talus  and  cuff  debris,   material  of 

avalanches. 
Modern  alluvium,  marsh  and  swamp 

(paludal)  deposits,  the  Champlain 

clays,  loess,  and  adobe  in  part. 
Wind-blown    material,    sand    dunes, 

adobe  and  loess,  in  part. 
Morainal  material,  drumlins,  eskers, 

osars,  etc. 


Sedentary  soils.  —  These  include  all  deposits  derived  by  the  processes  of  rock- 
weathering  or  from  organic  accumulation  in  place.  They  include  (1)  residual 
deposits,  derived  from  the  decay  of  the  immediately  underlying  rocks;  and  (2)  cumu- 
lose  deposits,  which  have  formed  in  place  from  the  accumulation  of  organic  matter 


242  ENGINEERING  GEOLOGY 

with  ordinarily  small  amounts  of  rock  waste,  such  as  many  of  the  peat  and  muck 
deposits  in  ponds  and  lakes. 

Varietal  names,  based  on  the  kind  of  rock  from  which  they  have  been  derived, 
may  be  given  to  residual  soils;  thus,  granite  soil,  limestone  soil,  etc. 

Transported  soils.  —  These  may  be  grouped  into  (1)  colluvial,  (2)  alluvial, 
(3)  aeolian,  and  (4)  glacial,  according  to  the  agent  involved  in  transportation. 
Colluvial  deposits  include  the  heterogeneous  masses  of  rock  waste  resulting  from 
the  transporting  action  of  gravity,  such  as  talus,  cliff,  and  avalanche  accumulations, 
etc.  Alluvial  deposits  have  been  formed  through  the  agency  of  running  water,  and 
are  usually  well  assorted,  and  therefore  bedded  or  stratified.  Aeolian  deposits,  as 
described  in  Chapter  II,  owe  their  origin  to  wind  action,  while  the  glacial  deposits 
are  the  result  mainly  of  ice  action  with  or  without  that  of  water  (Chapter  X) . 

According  to  texture,  soils  may  be  divided  into  sand,  sandy  loam,  loam,  clay 
loam,  and  clay.  A  loam  is  usually  defined  as  a  mixture  of  sand  or  clay  with  some 
organic  matter. 

Composition  of  Soils 

Soils  are  composed  of  mineral  and  organic  matter,  with  usually  the  former  pre- 
dominating, although  some  peat  and  muck  soils  may  contain  as  much  as  75  per 
cent  or  more  of  organic  matter.  Probably  the  average  in  organic  matter  in  most 
soils  is  less  than  3  or  4  per  cent. 

Mineral  matter.  —  This  varies  both  in  physical  character  and  chemical  com- 
position. Physically  the  soil  particles  vary  in  size,  shape,  weight,  and  color.  The 
chief  inorganic  constituents  in  most  soils  are  sand,  silt,  and  clay,  although  gravel 
and  larger  pieces  of  rock  may  be  present.  According  to  the  U.  S.  Bureau  of  Soils, 
they  are  graded  as  follows:  All  mineral  particles  from  2  mm.  to  0.5  mm.  in  diameter 
are  classed  as  sands;  silt  includes  particles  within  the  limits  of  0.05  and  0.005  mm.; 
and  all  particles  less  than  0.005  mm.  are  classed  as  clay.  Of  these  clay  exerts  the 
most  important  influence  in  determining  the  character  of  the  soil. 

Chemically  soils  vary  according  to  the  kind  and  proportion  of  the  various  minerals 
of  which  they  are  composed,  and  they  may  be  as  variable  as  the  rocks  from  which 
they  have  been  derived,  on  which  their  mineralogical  nature  largely  depends.  Soils 
contain  a  great  variety  of  minerals,  probably  all  the  common  rock-forming  ones  in 
many  cases,  and  any  mineral  commonly  occurring  in  rocks  may  be  found  in  soils, 
regardless  of  the  origin  of  the  particular  soil.  Unless  the  processes  involved  in  soil 
formation  are  entirely  mechanical,  there  is  a  tendency,  as  already  explained,  for  the 
more  soluble  constituents  of  the  rock  to  be  leached  out  in  the  change  to  soil,  which 
increases  the  relatively  insoluble  constituents,  such  as  quartz.  The  most  striking 
contrast  between  the  composition  of  the  parent  rock  and  its  derived  soil  is  best 
shown  in  the  purer  limestones,  which  weather  by  solution. 

Silica  in  the  form,  of  free  quartz  and  various  silicates,  alumina  as  hydrous  sili- 
cates, and  iron  as  hydrated  oxides,  make  up  from  80  per  cent  to  90  per  cent  of  the 
superficial  portions  of  most  deposits  (Merrill).  New  minerals  may  be  formed. 

Organic  matter.  —  The  organic  matter  in  soils  consists  of  the  remains  of  both 
plants  and  animals  in  various  forms  and  stages  of  decomposition,  about  which 
very  little  is  definitely  known.  Existing  in  the  form  of  humus,  the  organic  material 
in  soils  exerts  an  important  influence  upon  the  growth  of  plants.  Muck  and  peat, 
marsh  and  swamp,  and  meadow  types  of  soils  are  characterized  by  unusually  large 
percentages  of  organic  matter. 

Soil  Areas1 

For  the  purposes  of  soil  classification  the  U.  S.  Bureau  of  Soils  divides  the  United 
States  into  thirteen  subdivisions,  seven  of  which,  lying  east  of  the  Great  Plains,  are 
called  soil  provinces,  and  six,  including  the  Great  Plains  and  the  country  to  the  west, 
1  Summarized  from  Bull.  No.  96;  U.  S.  Bureau  of  Soils,  1913. 


ROCK-WEATHERING  AND  SOILS  243 

are  known  as  soil  regions.  A  soil  province  is  defined  as  an  area  having  the  same 
general  physiographic  expression,^  the  soils  in  which  have  been  produced  by  the 
same  forces,  and  throughout  which  each  rock  or  soil  material  yields  to  equal  forces 
equal  results.  A  soil  region  is  more  inclusive,  and  embraces  an  area  the  parts  of 
which  may  on  further  study  resolve  themselves  into  soil  provinces.  Soil  provinces 
and  soil  regions  are  essentially  geographic  features.  They  are  differentiated  on  the 
basis  of  geographic  features  rather  than  on  soil  character. 

The  soils  occurring  in  a  province  are  grouped  on  the  basis  of  certain  character- 
istics of  the  soils  themselves,  each  group  constituting  a  soil  series.  A  soil  series  is 
defined  as  a  group  of  soils  having  the  same  range  in  color,  the  same  character  of 
subsoil,  particularly  as  regards  color  and  structure,  broadly  the  same  type  of  relief 
and  drainage,  and  a  common  or  similar  origin.  A  soil  class  includes  all  soils  having 
the  same  texture,  such  as  sands,  clays,  loams,  etc.  A  soil  class  is  not  limited  in 
its  occurrence  to  a  soil  province,  but  the  same  class  occurs  in  all  the  provinces  or 
regions. 

The  soil  unit  or  the  soil  individual  is  the  soil  type;  that  is,  a  soil  which  through- 
out the  area  of  its  occurrence  has  the  same  texture,  color,  structure,  character  of 
subsoil,  general  topography,  process  of  derivation,  and  usually  derivation  from  the 
same  .material. 

The  soil  province  is  named  in  accordance  with  some  generally  accepted  termi- 
nology for  the  area  represented  or  according  to  the  process  by  which  its  soil  material 
was  formed.  A  soil  series  is  named  from  some  town,  village,  county,  or  natural 
feature  existing  in  the  area  when  it  was  first  encountered.  The  class  name  is  wholly 
descriptive. 

Taking  the  soils  as  a  whole,  so  far  as  they  have  been  classified  into  types,  the 
dominant  soils  of  the  United  States  are  the  silt  loams,  with  the  other  classes  follow- 
ing in  this  order:  Loams,  fine  sandy  loams,  clay  loams,  sandy  loams,  clays,  sands, 
and  fine  sands.  

List  of  References  on  Rock- Weathering  and  Soils 

1 .  Buckley,  E.  R.,  The  Properties  of  Building  Stones  and  the  Methods 
of.  Determining  their  Value,  Jour.  Geol.,  1900,  Vol.  VIII,  pp.  160-185, 
333-358,  526-567.  2.  Buckman,  H.  0.,  Chemical  and  Physical  Processes 
involved  in  Formation  of  Residual  Clay,  Trans.  Amer.  Ceramic  Society, 
XIII,  p.  336, 1911.  3.  Hilgard,  E.  W.,  Soils,  The  Macmillan  Company, 
N.  Y.,  1907,  593  pages.  4.  Merrill,  G.  P.,  The  Physical,  Chemical,  and 
Economic  Properties  of  Building  Stones,  Maryland  Geol.  Survey, 
1898,  Vol.  II,  pp.  47-123.  5.  Merrill,  G.  P.,  Stones  for  Building  and 
Decoration,  John  Wiley  &  Sons,  N.  Y.,  1903,  3d  edition,  551  pages. 
6.  Merrill,  G.  P.,  Rocks,  Hock-Weathering,  and  Soils,  The  Macmillan 
Co.,  N.  Y.,  1906,  400  pages.  7.  Van  Hise,  C.  R.,  A  Treatise  on  Meta- 
morphism,  Mon.  XLVII,  U.  S.  Geol.  Survey,  1904,  1286  pages. 
8.  Watson,  Thomas  L.,  Granites  and  Gneisses  of  Georgia,  Bull.  No. 
9-A,  Georgia  Geol.  Survey,  1902,  367  pages;  see  also  Bull.  G.  S.  A., 
1901,  vol.  12,  pp.  93-108. 

For  soils,  see  the  publications  of  the  Bureau  of  Soils,  U.  S.  Dept.  of 
Agriculture,  Washington,  D.  C.,  and  of  the  various  State  Agricultural 
Experiment  Stations. 


CHAPTER  V 
SURFACE  WATERS    (RIVERS) 

Introduction.  —  The  engineer  in  many  branches  of  his  work  is 
brought  face  to  face  with  the  work  of  rivers  past  or  present,  and  con- 
sequently needs  to  be  familiar  not  only  with  many  phases  of  the  work 
of  running  water,  but  especially  with  the  deposits  that  have  been  built 
up  by  streams.  The  living  streams  are  related  not  alone  to  one  kind 
of  engineering  work,  but  to  many. 

River  improvement,  surface  water  supply,  hydro-electric  power 
plants,  railroad  construction,  and  irrigation  are  all  connected  with  or 
affected  by  the  surface  flow  of  water  as  will  be  presently  explained. 

Stating  the  case  in  general,  by  way  of  introduction  we  may  say  that 
a  part  of  the  rain  water  which  falls  on  the  surface  gathers  together  to 
form  streams,  often  of  navigable  size.  These  streams  which  are  of 
varying  volume,  velocity  and  size  are  active  agents  of  erosion;  they 
carry  away  more  or  less  of  the  eroded  material,  and  deposit  it  again 
under  favorable  conditions. 

To  discuss  in  a  bald  and  theoretic  manner  the  way  in  which  they 
perform  their  work  may  be  of  scientific  interest,  but  unless  we  couple 
with  this  a  statement  of  its  bearing  on  engineering  work,  the  discussion 
loses  much  of  its  practical  significance. 

STREAM  FLOW 

Rainfall  and  run-off.  —  The  amount  of  rain  which  normally  falls 
in  any  given  region  varies  in  different  parts  of  the  country,  as  explained 
in  Chapter  VI,  and  only  a  portion  of  it  runs  off  on  the  surface  either 
directly  or  indirectly,  the  balance  being  disposed  of  partly  by  evapora- 
tion and  partly  by  seepage  into  the  ground  (see  further  in  Chapter  VI). 

That  portion  which  flows  off  the  surface  of  the  land  in  the  form  of 
visible  streams  is  known  as  the  run-off. 

Factors  controlling  run-off.  —  The  amount  of  run-off  is  affected  by: 
(1)  Amount  and  intensity  of  precipitation;  (2)  slope;  (3)  character  and 
condition  of  soil;  (4)  vegetation;  and  (5)  wind.  The  effects  of  these 
factors  are  as  follows: 

1.  A  heavy  shower  may  give  an  abundant  run-off,  but  a  light  one 

244 


SURFACE  WATERS   (RIVERS)  245 

may  be  absorbed  to  a  considerable  extent  by  the  soil,  before  much  run- 
off takes  place.  The  run-off  will  also  be  greater  during  a  steady  rain, 
because  in  such  event  the  soil,  unless  very  dry  and  porous,  becomes 
so  saturated  that  it  cannot  absorb  any  more  water,  and  the  entire 
precipitation  finds  its  way  to  the  streams.  So  too,  if  a  given  quantity 
of  rain  falls  in  a  short  time,  more  of  it  will  run  off,  than  if  the  same 
amount  were  precipitated  slowly,  and  the  soil  had  a  better  chance  to 
absorb  it.  A  frozen  soil  may  cause  similar  results.  Again,  if  a  warm 
rain  falls  on  snow,  the  latter  not  only  prevents  its  filtering  into  the  soil, 
but  the  melting  of  the  snow  adds  to  the  volume  of  the  streams.  The 
reverse,  however,  is  sometimes  found  to  occur,  as  when  a  heavy  snow 
fall  absorbs  the  rain,  and  lets  it  drain  off  gradually.  Melting  snows 
are  said  to  rarely  affect  large  streams,  but  the  same  is  not  true  of  smaller 
ones,  especially  in  hilly  or  mountainous  regions. 

2.  Water  will  drain  off  more  rapidly  on  a  steep  slope  than  on  a 
gentle  one.     Newell  states  that  a  rainfall  giving  as  high  as  30  to  40 
per  cent  run-off  on  the  steep  sides  of  a  mountain  range  may  not  produce 
more  than  3  or  4  per  cent  on  the  lower  levels  or  gently  rolling  plains. 

3.  Porous  soils  absorb  more  rain  than  dense  ones,  but  even  a  porous 
one  which  is  water  soaked  or  frozen  will  permit  rapid  surface  drainage. 

4.  Vegetation,  especially  forest  growth,  is  a  strong  deterrent  of  the 
surface  drainage,  and  exerts  a  beneficial  effect,  because  it  retains  the 
moisture  and  feeds  it  more  slowly  to  the  streams.1     Where  forests  have 
been  removed  from  the  watershed  of  a  river,  or  the  vegetation  de- 
stroyed in  other  ways,  the  rainfall  drains  off  rapidly,  and  the  stream 
may  be  subject  to  great  fluctuation. 

5.  Much  of  the  water  which  remains  on  the  surface  escapes  by  evapo- 
ration, but  the  rate  of  this  is  influenced  by  several  factors  such  as  the 
dryness  of  the  atmosphere,  temperature,  and  vegetation. 

Ratio  of  run-off  to  rainfall.  —  For  many  purposes,  irrigation  in 
particular,  it  is  desirable  to  know  the  ratio  of  run-off  to  rainfall,  but 
unfortunately  no  rule  can  be  made  to  apply  to  all  parts  of  the  country. 

On  some  watersheds2  of  the  eastern  and  more  humid  regions  of  the 
United  States,  having  a  rainfall  which  is  relatively  constant  in  quantity 
and  time,  there  appears  to  be  a  somewhat  consistent  ratio  between 
rainfall  and  run-off.  But  in  the  arid  west  no  such  constant  ratio  ap- 
pears to  exist. 

1  Chittenden,  H.  H.,  Relation  of  forests  to  stream  flow.     Trans.  Amer.  Soc.  Civ. 
Engrs..  1908,  and  Eng.  News,  Oct.,  1908. 

2  A  watershed  of  a  stream  includes  all  the  area  whose  drainage  runs  into  that 
stream. 


246 


ENGINEERING  GEOLOGY 


Run-off  from  different  watersheds.  —  The  table  given  below l  gives  the  run-off 
from  a  number  of  different  watersheds. 

MEAN  ANNUAL  RUN-OFF  FOR  VARIOUS  WATERSHEDS  IN  THE  UNITED  STATES 


River. 

Point  of  Measurement. 

Drainage 
area, 
square 
miles. 

Period. 

lun-off 
n  depth 
in  inch- 
es on 
drain- 
age 
area. 

Kern         

Bakersfield,  Cal  

2.340 

1896-1905 

4.36 

San  Joaquin 

Herndon,  Cal  

1,640 

1896-1901 

20  47 

Kinsrs 

Sanger  Cal 

1  740 

1897-1906 

20  38 

Sacramento 

Red  Bluff,  Cal  

4,300 

1902-1906 

24  06 

TJmatilla 

TJmatilla  Ore 

2  130 

Nov   1   1900  to 

3  94 

Will  amette 

Albany  Ore. 

4,860 

Dec.  31,  1900 
Jan   1   1899  to 

46  62 

Boise         

Boise,  Idaho  

2,610 

Dec.  31,  1908 
1895-1904 

15  60 

Green  

Green  River,  Wyo  

7,450 

May  1,  1896,  to 

4.81 

Laramie  
Red  

Uva,  Wyo  
Grand  Forks,  N.  Dak.  . 

3,180 
25,100 

Oct.,  31,  1906 

Mav,  1895,  to 
Oct.,  1903 

Sept.,  1902,  to 

1.10 
2.08 

Rio  Grande 

Rio  Grande,  N.  Mex.  .  . 

14,000 

Sept.,  1908 
Jan   1   1896,  to 

1  46 

Animas  

Durango,  Colo  

812 

Dec.  31,  1905 
July,  1895,  to 

14.86 

South  Platte 

Denver,  Colo  

3,840 

Dec.,  1905 
Jan   1,  1896,  to 

1  44 

Green  

Greenriver,  Utah  

38,200 

Nov.  30,  1906 
Jan.,  1895,  to 

3  17 

Logan 

Logan,  Utah  

218 

Dec.,  1908 
1S96-1900 

21  18 

Carson  

Empire,  Nev  

988 

1904-1906 
Nov.,  1900,  to 

6.25 

Truckee  . 

Vista,  Nev  

1,520 

Dec..  1906 

Sept.,  1899,  to 

9  18 

Humbolt 

Orleans   Nev 

13  800 

Dec..  1906 
Jan    1897  to 

0  25 

Dec.,  1906 

1  Compiled  by 
Survey  records. 


Newell  and  Murphy,  Irrigation  Engineering,  from  U.  S.  Geol. 


SURFACE  WATERS   (RIVERS) 


247 


MEAN  ANNUAL  RUN-OFF  FOE  VARIOUS  WATERSHEDS  IN  THE  UNITED  STATES 

— (Continued) 


River. 

Point  of  Measurement. 

Drainage 
area, 
square 
miles. 

Period. 

Run-off 
in  depth 
in  inch- 
es on 
drain- 
age 
area. 

Colorado 

Yuma,  Ariz 

225000 

Jan     1902  to 

1   15 

St.  Croix  

St.  Croix  Falls,  Wis.    . 

6,370 

Dec.,  1906 
1902-1904 

10  60 

Menominee  .  .  . 

Iron  Mountain,  Mich 

2420 

Sept     1902  to 

18  92 

Illinois 

Peoria   111 

13  200 

Sept.,  1906 
Apr   1    1903  to 

14  11 

Maumee  

Waterville,  Ohio 

6,110 

Jan.  30,  1906 
Dec.,  1898,  to 

13  61 

Scioto  

Columbus  Ohio 

1  050 

Jan.,  1902 
1889  to  July  1906 

10  43 

Duck  

Columbia,  Tenn. 

1,260 

Nov.  1    1904  to 

18  87 

Tennessee   . 

Chattanooga   Tenn 

21  400 

Dec.  31,  1908 
1899-1908 

23  63 

Tombigbee  

Columbus,  Miss.    . 

4,440 

1905-1908 

15  48 

Black  Warrior 

Cordova,  Ala 

1  900 

1900-1908 

19  37 

Alabama  

Selma,  Ala.  . 

15,400 

19CO-19Q8 

24  01 

Savannah  

Augusta,  Ga  

7,300 

1899-1908 

22  29 

Catawba  

Rock  Hill,  S.  C. 

2,990 

1895-1903 

25  21 

Tar  

Tarboro   N   C 

2  290 

1896-1900 

13  89 

Roanoke  

Randolph,  Va. 

3080 

1901-1905 

18  86 

Potomac. 

Pt  of  Rocks  Va 

9  650 

1895-1906 

14  40 

Oswego  

Oswego,  N  Y 

5  000 

1897-1901 

11  69 

Delaware 

Port  Jervis    N  Y 

3  250 

1904-1908 

22  20 

Susquehanna 

Binghamton   1ST  Y 

2400 

1901-1906 

28  88 

Hudson 

Alechanicsville  N  Y 

4  500 

1891-1900 

22  95 

Mohawk  

Dunsbaoh  Ferry    N  Y 

3  440 

1898-1907 

23  28 

248 


ENGINEERING  GEOLOGY 
DISCHARGE  OF  VARIOUS  RIVERS 


River. 

Discharge  in  Cubic  Feet 
per  Second. 

Ratio  of 
Minimum 
to 
Maximum. 

Extreme 
Range 
between 
High  and 
Low 
Water. 

Remarks. 

Mini- 
mum. 

Maxi- 
mum 

Annual 
Mean. 

Columbia  

48,500 
1,500 
30,000 
45,000 

65,666 
7,000 
15,000 
175,000 
1,400 

5,600 

1,390,000 
117,000 
1,200,000 
1,507,000 
1,617,000 
1,740,000 
650,000 
900,000 
260,000 
439,000 

67,000 
'  225,666 

1  to  28.  7 
1  to  23.  4 
1  to  40.0 
1  to  33.  5 

"1  to  26.8" 
1  to  92.  8 
1  to  60.0 
1  to  1.49 
1  to  313.5 

'"19.7"' 
42.5 
54.0 
52.5 
21.4 
19.0 
35.0 

"'ssie'" 

63.5 
59.4 

Measured  at  Dalles. 

In  dry  seasons  flow 
all  subsurface. 

Mississippi  at  St.  Paul  
Mississippi  at  St.  Louis  
Mississippi  at  Cairo  
Mississippi  at  Vicksburg  
Mississippi  at  New  Orleans  
Missouri  at  Sioux  City  
Missouri  at  mouth  
Niagara 

100,000 
219,850 

Ohio  at  Pittsburgh  
Ohio,    just    below    Kanawha 
River 

Ohio,    just    below    Kentucky 
River 

6,900 

Rio  Grande  at  El  Paso 

16,600 

330,000 
468,000 

1,500 
251,900 

St.  Lawrence 

185,000 
3,700 

1  to  1.78 
1  to  126 

""58;6"' 

Tennessee  at  Chattanooga  

In  comparing  the  run-offs  from  different  watersheds,  all  the  influencing  factors 
must  be  considered,  otherwise  serious  errors  may  result.  If  deductions  are  to  be 
made  from  such  comparisons,  it  is  important  not  only  to  compare  areas  containing 
similar  conditions,  but  having  approximately  the  same  size.  Such  deductions  can 
not  take  the  place  of  direct  measurements. 

Stream  formation.  —  As  the  rain  falls  on  the  surface,  that  portion 
which  runs  off,  becomes  rapidly  concentrated  along  definite  lines  due 
to  inequalities  of  the  surface,  thus  developing  a  series  of  rivulets,  which 
in  turn  converge  to  form  brooks,  and  these  to  form  large  streams  or 
rivers.  The  total  quantity  of  water  conveyed  to  the  sea  being  very 
large,  it  is  estimated1  that  rivers  carry  about  6500  cubic  miles  of 
water  to  the  ocean  annually. 

This  is  conveyed  by  rivers  of  varying  size  and  length.  Some  are 
mere  brooks,  others  are  mighty  streams,  occasionally  flowing  along 
with  irresistible  force.  They  are  not  only  more  numerous  in  regions 
of  abundant  rainfall,  but  have  a  larger  number  of  branches. 

Those  which  flow  throughout  the  year  are  known  as  permanent 
streams,  while  those  which  flow  during  but  a  part  of  the  year  are  tem- 
porary or  intermittent  streams,  and  many  of  those  in  arid  regions  are  of 
this  latter  class.  Streams  become  permanent  for  all  parts  of  their 
courses  sunk  below  the  level  of  the  upper  surface  of  groundwater, 
when  they  are  independent  of  the  run-off  of  showers  (see  further  under 
Chapter  VI).  Each  stream  performs  similar  work,  but  that  done  it 
differs  in  degree  and  constancy. 

1  Chamberlin  and  Salisbury,  Geology,  I. 


PLATE  XXXIX,  FIG.  1.  — Hillside  gullied  by  erosion,  Lyell  gullies,  near  Milledge- 
ville,  Ga.     (T.  L.  Watson,  photo.) 


FIG.  2.  —  Gravelly  character  of  material  carried  by  swiftly  flowing  stream.     (T. 

L.  Watson,  photo.) 

(249) 


250  ENGINEERING  GEOLOGY 

The  water  which  is  held  by  the  soil  and  slowly  drained  into  the 
streams  is  of  great  benefit  to  navigation,  since  it  keeps  up  the  supply, 
at  a  time  when  there  may  be  little  or  no  surface  run-off.  One  large 
river  with  its  tributaries  may  therefore  drain  a  very  large  area,  which 
is  called  its  basin. 

Some  rivers  show  great  irregularity  of  flow,  having  a  large  volume 
during  spring  and  early  summer,  and  running  almost  dry  in  autumn. 
But,  whatever  the  size  of  a  river,  its  behavior  is  governed  by  certain 
laws,  so  that  either  a  large  or  a  small  stream  may  exhibit  the  same 
sinuosities,  bars,  eddies,  or  floods. 

In  the  case  of  navigable  rivers,  it  is  necessary  to  maintain  a  free, 
unobstructed  channel,  and  since  much  of  the  work  done  by  the  current 
of  a  stream  is  injurious  to  the  permanence  of  such  conditions,  it  is  of 
the  highest  importance  for  an  engineer  engaged  in  river  improvement 
to  understand  the  nature  of  the  work  performed  by  river  currents,  so 
that  if  necessary  he  can  control  and  regulate  it.  Indeed,  the  work  of 
river  improvement  is  one  of  the  most  important  branches  of  civil 
engineering. 

In  studying  a  river  with  the  view  to  improving  it  for  navigation,  or  using  it  for 
water  power,  irrigation,  or  water  supply,  the  collection  of  data  regarding  rainfall, 
stream  discharge,  etc.,  should  extend  over  a  considerable  period  of  time. 

Plate  XL,  Figs.  1  to  4  are  of  interest  in  that  they  show  the  difference  in  behavior 
of  typical  rivers  of  the  eastern  humid  region  of  the  United  States  and  the  western 
arid  portion.  The  height  of  the  black  lines  illustrates  the  relative  quantity  of  water 
expressed  in  cubic  feet  per  second,  or  second  feet,  occurring  throughout  the  year. 

The  diagram  shows  that  the  greater  flow  of  the  Susquehanna  River  at  Harris- 
burg,  Pa.,  occurs  in  the  spring,  followed  by  a  summer  drought,  especially  in  late 
August  or  early  September.  On  the  Yadkin  River  at  Salisbury,  N.  C.,  on  the  con- 
trary, the  greatest  flow  is  due  to  short  quick  floods  in  late  summer  and  early  autumn, 
and  came  probably  from  heavy  storms  on  the  mountains. 

Interesting  comparison  is  afforded  by  the  diagram  of  the  Gila  River  at  Buttes, 
Ariz.,  which  shows  a  relatively  small  steady  flow  in  the  early  part  of  the  year,  fol- 
lowed by  erratic  floods  due  to  cloudbursts  on  the  drainage  basin.  Strongly  con- 
trasted with  this  is  the  large  and  comparatively  uniform  flood  in  the  Green  River 
at  Blake,  Ariz.,  which  is  typical  of  streams  coming  from  the  snow-clad  mountains, 
whose  melting  snows  supply  water  as  the  summer  heat  melts  them.1 

Measurement  of  water.  —  This  may  be  done  with  two  different 
objects  in  view:  (1)  measurement  of  supply,  and  (2)  measurement  of 
duty  requirement. 

"  Measurement  of  supply  is  for  the  purpose  of  determining  the  quan- 
tity of  water  available  for  irrigation,  power  development  and  domestic 
use.  It  includes  the  measurement  of  run-off  from  the  various  streams 
and  to  a  limited  degree  also  the  determination  of  underground  flow 

1  Newell  and  Murphy,  Principles  of  Irrigation  Engineering,  1913. 


SURFACE  WATERS   (RIVERS) 


251 


s  §  I  i  i  §  i  §  §  i  §  i  § 

g    8  3   S  3   S  8   8  S  Sf  S*  S!  « 

OQ 


;§§§§§§§§§  °- 

Jill  3"  1  2  S"  I  *  *  *  a 

PLATE  XL,  Diagrams  showing  volume  of  discharge:  FIG.  1.  —  Susquehanna 
River,  Harrisburg,  Pa.,  1896.  FIG.  2.  —  Yadkin  River,  Salisbury,  N.  C.,  1898. 
FIG.  3.  — Gila  River,  Buttes,  Ariz.  FIG.  4.  — Green  River,  Blake,  Utah,  1897. 
(After  Newell  and  Murphy,  Irrigation  Engineering.) 


252  ENGINEERING  GEOLOGY 

which  may  be  made  available  for  use  through  pumping  or  artesian 
flow.  Measurement  of  duty  requirement  includes  the  determination 
of  the  amount  used  for  irrigation,  power  development  and  other  pur- 
poses. Both  of  the  above  classes  of  measurements  are  necessary  in  an 
enterprise  involving  the  use  of  water;  the  first  to  determine  the  amount 
available  and  the  second  to  determine  the  extent  of  an  enterprise  which 
a  given  supply  will  furnish."  (Newell  and  Murphy.) 

Stream  measurement.  —  This  is  accomplished  by  measuring  the 
total  quantity  of  water  passing  a  given  point  in  a  stream,  and  from 
this  determining  the  run-off  from  the  watershed. 

Unit  of  measurement.  —  The  two  classes  used  represent  quantity 
and  rate  of  flow  respectively.  Units  of  quantity  are  the  gallon,  cubic 
foot  and  acre  foot.  The  first  two  may  be  employed  to  express  the 
quantity  of  water  stored  or  used  for  domestic  purposes,  but  the  last 
is  more  commonly  used  in  engineering  estimates  of  irrigation  work. 
The  acre  foot  is  the  amount  of  water  required  to  cover  1  acre,  1  foot 
deep,  and  is  equal  to  43,560  cubic  feet. 

The  rate  of  flow  is  the  quantity  of  water  flowing  through  a  pipe  or 
channel  in  a  given  unit  of  time,  usually  a  second.  The  miner's  inch 
and  the  second  foot  are  the  common  units.  A  miner's  inch  represents 
the  quantity  of  water  which  flows  through  an  opening  1  inch  square 
under  a  given  head,  usually  4  inches.  A  second  foot  can  be  defined  as 
the  delivery  of  1  cubic  foot  per  second  of  time.  This  is  a  more  definite 
unit  of  measurement  than  the  miner's  inch. 

Second  feet  per  square  mile  is  the  average  number  of  cubic  feet  of 
water  flowing  per  second  from  each  square  mile  of  area  drained,  it 
being  assumed  that  the  run-off  is  evenly  distributed. 

Run-off  in  inches  is  the  depth  to  which  the  drainage  area  would  be 
covered  if  all  the  water  flowing  from  it  in  a  given  period  were  con- 
served and  uniformly  distributed  over  the  surface. 

Convenient  equivalents.  —  The  following  is  a  list  of  convenient  equivalents  for 
use  in  hydraulic  computations: 

1  second-foot  equals  7.48  United  States  gallons  per  second;  equals  448.8  gallons 

per  minute;   equals  646,272  gallons  for  one  day. 
1  second-foot  equals  6.23  British  imperial  gallons  per  second. 
1  second-foot  for  one  year  covers  1  square  mile  1.131  feet  or  13.572  inches  deep. 
1  second-foot  for  one  year  equals  31,536,000  cubic  feet. 
1  second-foot  equals  about  1  acre-inch  per  hour. 
1  second-foot  for  one  day  covers  1  square  mile  0.03719  inch  deep. 
1  second-foot  for  one  30-day  month  covers  1  square  mile  1.116  inches  deep. 
1  second-foot  for  one  day  equals  1.983  acre-feet. 
1  second-foot  for  one  30-day  month  equals  59.50  acre-feet. 
100  United  States  gallons  per  minute  equals  0.223  second-foot. 
100  United  States  gallons  per  minute  for  one  day  equals  0.442  acre-foot. 


SURFACE  WATERS   (RIVERS)  253 

1,000,000  United  States  gallons  per  day  equals  1.55  second-feet. 

1,000,000  United  States  gallons  equals  3.07  acre-feet. 

1,000,000  cubic  feet  equals  22.95  acre-feet. 

1  acre-foot  equals  325,850  gallons. 

1  inch  deep  on  1  square  mile  equals  2,323,200  cubic  feet. 

1  inch  deep  on  1  square  mile  equals  0.0737  second-foot  per  year. 

1  foot  equals  0.3048  meter. 

1  mile  equals  1.60935  kilometers. 

1  acre  equals  0.4047  hectare. 

1  acre  equals  43,560  square  feet. 

1  acre  equals  209  feet  square,  nearly. 

1  square  mile  equals  2.59  square  kilometers. 

1  cubic  foot  equals  0.0283  cubic  meter. 

1  cubic  foot  equals  7.48  gallons. 

1  cubic  foot  of  water  weighs  62.5  pounds. 

1  cubic  meter  per  minute  equals  0.5886  second-foot. 

1  horse-power  equals  550  foot-pounds  per  second. 

1  horse-power  equals  76  kilogram-meters  per  second. 

1  horse-power  equals  746  watts. 

1  horse-power  equals  1  second-foot  falling  8.80  feet. 

1£  horse-power  equals  about  1  kilowatt. 

„,        .  .  .  .       Sec.-ft.  X  fall  in  feet 

To  calculate  water-power  quickly:    -  — — —  -  =  net  horse-power  on 

water  wheel  realizing  80  per  cent  of  theoretical  power. 

WORK  PERFORMED  BY  RIVERS  AND  ITS  ECONOMIC  APPLICATION 
The  work  performed  by  rivers  is  of  three  kinds,  as  follows: 

1.  Work  of  erosion,  which  is  mainly  of  a  mechanical  nature,  but  in 
part  is  chemical.     Through  it  the  river  carves  its  channel  of  variable 
size  in  either  hard  or  soft  rocks.     The  process  is  usually  slow,  except 
in  soft  materials,  when  under  favorable  conditions  the  process  may  be 
rapid  and  destructive. 

2.  Transportation,  by  means  of  which  the  stream  removes  more  or 
less  effectively  the  material  derived  from  erosion,  and  the  material 
supplied  to  it  in  other  ways. 

3.  Deposition,  or  the  dropping  of  the  material  which  it  has  carried  in 
variable  quantity  for  different  distances. 

One  problem  of  the  engineer  who  has  to  deal  with  the  work  of  running 
water  is  to  see  that  the  river  performs  these  several  functions  at  the 
proper  time  and  in  the  proper  place.  The  discussion  of  the  latter  will 
be  kept  separate  so  far  as  is  possible,  although  this  cannot  always  be 
done. 

Work  of  Erosion 

Erosion.  —  The  work  of  erosion  performed  by  rivers  may  be  me- 
chanical (corrasion),  and  chemical  (corrosion).  Both  may  be  going  on 
at  the  same  time,  but  the  former  is  usually  the  more  important  of  the 
two. 


254  ENGINEERING   GEOLOGY 

Corrasion.  —  Pure  water  does  but  little  erosive  work,  unless  it  is 
flowing  swiftly  over  unconsolidated  material  like  sand,  or  loose  soil, 
but  when  running  over  hard  rock,  even  though  its  velocity  is  high,  the 
water  alone  has  little  wearing  effect. 

A  heavy  rainstorm  falling  on  the  soil  of  a  hillside  will  sometimes 
wash  out  a  large  gully  in  a  short  time  (Plate  XXXIX,  Fig.  1),  and 
neglect  to  prevent  or  stop  this  results  in  the  removal  of  much  material 
from  the  surface  in  some  areas,  and  deterioration  in  land  values  (Refs. 
4  and  6).  In  all  cases  the  damage  is  serious  whether  it  involves  the 
erosion  of  farm  lands  or  railway  embankments.  Earth  dams  built  of 
residual  clay  (q.v.)  are  also  liable  to  injury  from  this  cause. 

In  contrast  to  this  we  have  the  case  of  the  swift,  but  sediment-free 
Niagara  River  flowing  over  hard  lichen-covered  rocks,  and  yet  not 
having  enough  erosive  power  to  remove  the  green  vegetable  growth. 

The  corrasive  work  of  a  stream  is  performed  chiefly  with  the  aid  of 
the  sediment  which  it  carries.  This  consists  of  mineral  matter  ranging 
all  the  way  from  fine  clay  to  coarse  stones  in  different  streams,  the 
grains  acting  like  cutting  tools.  A  sluggish  stream  carries  only  fine 
sediment,  while  a  mountain  torrent  carries  or  rolls  along  stones  of  large 
size  (Plate  XXXIX,  Fig.  2). 

It  can  be  easily  seen  that  the  amount  of  mechanical  wear  which  a 
stream  accomplishes  depends  on  the  character  of  the  rock,  stream 
velocity,  and  load  of  sediment.  Both  the  sediment  carried  in  sus- 
pension and  that  rolled  along  the  bottom  will  wear  the  stream  channel. 
A  swift  sediment-laden  stream  cuts  its  channel  with  comparative 
rapidity,  but  at  a  different  rate  in  different  kinds  of  rock,  and  assum- 
ing the  sides  and  bottom  of  the  channel  to  be  of  equal  resistance  does 
its  main  work  of  cutting,  vertically.  A  slow  stream  cuts  more  actively 
laterally,  and  does  not  deepen  its  valley  much.  As  a  result  of  this  the 
sluggish  stream  is  likely  to  develop  flats. 

Corrosion.  —  The  work  of  solution  or  corrosion  performed  by  a 
river  is  usually  of  secondary  importance,  except  in  limestone  areas. 
All  river  waters  contain  dissolved  mineral  matter,  but  it  is  probable 
that  most  of  this  is  contributed  to  the  river  by  underground  waters. 
Some  mineral  matter,  however,  is  dissolved  from  the  sides  and  bottom 
of  the  river  channel,  especially  where  the  latter  is  of  soluble  rock  like 
limestone,  and  the  water  is  somewhat  acid  in  character. 

Factors  governing  rate  of  erosion.  —  The  main  factors  affecting 
the  erosive  power  of  a  river  are,  slope,  character  and  structure  of  rock, 
and  climate. 

The  steeper  the  grade,  the  higher  the  velocity,  and  the  greater  the 


SURFACE  WATERS   (RIVERS) 


255 


SKETCH  MAP 

LOWER   COLORADO  RIVER 

AND 
SALTON  BASIN 

OCT.  1,  1906 


FIG.  125.  —  Map  of  Salton  Sink  and  Imperial  Valley,  California.  Shows  points  at 
which  river  broke  through  its  banks.  (From  Thomas  and  Watt,  Improve- 
ment of  Rivers.) 

transporting  power  of  the  river;   hence,  other  things  being  equal,  the 
larger  the  amount  of  erosion  it  is  capable  of  accomplishing. 

Hard  and  firm  rocks  resist  erosion  more  than  loose  and  unconsoli- 
dated  ones.  Some  years  ago  when  the  Colorado  River  broke  through 
its  banks  and  flowed  down  into  the  Imperial  Valley  (Fig.  125),  the 


256  ENGINEERING  GEOLOGY 

rapidity  and  depth  of  erosion  accomplished  by  the  New  River  flowing 
over  sandy  beds  was  astonishing.  "Near  the  town  of  Imperial  early  in 
1906,  the  river  was  flowing  in  a  shallow  depression,  but  by  August  a 
chasm  had  been  cut  there  to  a  depth  of  80  feet  and  with  a  width  of 
1200  feet"  (Thomas  and  Watt). 

But  even  very  hard  rock  may  be  worn  with  comparative  rapidity  if 
it  is  traversed  by  a  swift  stream  transporting  resistant  and  sharp  angu- 
lar grains.  Cases  of  this  are  frequently  seen  in  sluiceways  lined  with 
vitrified  brick,  and  used  for  carrying  off  sand,  ore  tailings,  or  granulated 
slag. 

Stratified  rocks,  especially  thinly-bedded  ones,  and  much-jointed 
rocks,  are  more  easily  eroded  than  massive  ones,  and  if  the  beds  are 
tilted  they  succumb  more  readily  than  if  they  are  horizontal. 

In  dealing  with  the  improvement  and  regulation  of  navigable  rivers 
the  engineer  is  concerned  with  the  erosion  of  soft  rather  than  with  hard 
material,  and  frequently  has  to  guard  against  strong  scouring  action  of 
streams  during  flood  periods.  One  case  illustrative  of  this,  was  that  of 
a  bridge  constructed  across  the  Saskatchewan  River  in  Canada.  Fifty- 
foot  piles  were  driven  into  the  sandy  bottom  of  the  river  to  serve 
as  supports  for  the  piers.  Shortly  after  their  completion  the  June 
floods  scoured  out  the  bottom  to  such  an  extent  that  the  piles  were 
carried  away.  They  were  replaced  by  eighty-foot  piles,  the  river  bottom 
covered  with  matting,  on  which  was  dumped  riprap,  and  then  they 
remained. 

Depth  of  erosion.  —  A  stream  at  first  cuts  vertically,  that  is  down- 
ward, and  if  no  other  natural  agents,  such  as  weathering,  were  co- 
operating with  it,  the  valley  would  in  the  beginning  have  vertical  walls, 
with  the  stream  completely  covering  the  bottom  of  the  valley. 

Cutting  thus  downward,  a  stream  will  tend  to  erode  its  valley  until 
it  reaches  sea-level,  or  the  level  of  some  other  body  of  water  into  which 
it  flows.  This  lowest  sea  level  to  which  running  water  will  usually 
wear  a  land  surface  is  known  as  the  base  level.  Some  large  rivers,  like 
the  Mississippi,  carve  their  channels  somewhat  lower  than  sea  level. 
A  temporary  base  level  is  established  in  those  streams  emptying  into 
inland  bodies  of  water  (lakes)  which  are  elevated  above  sea  level. 
From  what  has  just  been  said,  it  must  not  be  understood  that  the  en- 
tire length  of  the  stream  reaches  base  level  at  the  same  time,  since  for  a 
long  period,  only  the  lower  part  of  its  grade  may  be  cut  down  to  this 
plane.  On  the  contrary,  the  profile  of  a  stream  which  has  reached 
base  level  in  the  lower  portion  of  its  course,  is  that  of  a  parabolic  curve. 

The  head  of  the  valley  gradually  works  inland,  and  continues  to 


SURFACE  WATERS   (RIVERS)  257 

grow  by  headward  erosion  until  it  reaches  a  point  "  where  erosion 
towards  the  valley  in  question  is  equal  to  erosion  in  the  opposite  direc- 
tion." (Chamberlin  and  Salisbury.)  But  while  the  stream  is  deepen- 
ing its  valley,  the  latter  is  also  being  widened  at  a  variable  rate  by 
weathering  and  side  wash.  Particles  of  soil  and  rock  are  dislodged 
from  the  valley  sides  by  the  weathering  agents,  and  carried  down  into 
the  stream  either  by  gravity  or  rain  wash.  Some  of  the  material  may 
be  fine  enough  to  be  removed  at  once  by  the  stream,  but  other  portions 
may  not  be  carried  away  until  a  flood 
comes. 

After  a  stream  has  reached  its  base 
level,  it  begins  to  cut  laterally,  thereby 
broadening  its  valley,  and  in  the  valley 
thus  broadened  the  stream  meanders 

or  swings  from  side  to  side  (Plate  XLI, 

-p.       ..  N  FIG.  126.  —  River  curve  indicating 

place  of  greatest  erosion,  on  bend. 

Where  several  streams  have  cut  down         (Thomas  and  Watt.) 
to  base  level,  and  begun  to  meander, 

the  divides  separating  their  valleys  are  gradually  worn  away,  and  the 
land  surface  reduced  to  an  almost  featureless  plain,  at  or  near  sea  level, 
known  as  a  peneplain  (see  further,  p.  278). 

Character  of  meandering  streams.  —  Meandering  streams  usually 
have  a  low  velocity,  and  are  easily  deflected,  so  that  if  the  bank  is  more 
easily  eroded  at  one  point  than  another,  it  is  sure  to  be  cut  into.  If 
now  the  stream  is  directed  against  one  point  in  the  bank,  it  cuts  in 
there,  and  the  current  striking  this  bank  obliquely,  is  deflected  toward 
the  opposite  bank,  and  develops  a  curve  there  (Plate  XLII).  This 
action  once  started,  continues,  resulting  in  the  concave  bank  being 
eroded  more  and  more  and  the  curves  or  meanders  becoming  con- 
tinually more  accentuated. 

There  is  also  a  marked  difference  in  the  velocity  of  the  current  along 
the  two  banks  of  the  bend.  Thus  it  may  be  5  feet  per  second  close  to 
the  concave  shore  and  only  1  foot  per  second  on  the  convex  one.  This 
will  naturally  result  in  the  dropping  of  sediment  on  the  convex  side, 
and  crowding  the  current  farther  towards  the  concave  shore  (Plate 
XLI,  Fig.  2). 

As  the  result  of  such  shifting  of  the  Mississippi  River  at  Memphis, 
in  a  period  of  fifteen  years,  the  left  bank  had  increased  its  area  106 
acres,  with  a  maximum  increase  in  width  of  2300  feet,  and  parts  of  the 
former  channel  silted  up  45  feet  in  one  year.  The  change  of  the  flow 
necessitated  protection  of  the  right  bank  for  some  distance. 


PLATE  XLI,  FIG.  1.  —  Patuxent  River,  Maryland.  A  river  flowing  nearly  at 
base  level.  Note  the  sediment  depositing  on  convex  side  of  bends  and  sup- 
porting marsh  growth.  (H.  Ries,  photo.) 


FIG.  2. —  Saskatchewan  River,  near  Medicine  Hat,  Alberta.  On  concave  side  of 
curve,  river  has  undermined  cliffs,  while  on  convex  side,  deposition  has  taken 
place.  (H.  Ries,  photo.) 

(258) 


SURFACE  WATERS   (RIVERS)  259 

The  extreme  curvatures  of  river  channels  thus  developed  are  termed 
ox-bows  (Plate  XLII).  When  portions  of  adjoining  curves  almost 
touch,  the  river  may  become  straightened  by  artificial  or  natural  means, 
and  a  cut-off  be  formed  (Plate  XLII) .  The  former  consists  in  exca- 
vating a  channel  to  connect  neighboring  parts  of  adjoining  ox-bows, 
as  in  the  case  of  the  Dutch  Gap  on  the  James  River,  below  Richmond, 
Va.  The  latter  may  be  accomplished  in  two  ways:  (1)  Either  by  ex- 
treme development  of  the  curves  the  strip  of  land  between  two  adjoining 
ones  may  become  so  thin  as  to  break  through,  or  (2)  during  periods  of 
flood,  when  the  river  covers  most  of  the  flood  plain,  the  main  current, 
in  preference  to  following  the  regular  channel,  may  flow  across  the 
neck  of  land  (Plate -XLII),  and  cut  a  new  channel,  which  the  river 
will  then  follow  during  normal  periods.  If  the  abandoned  channel 
curve  becomes  separated  from  the  main  stream  by  sediment,  and  con- 
tains stagnant  water  it  is  called  an  ox-bow  lake  (Plate  XLII).  Ox-bow 
lakes  are  common  along  the  middle  and  lower  courses  of  the  Mississippi 
River. 

Shoals,  bends,  and  crossings.  —  To  the  engineer  the  behavior  of 
rivers  flowing  on  an  alluvial  plain,  is  a  matter  of  some  importance, 
especially  if  he  is  engaged  in  their  regulation  and  improvement.  From 
his  viewpoint  the  river  often  consists  of  a  series  of  bends,  connected  by 
straight  reaches.  The  main  current  or  volume  of  flow  follows  the 
concave  shore,  until  a  straight  part  of  the  channel  is  reached,  when  it 
crosses  over  gradually  to  the  beginning  of  the  next  bend.  This  is 
known  as  a  crossing  (Plate  XLII) .  Deep  water  is  found  along  the  con- 
cave bank,  the  deepest  spot  being  usually  below  the  point  of  sharpest 
curvature.  On  approaching  the  crossing,  the  flow  spreads  out,  and  as 
there  is  here  a  wider  cross  section  as  well  as  an  absence  of  eddies  the 
sediment  settles. 

The  wider  the  channel  the  greater  the  slackening,  and  the  larger  the 
amount  of  sediment  deposited.  Crossings  therefore  are  found  at  low 
water  to  be  shallow  and  of  uncertain  depths.  In  their  widest  parts  the 
sediment  may  build  up  into  bars  or  islands.  Occasionally  the  straight 
reaches  of  a  river  being  narrow,  keep  free  from  sediment,  as  the  volume 
of  water  scours  them,  so  that  they  retain  their  same  cross  section  from 
year  to  year. 

Scour.  —  During  flood  periods  much  sediment  is  dropped  in  the 
crossings,  and  when  the  river  falls  again  to  a  low  stage  it  makes  an 
effort  to  remove  it.  It  can  do  this,  however,  but  incompletely,  since  a 
higher  velocity  is  required  to  pick  the  particle  of  sediment  up  than  is 
necessary  to  transport  it. 


260 


ENGINEERING   GEOLOGY 


The  deepest  spot 

ts  usually  found  below  the 

point  or  Maximum  Curvature 


SECTION  B-B 


SECTION  C-C 


PLATE  XLII.  —  Plan  and  sections  showing  typical  features  of  a  meandering  river. 
(After  Thomas  and  Watt.) 


SURFACE  WATERS   (RIVERS)  261 

With  gravel  shoals  the  current  must  be  fast  enough  to  roll  the  pebbles 
down  the  slope.  With  sand  shoals,  "the  function  of  the  flow  over  the 
bottom  sets  up  a  series  of  small  eddies  transverse  to  the  current,  and 
these  in  turn  throw  up  sand  ripples  or  ridges  perhaps  an  inch  or  two 
in  height,  with  a  short  down-stream  slope  and  a  long  up-stream  one, 
against  which  the  water  beats,  disturbing  the  particles  and  carrying 
them  down  stream  (Ref.  15)." 

The  material  which  the  falling  river  has  to  remove  is  of  varying 
compactness.  If  such  material  has  settled  it  may  have  become  so  com- 
pacted as  to  resist  the  scouring  action  of  the  current,  and  even  deflect 
it.  The  scouring  process  is  a  slow  one,  and  if  the  river  falls  rapidly, 
the  bar  may  become  an  obstruction  to  navigation,  before  there  has  been 
enough  time  to  permit  its  removal  by  natural  processes. 

The  effect  of  scour  varies  as  the  square  of  the  velocity,  and  is  depend- 
ent on  the  depth  of  flow.  A  river  with  a  discharge  10  feet  deep  and  4 
feet  per  second  velocity,  pressing  on  its  bed  with  a  weight  of  625  pounds 
per  square  foot  (10  feet  of  water  at  62 J  pounds  per  foot),  will  have  a 
much  stronger  scouring  power  than  a  brook  of  equal  velocity  only  1 
foot  deep,  which  would  exert  a  pressure  of  only  62J  pounds  per  square 
foot. 

Eddies  and  currents.  —  These  are  most  strongly  developed  during 
periods  of  flood,  and  at  such  times  the  main  channel  in  both  the  bends, 
and  even  part  way  down  the  crossings  is  strongly  agitated  by  swirls 
and  currents,  of  which  the  "boils"  or  vertical  eddies  are  the  most 
peculiar.  These  produce  an  upthrow  of  water  to  the  surface,  causing 
it  to  boil  as  the  water  does  over  a  great  spring. 

It  is  stated  that  on  the  Mississippi  during  floods  "  these  boils  at 
times  spread  half  way  across  the  river,  and  may  reach  a  height  at  their 

Arrows  fly  with  Current. 
Suction  "Eddy 


FIG.  127.  —  Sketch  of  a  suction  eddy.     (After  Thomas  and  Watt.) 

center  of  some  5  feet  above  the  normal  surface  of  the  flow.  The  re- 
sulting disturbance  of  the  water  is  tremendous  and  very  dangerous  to 
boats,  especially  when  towing;  indeed  more  than  one  case  is  known 


262 


ENGINEERING  GEOLOGY 


where  craft  have  been  caught  and  sunk  by  these  sudden  cross  currents/'" 
(Ref.  15.) 

Two  other  types  of  eddies  are  those  known  as  suction  and  pressure 
eddies.  The  former  (Fig.  127)  are  said  to  be  due  to  the  pressure  in  a 
caving  bank  of  a  hard  stratum,  which  is  eroded  slowly  and  which  grad- 
ually forms  a  "false  point"  projecting  into  and  deflecting  the  current. 
These  suction  eddies  may  be  several  hundred  feet  wide  and  long.  A 
pressure  eddy  is  commonly  caused  by  a  sudden  change  in  direction  of 


FIG.  128.  —  Sketch  of  a  pressure  eddy.     (After  Thomas  and  Watt.) 

the  current  (Fig.  128).  There  is  usually  a  suction  eddy  on  the  opposite 
bank. 

Erosion  of  banks.  —  The  erosion  of  river  banks  is  the  primary 
cause  of  caving  or  slipping.  It  may  be  due  to:  (1)  Water  eating  into 
the  base  of  the  bank;  (2)  the  presence  of  an  easily  eroded  layer  of  sand 
at  the  base  of  the  bank,  whose  removal  robs  the  latter  of  its  support; 
and  (3)  the  sudden  fall  of  the  river  leaving  a  saturated  bed  unable  to 
support  the  overlying  load  of  the  bank.  Erosion  is  most  active  on  the 
concave  bank,  or  where  there  are  eddies,  and  in  either  case  goes  on 
chiefly  during  periods  of  high  water. 

Figure  129a-c  shows  the  successive  stages  of  bank  erosion.  Figure 
126  shows  the  action  of  the  current  on  a  bend.  Figure  129a  represents 
the  initial  condition  of  the  bank.  Figure  1296  shows  erosion  in  progress, 
with  some  caved  material  forming  a  temporary  protection,  which  is 
soon  washed  away.  In  Figure  129c  the  bank  has  been  cut  back  to  a 
vertical  surface,  just  before  breaking.  Needless  to  say  saturation  of 
the  bank  by  rain  water  will  hasten  its  collapse.  Figure  129d  shows  a 
basal  layer  of  sand  whose  saturation  and  outflow  leaves  the  bank  un- 


SURFACE  WATERS   (RIVERS) 


263 


129c 


supported.     Figure  129e  shows  the  profile  which  is  likely  to  develop  in 
the  presence  of  hard  rock  or  tough  clay. 

Protection  against  such  erosion  may  be  had  by  grading  the  bank 
to  a  flat  slope,  and  covering  its  surface 
to  high-water  mark  by  non-erodable  ma- 
terials, or  by  breaking  the  attack  of  the 
water  by  spur  revetments  or  spur  dikes,  or 
guarding  the  toe  with  a  longitudinal  dike 
of  stone  or  piles.  Railway  embankments 
constructed  along  rapidly  flowing  streams 
are  not  infrequently  undermined  unless 
properly  protected. 

Ice  may  also  play  a  part  in  the  erosion  of 
a  stream  channel.  If  the  banks  are  of  soft 
material,  floating  ice  rubbing  into  them  will 
remove  more  or  less  of  the  sand  or  clay. 
Or  again  ice  that  has  become  attached  to 
the  shore  when  the  river  freezes  over  may, 
when  torn  loose  by  the  spring  floods,  detach 
material  from  the  banks. 

Levees.  -  -  When  a  river  overtops  its 
banks  during  periods  of  flood,  and  spreads 
over  the  flood  plain,  the  sediment  is  de- 
posited most  actively  on  that  part  of  the 
plain  adjoining  the  river  channel.  As  a  re- 
sult of  this,  low  alluvial  ridges  or  natural 
levees  are  built  up,  which  may  be  a  few  or 
many  feet  in  width. 

In  many  regions  the  height  of  these  nat- 
ural levees  has  been  increased  by  artificial 
means,  high  embankments  being  sometimes 
constructed  to  protect  the  alluvial  plain 
from  overflow  during  periods  of  flood.  The 
most  extensive  example  of  levee  work  in 
the  United  States  is  that  of  the  Mississippi 
and  its  tributaries  below  Cairo.1 

Irregular  hardness  of  river  bed.  —  It 
is  a  well-known  fact  that  the  grade  or 


Clay  or 
Rock 


129e 

FIG.  129.  —  Sections  showing 
successive  stages  of  bank 
erosion,  (a)  Initial  condi- 
tion of  bank;  (6)  erosion  in 
progress;  (c)  bank  cut  back 
to  vertical;  (d)  saturated 
basal  sand  layer,  whose 
outflow  leaves  bank  unsup- 
ported; (e)  profile  developed 
with  hard  rock  or  tough  clay. 
(From  Thomas  and  Watt, 
Improvement  of  Rivers.) 


slope  of  a  river  channel  is  not  always  free  from  irregularities.     On  the 

1  For  an  excellent  treatment  of  the  subject  of  levee  construction  and  maintenance 
see  Thomas  and  Watt.     Improvement  of  Rivers,  2d  ed.,  pt.  I,  p.  243,  1913. 


264  ENGINEERING  GEOLOGY 

contrary  it  may  show  irregularities  resulting  in  the  development  of 
rapids,  falls,  etc. 

Falls  and  rapids. — These  are  caused  by  irregularities  in  the  hardness 
of  the  rock  in  the  river  channel,  and  form  where  streams  pass  from  a  more 
resistant  to  a  less  resistant  rock  (Plates  XLIII,  Fig.  1  and  XLVII,  Fig.  2). 

Thus,  if  we  have  a  hard  stratum  outcropping  in  the  bed  of  a  stream, 
with  softer  beds  below  it,  the  greater  wear  of  the  latter  develops  suf- 
ficient inequality  of  bed  to  produce  rapids.  With  progressing  erosion 
and  increasing  steepness  of  the  stream  bed,  the  rapids  change  to  falls. 
Continued  erosion  of  the  soft  layer  undermines  the  hard  one  and  the 
falls  migrate  upstream,  and  although  this  movement  is  slow,  plotted 
surveys  extending  over  a  series  of  years  often  show  it  clearly.  If,  how- 
ever, the  resistant  rock  is  vertical  (Plate  XLIII,  Fig.  1)  and  strikes 
across  the  stream,  the  falls  may  remain  stationary  until  the  hard  layer 
is  removed  by  erosion. 

A  waterfall  may  be  formed  in  stratified  rocks  by  the  presence  of 
hard  beds  interstratified  with  softer  ones,  or  in  other  cases  the  develop- 
ment of  the  fall  may  be  due  to  the  existence  of  a  hard  dike  or  sill  of 
igneous  rock.  Waterfalls  may  originate  in  other  ways,  but  the  above- 
mentioned  causes  are  the  commonest. 

To  the  engineer  the  existence  of  falls  and  rapids  is  of  importance, 
because  the  drop  of  the  stream  in  a  short  distance  permits  its  utiliza- 
tion for  power  purposes,  and  hydro-electric  plants  are  being  constructed 
at  many  localities  where  such  powers  can  be  profitably  developed. 

Waterfalls  and  rapids,  on  the  other  hand,  frequently  break  the  navi- 
gable continuity  of  a  stream,  and  have  to  be  passed  in  different  ways. 
Falls  can  be  passed  sometimes  by  a  series  of  locks,  while  rapids  if  not 
too  steep,  can  be  overcome  by  blasting  out  the  rock  ledges  in  the  stream 
bed,  thus  making  a  navigable  channel.  This  was  done  for  example  on 
the  Danube  River  in  lower  Hungary.1 

Potholes.  —  In  eddies  and  also  at  the  foot  of  cascades  where  the 
water  has  a  swirling  motion,  the  stones  lying  on  the  bottom  are  whirled 
around,  and  excavate  cylindrical  holes  known  as  potholes,  which  are 
often  well  preserved  in  the  solid  rock.  They  vary  in  depth  and  diame- 
ter, some  being  of  large  size. 

Work  of  Transportation 

Transportation  of  sediment.  —  A  river  may  transport  mineral 
matter  in  suspension  or  in  solution.  The  sediment  moved  mechanically 
by  a  river  is  either  carried  in  suspension  or  rolled  along  the  bottom. 

1  See  Thomas  and  Watt,  Improvement  of  Rivers,  I. 


If 

25  f£3 


s 

o 

I 


266 


ENGINEERING  GEOLOGY 


The  transporting  power  of  a  stream  depends  on  its  velocity,  and  is 
expressed  by  the  equation 

T  oc  7<5 

in  which  T  equals  the  transporting  power  and  F,  the  velocity.  If 
then  the  velocity  is  doubled,  the  transporting  power  is  increased  64 
times.  But  the  velocity  depends  on  grade,  volume  and  load.  That  is, 
the  steeper  the  slope  the  greater  the  velocity;  the  greater  the  volume 
of  flow  for  a  given  slope,  the  higher  the  velocity.  Increased  load  tends 
to  diminish  the  velocity.  The  last  is  shown  by  the  fact  that  the 
velocity  of  a  muddy  stream  is  not  as  high  as  when  the  same  stream  is 
free  from  mud. 

Amount  of  sediment  transported.  —  The  quantity  of  sediment 
transported  by  different  rivers  varies,  owing  partly  to  the  variable 
quantity  of  debris  supplied  to  different  streams,  and  partly  to  their 
varying  velocity.  A  swift  stream  flowing  from  a  lake  may  even  carry 
very  little  sediment,  because  the  latter  acts  as  a  settling  basin  to  sepa- 
rate the  sediment  from  the  water  before  it  leaves  it. 

In  the  same  stream  the  quantity  of  sediment  carried  will  vary  with 
the  volume  and  velocity  of  the  stream  during  different  periods.  In- 
deed, the  quantity  of  sediment  per  cubic  foot  of  water  may  not  be  the 
same  in  all  parts  of  the  stream's  channel.  Consequently,  in  making 
observations  on  the  amount  of  sediment  in  a  river,  it  is  important  to 
take  samples  of  the  water  from  different  depths,  and  at  different  points 
of  the  section.  The  following  tables  are  of  interest  in  this  connection. 

PERCENTAGE  OF  MATERIAL  CARRIED  IN  SUSPENSION  BY  VARIOUS  RIVERS l 


Height 

Drainage 

Mean 

Ratio 

nf 

in  feet 
of  col- 

Thickness 
of  sedi- 

River. 

area 
in 
square 
miles. 

discharge 
(in  cubic 
feet)  per 
second. 

Total 
tons 
annually. 

OI 

sediment 
to 
water 
by  weight. 

umn  of 
sediment 
with  a 
base  of 

ment  in 
inches 
if  spread 
over  drain- 

one square 

age  area. 

mile. 

Potomac  

11,043 

20,160 

5,557,250 

1  :  3,575 

4.0 

0.00433 

Mississippi  

1,244,000 

610,000 

406,250,000 

1  :  1,500 

241.4 

0.00223 

Rio  Grande  

30,000 

1,700 

3,830,000 

1:291 

2.8 

0.00116 

Uruguay 

150,000 

150,000 

14,782,500 

1  :  10,000 

10.6 

0  00085 

Rhone     .    . 

34,800 

65,850 

36,000,000 

1  :  1,775 

31.1 

0.01075 

Po  

27,100 

62,200 

67,000,000 

1  :  900 

59*0 

0^01139 

Danube.  . 

320,300 

315,200 

108,000.000 

1  :  2,880 

93.2 

0  00354 

Nile  

1,100,000 

113,000 

54,000^000 

1  :  2,050 

38  8 

0  .  00042 

Irrawaddy  

125,000 

475,000 

291,430,000 

1  :  1,610 

209.0 

0^02005 

Mean  

334,693 

201,468 

109,649,972 

1  :  2,731 

76.65 

0.00614 

Babb,  Science,  XXI,  p.  343,  1893. 


SURFACE  WATERS   (RIVERS) 
SEDIMENT  TRANSPORTED  BY  VARIOUS  RIVERS 


267 


River. 

Grammes  per  cubic 
meter. 

Grains  per  gallon. 

Total  cubic 
yards  dis- 
charged per 
annum. 

Min. 

Max. 

Mean. 

Min. 

Max. 

Mean. 

Arkansas                 

26,000,000 
78,500,000 

518,500,000 
413,000,000 
47,800,000 
27,500,000 

Danube               

100 

200 
530 

48 

1100 

2560 
5640 
1500 

283 

800 

sis 

870 

6 

11 
31 
3 

64 

150 

330 

87 

16 
46 

Mississippi,  Cairo  to  Gulf 
of  Mexico 

Missouri 

Nile                          

18 
51 

Rhone 

Relation  of  size  of  particles  to  current  velocity.  —  Experiments 
made  to  determine  the  relation  between  the  size  of  particles  trans- 
ported and  current  velocity  give  rather  uncertain  results  because  of 
local  conditions,  such  as  the  volume  of  discharge. 

A  river  with  a  fall  of  one  foot  per  mile  can  transport  a  large  amount 
of  heavy  sediment,  while  a  brook  with  similar  fall  can  hardly  carry 
silt.  It  has  been  noticed  also,  that  while  a  current  of  given  velocity 
may  carry  silt  in  suspension,  a  somewhat  higher  speed  is  required  to 
erode  the  same  material,  and  start  it  moving. 

Du  Buat1  gives  the  following  ratios  between  size  of  materials  and 
velocities  : 

Feet  per  second. 

Potter's  clay  .................................................  0.  26 

Sand,  deposited  by  clay  ......................................  0  .  54 

Large,  angular  sand  ..........................................  0.71 

Gravel,  size  of  peas  ...................  ,  ......................  0.  53 

Gravel,  size  of  beans  .........................................  1  .  07 

Round  pebbles,  large  as  thumb  ................................  2.13 

Angular  flint  stone,  of  size  of  hen's  eggs  ........................  3  .  20 


The  velocities,  sufficient  to  move  gravel  of  different  sizes  on  the 
Loire  River,  were  found  to  be: 

Feet  per  second. 

Gravel  0.04  in  diameter  .......................................  1.64 

Gravel  0  .  16  in  diameter  ......................................  3  .  28 

Gravel  0.39  in  diameter  .....................  .................  4.92 

Gravel  0.  69  in  diameter  ......................................  6.  56 

It  is  probable  that  higher  velocities  than  those  given  by  Du  Buat 
would  be  required  in  each  case  to  move  the  respective  sizes. 

In  irrigation   canals   leading  from  the  Nile  it  was  found  that  a 

velocity  of  2  feet  or  less  per  second  caused  suspended  silt  to  settle;  2.3 

feet  per  second  caused  no  deposit;  while  from  4  to  5  feet  per  second 

produced  scour.     A  rate  of  3j  feet  per  second  seemed  to  prevent  both 

1  Quoted  by  Thomas  and  Watt. 


268  ENGINEERING  GEOLOGY 

deposit  and  scour.  According  to  Buckley,  material  in  place  will  usually 
resist  the  following  velocities  per  second:  Sandy  soil,  from  1  to  2J  feet; 
ordinary  clay,  3  feet;  compact  clay,  from  5  to  6  feet;  gravel  and  pebbles, 
from  5  to  6  feet. 

Relation  of  sediment  to  cross-section  and  slope.  —  The  principles 
involved  in  the  transportation  of  sediment  by  rivers  make  the  chief 
law,  which  governs  their  behavior. 

The  burden  of  sediment  may  vary  from  mile  to  mile,  but  it  usually 
remains  in  exact  proportion  of  the  water  required  to  carry  it.  A  stream 
may  therefore  deposit  at  one  point  and  scour  at  another;  or  accelera- 
tion of  the  current  in  a  given  stretch  along  its  course  may  initiate 
scour,  where  previously  deposition  occurred. 

"  There  should  be  in  every  part  of  a  river  a  combined  proportion  be- 
tween the  discharge,  the  velocity,  and  the  cross-section  of  the  bed,  or 
the  amount  of  erosion  affected  by  the  stream"  (Ref.  15).  When  a  river 
rises  in  flood,  therefore,  it  should  deposit  or  scour  the  channel  to  the  ex- 
tent necessary  to  permit  the  passage  of  the  water  and  its  load  of  sedi- 
ment. This  exact  condition  is  not  always  attained. 

Various  factors  prevent  the  stream  from  reaching  a  condition  of 
exact  equilibrium.  Among  these  are:  (1)  Local  disturbances  due  to 
spurs  of  rock  or  clay;  (2)  variations  in  rapidity  of  rise  and  fall;  (3)  time 
elapsing  between  floods;  and  (4)  effect  of  local  rains,  etc. 

Measurements  in  low  water  show,  however,  that  where  considerable 
time  has  elapsed  after  a  flood,  the  bends,  where  the  water  runs  slowly 
in  the  low  season,  tend  to  silt  up  in  proportion  as  the  shoals,  where  the 
water  runs  fast,  tends  to  erode.  As  a  case  in  point,  measurements 
taken  on  the  Brazos  River  in  Texas,  over  a  distance  of  a  few  miles, 
comprising  several  bends  and  shoals,  showed  that  the  comparative 
areas  of  the  bend  and  of  the  shoals  sections  did  not  differ  by  more  than 
10  per  cent. 

Change  of  shape  of  cross-section.  —  Since  in  some  rivers  the  channel 
may  shift  from  one  side  to  the  other,  due  to  deflecting  causes  of  different 
kinds,  it  follows  that  scour  may  be  going  on  one  year  at  a  point  where 
deposition  was  going  on  the  previous  year.  The  cross-section  of  the 
stream  bed  may  consequently  vary  appreciably  from  year  to  year. 

Slope  of  streams.  —  As  already  stated  the  valley  slope  determines 
the  stream's  velocity,  and  other  things  being  equal  we  have  greater 
erosion  with  higher  velocity.  In  general,  a  river  has  the  steepest  slope 
nearest  its  head,  and  least  slope  at  its  mouth,  but  aside  from  this  there 
may  be  many  local  irregularities;  and  since  a  rise  in  the  river  causes 
increased  velocity  and  slope,  its  bed  and  banks  may  change. 


SURFACE  WATERS   (RIVERS)  269 

Tributaries  exert  an  important  local  influence  on  a  river's  slope.  If 
a  tributary  brings  in  much  sediment,  the  main  stream  may  receive 
more  load  than  it  can  transport,  and  the  sediment  is  deposited  down- 
stream, steepening  the  river  bed  in  that  direction,  but  reducing  it 
above.  "As  a  result  deposits  occur  and  the  bed  of  the  river  is  raised 
until  equilibrium  is  again  reached  between  velocity  and  sediment.  A 
sediment-free  tributary  adds  to  the  volume  of  the  main  stream,  and  it 
begins  to  scour  until  it  has  its  full  load  of  sediment.  This  results  in  the 
slope  becoming  less  than  above  the  confluence  of  the  two  streams." 

"  According  to  this  there  should  be  found  corresponding  differences  of  slope  at 
the  junctions  of  the  Mississippi,  the  Missouri,  and  the  Ohio  rivers.  The  upper 
Mississippi,  compared  with  the  Missouri,  is  comparatively  clear,  while  the  volume 
of  sediment  of  the  latter  is  enormous.  The  former  river,  before  joining  its  muddy 
tributary,  should  have  the  lesser  slope;  after  the  two  streams  have  mingled  the 
proportion  of  sediment  to  the  volume  of  the  water  is  reduced  for  the  one  and  in- 
creased for  the  other;  the  slope  should  therefore  become  a  mean  proportioned  to 
the  sediments  and  volumes,  and  should  become  less  than  that  of  the  Missouri,  and 
greater  than  that  of  the  Mississippi,  before  their  confluence.  As  the  combined  flow 
of  the  two  rivers  approaches  the  Ohio,  the  sediment  becomes  reduced  by  grinding, 
and  the  slope  becomes  less.  When  the  Ohio  is  reached  the  former  conditions  are 
reversed;  the  tributary  is  somewhat  the  clearer,  and  the  main  river  somewhat  the 
more  muddy.  Above  their  confluence,  therefore,  the  slope  of  the  Ohio  should  be 
less  and  the  slope  of  the  Mississippi  greater;  after  their  junction  the  proportion  of 
sediment  to  volume  is  lessened  for  the  Mississippi  and  increased  for  the  Ohio;  the 
resulting  slope  should  therefore  be  less  than  that  of  the  Mississippi  and  greater  than 
that  of  the  Ohio  before  their  junction.  Such  conditions  actually  exist."  The  slope 
of  a  river  often  indicates  the  character  of  the  material  which  forms  the  bed  (Ref.  15). 

Work  of  Deposition 

Alluvial  plains.  —  As  already  pointed  out,  a  stream  that  has  cut 
down  to  base  level  or  grade,  begins  to  erode  laterally,  widening  its 
valley,  and  developing  a  curving  or  meandering  course.  The  stream 
itself  does  not  occupy  the  entire  width  of  the  valley  and  is  bordered 
by  a  flat  of  varying  width.  During  periods  of  flood  the  river  overflows 
this  flat,  and  as  the  velocity  of  the  stream  is  reduced  over  the  over- 
flowed flat  it  may  deposit  much  of  its  load  of  sediment  on  such  areas. 
From  time  to  time  this  is  added  to,  and  the  surface  thus  built  up  or 
aggraded  constitutes  an  alluvial  plain  (Plate  XLIV,  Fig.  1). 

With  further  development  of  the  valley  the  flood  plain  extends 
farther  up  stream,  while  at  the  same  time  its  older  parts  may  grow 
wider  due  to  lateral  erosion  of  the  river.  In  some  cases  a  flood  plain 
may  be  formed  by  deposition  alone,  as  when  a  stream  becomes  over- 
loaded while  its  valley  is  still  narrow. 

Flood  plains  may  also  be  caused  by  either  natural  or  artificial  ob- 
structions. A  case  of  the  former  would  be  where  a  stream  flows  over 


PLATE  XLIV,  FIG.  1.  —  A  flood  plain.     View  along  Danube  River  in  Servia. 

(H.  Ries,  photo.) 


.. 


FIG.  2.  —  Section  of  ancient  delta,  Fishkill,  N.  Y.     (H.  Ries,  photo.) 
(270) 


SURFACE  WATERS   (RIVERS)  271 

resistant  ledges,  which  act  much  like  dams,  checking  the  current  above 
them,  and  favoring  the  deposition  of  sediment.  An  artificial  barrier 
or  dam  would  produce  similar  results. 

In  some  cases  flood  plains  attain  remarkable  size,  and  extend  up- 
stream for  great  distances.  The  flood  plain  of  the  Mississippi  has  a 
width  ranging  from  more  than  20  miles  at  Helena,  Ark.,  to  about  80 
miles  near  Greenville,  Miss.1 

Deltas.  —  When  a  sediment-laden  stream  enters  a  body  of  quiet 
water,  its  current  is  checked,  and  much  of  the  material  which  it  carries 
will  be  dropped  at  once  (Plate  XLV).  The  finer  material  may  be 
carried  farther  from  the  mouth  of  the  river  before  it  settles.  Such  de- 
posits are  termed  deltas,  and  their  extensive  development  at  the  mouths 
of  some  navigable  rivers  calls  for  considerable  attention  from  the 
engineer  engaged  in  river  improvement,  requiring  the  devising  of 
satisfactory  means  for  maintaining  an  open  safe  channel  way  across 
the  delta  to  the  sea.  The  cause  of  this  trouble  will  be  better  appre- 
ciated after  the  formation  of  the  delta  has  been  described. 

The  top  of  the  delta  deposit  is  comparatively  flat,  or  to  be  more 
exact,  the  surface  slopes  gently  seaward  so  long  as  the  river  current  is 
as  deep  as  the  standing  water  into  which  it  is  discharging,  but  beyond 
this  the  delta  surface  has  a  depositional  slope.  The  result  is  the  con- 
struction of  a  delta  platform  with  a  relatively  flat  top,  and  frontal 
slope  of  varying  inclination. 

As  deposition  continues,  the  delta  platform  is  built  up  (aggraded), 
and  at  the  same  time  its  margin  is  extended  seaward.  The  landward 
margin  is  gradually  built  up  above  sea  level,  and  this  land  portion  is 
also  gradually  extended  outward. 

In  the  beginning  the  main  flow  of  the  stream  across  the  aggraded 
delta  platform  will  be  more  or  less  in  line  with  the  main  channel  of  the 
river  above  its  mouth,  but  the  current  will  shift  somewhat  to  left  and 
right,  and  yet  since  these  shifting  currents  are  of  lower  velocity  than 
the  main  one,  there  will  be  a  tendency  for  the  sediment  to  build  up  on 
either  side  of  the  main  channel  forming  natural  levees.  The  main 
stream  then  finds  itself  flowing  in  a  natural  trench  which  it  gradually 
extends  seaward,  but  at  the  same  time  is  filling  up  by  further  deposi- 
tion. Its  capacity  to  hold  the  flow  of  the  main  stream  thus  becomes 
reduced,  and  the  latter  finally  breaks  through  the  levee  at  some  point, 
the  greater  portion  of  the  flow  following  a  new  channel,  which  passes 
through  the  same  changes  as  the  first  one  did.  The  main  stream  then, 
if  left  to  itself,  will  shift  from  one  part  of  the  delta  plain  to  another. 
1  Mississippi  River  Commission,  1887. 


272 


ENGINEERING  GEOLOGY 


X.  B.  Th.  Bondl.*..  u.  In  r«ct. 


PLATE  XLV.  —  Plan  of  Mississippi  delta.     (From  Thomas  and  Watt,  Improve- 
ment of  Rivers.) 


SURFACE  WATERS   (RIVERS) 


273 


The  problem  of  the  engineer,  therefore,  is  to  maintain  one  of  these  channel  ways 
in  a  navigable  condition  out  to  sea.  This  may  be  done  for  example  by  "  prolonging 
one  of  the  delta  channels  by  parallel  jetties  out  to  the  bar,  so  that  the  prolonged 
current,  being  concentrated  across  the  bar,  may  scour  a  deeper  channel,  and  carry 
its  burden  of  sediment  into  deeper  water  farther  out. 

"  One  of  the  minor  outlets  should  be  selected  for  improvement,  if  its  delta  channel 
is  adequate,  or  can  easily  be  made  adequate  for  the  requirements  of  navigation; 
and  the  discharge  of  the  other  outlets  should  not  be  interfered  with.  The  advance 
of  the  delta  at  one  of  the  minor  outlets  is  slower,  and  the  distance  out  to  the  bar  is 
less,  and  consequently  the  jetty  works  are  less  costly;  whilst  an  increased  discharge, 
produced  by  impeding  the  flow  through  the  other  outlets,  would  also  increase  the 
volume  of  sediment,  and  therefore  quicken  the  rate  of  advance  of  the  delta,  and 
hasten  the  necessity  of  prolonging  the  jetties. 

"  The  success  of  the  jetty  system  depends  on  a  rapid  deepening  of  the  sea  in 
front;  on  the  fineness  and  lightness  of  the  sediment  brought  down;  and  on  the  ex- 
istence of  a  littoral  current,  its  velocity,  and  the  depth  to  which  it  extends.  Any 
erosive  action  of  winds  and  waves  along  the  shores  of  the  deltas  is  favorable  to  the 
system,  and  also  any  reduction  in  the  density  of  the  sea  water,  such  as  may  be 
found  in  an  inland  sea. 

"  If  the  sea  bottom  is  flat;  if  a  large  proportion  of  the  sediment  is  dense  so  that 
it  is  carried  along  the  bed  of  the  river  or  close  to  it;  if  the  outlet  faces  the  prevalent 
winds;  and  if  no  littoral  current  exists;  it  is  possible  that  an  improvement  of  the 
outlet  may  not  be  practicable.  Then  recourse  must  be  had  to  a  side  canal,  start- 
ing off  from  the  river  some  distance  up,  and  entering  the  sea  beyond  the  influence 
of  the  alluvium  of  the  river. 

"  The  bars  in  front  of  the  outlets  of  tideless  rivers  being  formed  by  the  deposit 
from  the  river,  vary  in  form  according  to  the  nature  of  the  sediment  brought  down. 
When  the  material  is  composed  of  particles  of  very  variable  density,  it  is  gradually 
sifted  as  the  velocity  of  the  current  decreases,  and  gives  a  flat  sea  slope  to  the  bar. 
When  on  the  contrary  most  of  the  material  is  heavy,  the  bar  has  a  flat  river  slope, 
as  in  the  first  case,  formed  by  the  gradual  arrest  of  the  sediment  rolled  along  the 
bottom;  but  as  little  of  the  material  is  carried  beyond  the  crest  of  the  bar,  the  sea 
slope  is  steep. 

"  The  jetty  system  does  not  constitute  a  permanent  improvement,  for,  sooner  or 
later,  in  proportion  as  the  physical  conditions  are  unfavorable  or  the  reverse,  a  bar 
is  formed  further  out,  and  a  prolongation  of  the  jetties  becomes  necessary." l 


FIG.  130.  —  Section  of  delta  showing:  (a)  top-set  beds;  (6)  fore-set  beds;  (c)  bottom- 
set  beds. 

Structure  of  deltas.  —  In  plan  deltas  are  somewhat  triangular  re- 
sembling the  Greek  letter  A. 

1  Thomas  and  Watt,  I,  p.  311,  1913. 


274  ENGINEERING  GEOLOGY 

In  section  the  structure  is  as  shown  in  Fig.  130.  Here  we  see  a  series 
of  inclined  layers,  the  fore-set  beds,  which  accumulated  as  the  sediment 
rolled  down  the  steep  frontal  slope  of  the  delta.  The  finer  material 
carried  farther  out  constitutes  the  bottom-set  beds,  and  these  are  grad- 
ually covered  by  the  fore-set  layers  as  the  delta  is  built  seaward.  At 
the  same  time  material  is  being  laid  down  in  horizontal  layers  on  top 
of  the  delta,  forming  the  top-set  beds. 

Conditions  favorable  to  delta  formation.  —  All  streams  do  not  build 
deltas.  Their  absence  may  be  due  to  lack  of  sediment,  or  to  waves 
and  shore  currents  which  carry  off  the  sediment  as  soon  as  the  streams 
deliver  it.  A  third  cause  for  the  apparent  failure  of  delta  formation 
may  be  the  great  depth  of  the  water  into  which  the  stream  discharges, 
in  which  case  it  might  take  the  sediment  a  long  time  to  build  up  suffi- 
ciently to  shallow  the  water. 

Tidal  seas  then  are  usually  opposed  to  delta  formation,  although 
they  are  sometimes  formed  as  at  the  mouth  of  the  Yukon;  at  the 
Mackenzie,  with  three  feet  tidal  fall;  the  Niger,  with  four  feet;  the 
Hoang-Ho,  with  eight  feet;  and  the  Brahmaputra  and  Ganges,  with 
sixteen  feet.1 

Lakes,  bays,  gulfs,  and  inland  seas,  where  wave  action  and  tidal 
currents  are  likely  to  be  weak,  are  favorable  for  delta  formation.  Deltas 
are  absent  usually,  or  formed  only  at  the  heads  of  bays,  along  coasts 
that  have  been  recently  depressed,  as  in  the  Atlantic  Coast  region  at 
present. 

Extent  of  deltas.  —  The  deltas  of  large  rivers  are  sometimes  of 
vast  extent,  and  are  advancing  seaward  at  a  rapid  rate. 

The  Mississippi  delta  which  has  a  length  of  over  200  miles,  and  an 
area  of  over  12,000  miles  is  said  to  be  advancing  into  the  Gulf  of  Mexico 
at  a  rate  of  about  300  feet  per  year.  Its  depth  at  New  Orleans  is  esti- 
mated at  from  700  to  1000  feet.2  The  Yukon  delta  has  a  sea  margin  of 
70  miles,  and  a  length  of  100  miles.  The  Hoang-Ho  delta  heads  about 
300  miles  from  the  coast,  and  has  a  seaward  border  of  about  400  miles.3 

Of  historic  as  well  as  practical  interest  is  the  fact  that  some  towns 
which  were  formerly  sea-ports  are  now  inland  cities,  because  of  delta 
growth.  Thus  Adria,  formerly  a  port  which  gave  its  name  to  the 
Adriatic  Sea,  is  now  located  14  miles  inland,  because  of  the  outgrowth 
of  the  Po  delta.  The  latter  is  said  to  have  advanced  about  50  feet 

1  Chamberlin  and  Salisbury,  I,  p.  202,  1905;  Davis,  Physical  Geography,  p.  294. 

2  Humphrey  and  Abbott,   Physics  and  Hydraulics  of  the  Mississippi  River; 
Corthell,  National  Geographic  Magazine,  VIII,  p.  351,  1892. 

3  Dana,  Manual  of  Geology,  4th  ed.,  p.  198. 


SURFACE  WATERS   (RIVERS)  275 

per  year,  but  more  lately  the  growth  has  been  more  rapid  due  to  arti- 
ficial embankments.1 

Fossil  deltas.  —  Subsequent  to  the  formation  of  a  delta,  the  waters 
of  the  lake  or  sea  in  which  it  originated  may  be  drained  off,  either  due 
to  the  cutting  down  of  the  lake  outlets,  removal  of  the  retaining  dam 
(such  as  a  glacier),  or  elevation  of  the  land,  as  in  the  case  of  the  sea  or 
an  estuary.  These  old  deltas  are  often  readily  recognized  by  their  flat 
tops,  lobate  fronts,  and  characteristic  structure. 

Along  the  Hudson  River  valley  in  New  York  state  many  splendid 
fossil  deltas  (Plate  XLIV,  Fig.  2)  are  found.  Others  are  seen  along 
the  old  shore  lines  of  the  Great  Lakes  hi  the  north  central  states,  etc. 

The  fossil  deltas  often  serve  as  important  sources  of  sand  or  gravel 
for  structural  work,  filter  beds,  etc.  In  railroad  construction  they  are 
sometimes  drawn  upon  for  material  to  make  fills  across  valleys.  The 
flat  tops  of  the  old  deltas  often  serve  as  splendid  sites  for  towns,  shops 
or  factories. 

River  terraces.  —  Many  streams  are  bordered  by  natural  benches 
or  terraces  (Plate  XL VI,  Fig.  1),  which  are  usually  somewhat  narrow, 
but  may  often  have  considerable  length  parallel  with  the  river.  One 
or  several  of  these  terraces  are  often  present,  and  form  benches  on 
one  side  of  the  stream  valley,  or  there  may  be  corresponding  ones  at 
the  same  level  on  the  opposite  side,  although  this  is  not  always  the  case. 
In  some  cases  these  terraces  are  so  level  and  well  developed  as  to  make 
the  layman  suspicious  of  their  origin  by  natural  processes.  Terraces 
usually  represent  the  remnants  of  flood  plains. 

Flood-plain  terraces.  —  The  origin  of  these  was  described  on  an 
earlier  page.  To  the  statement  there  made  should  be  added  that  a 
flood-plain  terrace  may  be  chiefly  of  solid  rock,  of  unconsolidated  sedi- 
ment, or  of  rock  covered  by  a  variable  thickness  of  flood-plain  deposit. 
The  material  when  unconsolidated  varies  from  sandy  clay  to  gravel 
and  large  stones. 

As  a  river  cuts  its  channel  deeper  it  may  no  longer  cover  the  flood- 
plain  terrace  in  period  of  flood,  but  sometimes  develops  new  terraces 
at  successively  lower  levels.  This  cutting  down  of  the  stream  might 
be  due  to  different  causes. 

The  material  underlying  flood-plain  terraces  is  often  drawn  upon  for 
filling  or  for  structural  sand.  When  sufficiently  fine-grained  and 
clayey  it  serves  as  a  source  of  brick-making  earth.  The  terrace  ma- 
terial is  at  times  also  sufficiently  permeable  but  retentive  to  hold  ground- 
water  for  shallow  wells. 

1  Geikie,  Textbook  of  Geology,  3rd.  ed.,  p.  402. 


PLATE  XLVI,  FIG.  1.  —  High  river  terrace,  Orizaba,  Mexico.     (H.  Ries,  photo.) 


FIG.  2.  —  View  of  Hudson  River  valley,  looking  north  from  West  Point,  N.  Y. 
The  river  here  flows  through  a  deep  gorge  that  has  been  depressed  below  sea 
level,  and  partly  filled  by  sediment  and  glacial  drift.  (H.  Ries,  photo.) 

(276) 


SURFACE  WATERS   (RIVERS)  277 

Outlets  of  rivers.  —  The  outlets  of  rivers  are  of  three  general 
types:1  (1)  Those  which  discharge  directly  or  indirectly  into  seas  where 
the  range  of  tide  and  the  violence  of  the  storms  are  limited,  such  as 
the  Danube,  the  Nile,  the  Mississippi,  certain  rivers  flowing  into  the 
Baltic,  etc.;  (2)  those  which  discharge  through  estuaries,  such  as  the 
Thames,  the  Seine,  and  the  St.  Lawrence;  (3)  those  which  discharge 
directly  into  oceans  and  are  exposed  to  all  the  changes  produced  by 
sand  drift,  tidal  effects,  etc.,  such  as  most  of  the  rivers  of  the  Atlantic 
and  Pacific  coasts  of  the  United  States.  Of  these  three  types  the  third 
is  perhaps  the  most  difficult  to  improve. 

Bars  at  mouth  of  rivers.  —  Bars,  sometimes  constituting  a  trouble  or  even  menace 
to  navigation,  are  found  at  the  mouths  of  nearly  all  rivers.  They  may  be  formed 
in  several  ways: 

1.  In  the  case  of  sediment-bearing  rivers  like  the  Mississippi,  Nile,  Amazon,  etc., 
or  rivers  entering  lakes  or  inland  seas,  the  checking  of  the  current  on  entering  still 
water  causes  it  to  drop  its  load  resulting  in  the  formation  of  a  bar. 

2.  Where  a  river  enters  a  lagoon  or  bay  of  a  tidal  sea,  the  bar  may  be  formed 
by  wind  and  waves  driving  sediment  across  the  mouth  (see  Bars,  under  Ocean 
Waves  and  Currents,  Chapter  VIII),  and  the  river  channel  is  kept  open  by  tidal 
currents. 

3.  The  formation  of  a  bar  across  the  mouth  of  a  tidal  estuary  may  be  due  to 
eddies  and  still  water  produced  by  ebb  and  flood  currents  at  the  entrance,  or  to 
littoral  or  shore  currents  which  drift  material  across  the  mouth  of  the  estuary. 

DRAINAGE  FORMS  AND  MODIFICATIONS 

Development  of  valleys  and  tributaries.  —  A  valley  usually  has  its 
beginning  in  a  gully  formed  by  ram-wash.  This  serves  as  a  line  of  con- 
centration for  more  surface  water  during  successive  storms,  and  so 
becomes  enlarged,  being  washed  out  deeper  each  time.  At  the  same 
time  it  may  be  lengthened  by  headward  growth  (erosion)  and  widened 
by  rain-wash  from  the  sides.  Irregularities  of  slope  are  likely  to  pro- 
duce sinuosities  in  the  stream,  which  are  the  beginnings  retained  by  the 
valley  when  it  has  developed  more.  Since  the  water  flowing  down  the 
slopes  of  a  gully  follows  lines  of  depression,  so  branch  gullies  originate 
from  similar  inequalities  of  slope  or  hardness  of  rocks,  and  these  tribu- 
taries develop  in  the  same  manner  as  the  main  stream.  Tributaries  as 
a  rule  join  their  main  stream  with  the  acute  angle  up-stream. 

Although  a  valley  may  develop  up-stream,  that  is  headward,  it  will 
continue  until  it  reaches  a  point  where  erosion  from  the  opposite  direc- 
tion counterbalances  it.  If,  however,  erosion  on  opposite  sides  of  a 
divide  is  unequal,  the  latter  will  slowly  move  towards  the  side  of  less 
rapid  erosion. 

1  Thomas  and  Watt,  I,  p.  309,  1913 


278  ENGINEERING  GEOLOGY 

If  on  a  new  land  surface  we  have  a  series  of  somewhat  parallel  gullies 
developed,  these  will  tend  to  concentrate  the  drainage.  A  gully  widens 
by  water  entering  from  the  sides,  and  lengthens  by  wash  at  its  upper  end. 
Every  gully,  however,  does  not  develop  into  a  stream  valley,  for  if 
one  deepens  and  widens  more  rapidly  than  a  neighboring  one,  the  latter 
may  become  absorbed  or  eliminated  by  the  destruction  of  the  ridge 
between  them.  Moreover,  those  gullies  which  develop  headward  more 
rapidly  will  send  out  tributaries,  and  cut  off  the  up-slope  supply  of 
those  which  did  not  work  headward  as  fast. 

If  on  a  new  land  surface  we  have  developed  a  series  of  somewhat 
parallel  valleys,  they  will  occupy  a  series  of  trenches,  separated  by 
elevations  as  yet  not  much  dissected  by  erosion,  although  a  few  tribu- 
taries may  have  developed.  As  the  stronger  streams  deepen  and 
widen  their  valleys  these  inter-stream  areas  become  narrower.  At  the 
same  time  the  tributaries  increase  in  number  and  intersect  the  inter- 
stream  areas,  cutting  them  into  a  series  of  cross  ridges.  By  a  con- 
tinuation of  this  process  these  ridges  separating  the  valleys  become 
obliterated  by  erosion  and  weathering,  resulting  in  reducing  the  land 
surface  to  a  nearly  common  level  and  in  the  development  of  a  peneplain. 

If  a  drainage  system  develops  on  a  series  of  rocks  of  unequal  hard- 
ness, the  hardest  rocks  will  resist  erosion  most,  so  that  they  remain  as 
ridges  even  after  the  soft  rocks  have  been  leveled  down. 

If  a  stream  crosses  a  tilted  bed  of  hard  rock  lying  between  softer 
ones,  the  valley  will  widen  more  both  above  and  below  the  hard  bed 
than  it  does  where  the  stream  crosses  it.  If  the  hard  beds  are  vertical, 
so  that  their  outcrop  does  not  shift  as  erosion  proceeds,  a  narrows  is 
developed. 

The  formation  of  gullies  may  begin  without  much  regard  to  the 
degree  of  hardness  of  the  rocks,  but  with  further  development  the  rela- 
tion of  streams  to  rock  structure  often  becomes  emphasized.  Thus  a 
stream  flowing  over  a  soft,  less  resistant  rock,  deepens  its  valley  more 
rapidly  than  one  flowing  over  hard  rock.  More  rapid  erosion  also  takes 
place  when  a  stream  flows  across  rocks  of  unequal  hardness,  than  over 
rocks  which  are  all  hard. 

As  times  goes  on  the  streams  show  a  tendency  to  follow  the  softer 
formations,  so  that  the  harder  ones  become  divides,  and  there  is  thus 
an  adjustment  of  the  streams  to  rock  structure.  Joint  planes,  because 
they  are  lines  of  weakness,  may  also  exert  a  guiding  influence  on  stream 
drainage. 

Piracy.  —  Neighboring  streams  do  not  always  develop  with  equal 
rapidity,  because  of  unequal  conditions,  such  as  difference  in  slope, 


SURFACE  WATERS   (RIVERS) 


279 


character  of  rock,  size  of  streams,  etc.  One  stream  gains  the  advantage 
over  the  other  by  more  rapid  development  through  headward  erosion, 
so  that  the  more  able-bodied  stream  constantly  pushes  the  divide  into 
the  territory  of  its  weaker  neighbor,  and  its  headwaters  finally  cut  into 
the  upper  reaches  of  the  other.  Thus  the  head  of  the  second  stream 
or  even  one  of  its  tributaries  becomes  diverted  into  the  channel  of  the 
first.  This  is  known  as  piracy. 

As  shown  in  Fig.  131,  Beaverdam  Creek  once  flowed  across  the 
Blue  Ridge,  which  at  Snickers  Gap  is  of  hard  rock.     The  stream  was 


FIG.  131.  —  Stream  piracy.     (After  Willis.) 

unable  to  deepen  its  bed  across  the  hard  rock  of  the  ridge  as  rapidly  as 
the  larger  Potomac  lowered  its  channel  across  similar  rock.  The  result 
was  that  the  head  of  a  tributary  of  the  Potomac  worked  back  and 
tapped  Beaverdam  Creek.  By  this  process  the  water  gap  (at  Snickers 
Gap)  became  a  wind  gap. 

Young  and  old  topography.  —  Narrow  and  steep-sided  valleys  cut 
in  a  land  area  of  a  humid  region  are  said  to  be  young,  and  the  territory 
traversed  by  them  is  in  its  topographic  youth.  Young  streams  are  usually 
swift,  they  cut  vertically  rather  than  horizontally,  and  their  grade  is 
often  interrupted  by  rapids  and  falls.  At  this  stage  the  stream  has 
acquired  but  few  tributaries.  Valleys  approaching  base  level  develop 
flats.  As  these  flats  widen,  and  the  tributaries  increase  in  number  and 


280 


ENGINEERING   GEOLOGY 


size,  the  valley  slopes  become  gentle,  and  the  topography  is  said  to  be 
mature.  In  Plate  XL VII,  Fig.  1,  we  see  a  young  valley  tributary  to  a 
mature  one. 

Old  streams  usually  have  a  low  grade,  and  a  sluggish  current.  They 
erode  during  floods,  and  deposit  their  load  and  fill  their  channels  at 
other  times.  Meandering  is  a  characteristic  feature  of  old  streams,  as 
illustrated  in  the  Mississippi. 

Formation  of  canyons.  —  A  high  altitude  is  favorable  to  the  de- 
velopment of  swiftly-flowing  streams  and  deep  valleys,  and  if  the  con- 
ditions promoting  widening  are  absent,  the  valley  will  be  narrow.  In 
arid  climates  the  conditions  are  usually  favorable  to  the  development 


DANSKAMMER 


FIG.  132.  —  Sections  across  the  Hudson  River  Valley.  A,  the  Danskammer  cross- 
ing; B,  the  Storm  King  crossing;  C,  the  Little  Stony  Point  crossing;  D,  Arden 
Point  crossing.  (After  Kemp,  Amer.  Jour.  Science.) 

of  deep  narrow  valleys  or  canyons.  Firm  rock  is  also  a  condition  favor- 
ing their  growth.  The  Colorado  canyon  is  one  of  the  finest  examples 
of  its  kind  known.  A  small  canyon  is  usually  termed  a  gorge  (Plates 
XLVII  and  XI). 

Buried  channels.  —  The  drainage  systems  of  a  region  are  some- 
times seriously  disturbed  by  natural  processes.  Thus  lava  flows  may 
obliterate  the  river  valleys  of  an  area,  and  necessitate  the  establish- 
ment of  new  ones,  but  of  more  practical  importance  to  the  engineer 
perhaps,  because  of  extensive  areas  affected,  is  the  displacement  of 
drainage  by  glacial  action.  Prior  to  the  advance  of  the  continental  ice 


PLATE  XLVTI,  FIG.  1.  —  View  looking  west  down  Fall  Creek  gorge,  Ithaca,  N.  Y. 
A  post-Glacial  gorge  cut  in  shales.  In  the  distance  is  seen  the  valley  at  the  head 
of  Cayuga  Lake;  a  mature  valley  with  gently  sloping  sides,  and  filled  in  by  drift 
and  delta  deposits  to  a  depth  of  over  400  feet. 


FIG.  2.  —  View  looking  east  up  Fall  Creek  gorge,  Ithaca,  N.  Y. 

horizontal  strata. 


Falls  flowing  over 

(281) 


282  ENGINEERING  GEOLOGY 

sheet  in  recent  geologic  times,  there  were  well-established  drainage  sys- 
tems. In  many  cases  the  pre-Glacial  river  valleys  were  completely 
filled  with  glacial  drift,  so  that  after  the  ice  withdrew  these  rivers  had 
to  cut  new  valleys.  In  some  cases  these  new  (post-Glacial)  valleys 
were  cut  in  the  drift  filling  of  the  old  or  pre-Glacial  valley  (Fig.  191), 
in  others  the  river  has  cut  a  gorge  in  the  rock  at  one  side  of  the  buried 
channel  (Plate  XL VII,  Figs.  1  and  2),  and  in  still  other  cases  a  part  of 
the  present  valley  is  excavated  in  the  glacial  filling  and  a  part  in  the 
solid  rock. 

These  buried  channels  are  not  always  known  to  the  engineer,  and 
when  encountered,  as  they  sometimes  are,  in  tunneling  operations,  they 
may  be  a  source  of  both  surprise  and  trouble.  Several  buried  channels 
were  encountered  in  the  construction  of  the  Catskill  aqueduct  for  New 
York  City,  and  are  referred  to  in  Chapter  X. 

A  remarkable  case  of  a  partly  buried  valley  is  that  of  the  Hudson 
River.  In  recent  geologic  times  the  land  of  the  Atlantic  coast  stood  much 
higher,  so  that  the  Hudson  River  carved  a  deep  gorge,  whose  continua- 
tion can  be  traced  by  a  trench  on  the  sea  bottom  some  distance  beyond 
New  York  bay.  Similar  valleys  carved  in  the  submerged  continental 
shelf  bordering  the  Atlantic  Coastal  Plain  have  been  traced  opposite 
the  present  mouths  of  several  of  the  pre-Glacial  streams  of  the  eastern 
United  States.  Subsequently  the  land  was  depressed  lower  than  it  is 
now  and  the  Hudson  River  gorge  was  filled  by  clay  and  sand  brought 
down  by  the  river,  and  in  part  by  glacial  drift  (Fig.  132).  This  was  fol- 
lowed by  a  slight  re-elevation,  bringing  the  estuary  clays  about  200  feet 
above  present  sea  level  in  the  Highlands.  When  it  became  necessary 
recently  to  carry  the  new  aqueduct  under  the  Hudson  (Plate  XL VI, 
Fig.  2)  by  means  of  an  inverted  siphon,  the  engineers  found  it  necessary 
to  go  nearly  1000  feet  below  the  river  level  in  order  to  cross  in  the  rock 
bottom. 

FLOODS  AND  DAM  FOUNDATIONS 

Floods  and  their  regulation.  —  A  river  which  is  irregular  in  its  dis- 
charges may  cause  trouble:  First,  by  having  a  deficiency  of  water  for 
navigation,  power  or  other  purposes  in  dry  weather,  and  second,  by 
discharging  an  excess  during  another  period,  the  volume  being  danger- 
ous to  navigation  and  injurious  to  property. 

The  prevention  of  damage  by  floods  is  a  subject  to  which  engineers 
and  others  have  given  considerable  thought,  and  on  which  much  money 
has  been  expended,  sometimes  with  but  little  reward. 

The  causes  of  disastrous  floods  are:   (1)  Excessive  rainfall;  (2)  rapid 


SURFACE  WATERS   (RIVERS)  283 

melting  of  accumulated  snow;  (3)  failure  of  reservoirs;  (4)  formation 
and  failure  of  ice  jams;  and  (5)  the  breaking  of  levees.  These  may 
act  singly  or  jointly,  and  the  great  problem  is  the  prevention  of  flood 
damage  by  causes  (1)  and  (2). 

As  stated  by  Thomas  and  Watt,1  "  Nature  has  indicated  one  satis- 
factory method  of  improving  the  navigability  of  water  courses,  in  the 
lakes  which  lie  at  the  foot  of  mountainous  regions  and  from  which 
rivers  flow.  By  them  the  length  of  the  navigable  season  is  increased 
and  the  damage  from  floods  is  decreased,  and  the  lesson  taught  is  that 
where  artificial  lakes  or  reservoirs  can  be  constructed  near  the  sources 
of  streams,  the  waters  falling  in  the  various  basins  leading  to  these 
reservoirs  may  be  usefully  stored  up.  Not  only  will  the  excess  of  water 
thus  be  held  back  while  that  entering  lower  down  is  making  its  escape, 
thereby  preventing  a  flood,  but  it  may  be  drawn  out  as  required  by 
the  necessities  of  navigation  and  to  its  great  benefit. 

The  best  example  of  natural  reservoirs  known  in  the  world  is  the 
chain  of  Great  Lakes,  which  exercises  a  complete  control  over  the 
St.  Lawrence  River.2 

Since  the  natural  method  of  control  seems  to  work,  an  artificial  method,  by  the 
construction  of  artificial  reservoirs,  on  the  tributaries  of  a  large  main  stream,  sug- 
gests itself,  and  while  it  appears  practicable  in  the  case  of  small  rivers,  the  cost  in- 
volved seems  to  many  to  prohibit  its  application  to  large  river  systems. 

A  commission  which  was  appointed  by  the  city  of  Pittsburgh  to  look  into  the 
matter  of  reducing  floods  on  the  Allegheny  and  Monongahela  rivers,  recommended 
building  seventeen  reservoirs  in  the  water  shed  above  the  city  at  an  expense  of 
§20,000,000.  Such  reservoirs  it  was  claimed  would  not  only  take  up  the  surface 
water  during  floods,  but  in  tune  of  drought  the  water  in  them  could  be  let  out  to 
raise  the  level  of  the  river  the  necessary  amount. 

One  of  the  most  disastrous  floods  in  recent  years  was  that  of  the  Ohio  Valley  in 
March,  19 13,3  which  caused  over  $200,000,000  damage.  This  flood  was  not  an 
isolated  one  for  the  Ohio  River  has  overflowed  its  banks  at  some  points  every  year 
since  1873. 

To  have  controlled  these  floods  by  reservoirs  would  involve  holding  back  tre- 
mendous volumes  of  water.  Taking  the  floods  at  Cincinnati,  for  example,  it  is 
found  that  to  have  kept  the  highest  flood  on  record  at  that  city  below  the  danger 
line,  would  have  necessitated  holding  back  above  Cincinnati  226,000  million  cubic 
feet  of  water,  representing  the  dangerous  crest  or  top  of  the  flood.  The  capacity  of 
the  forty-three  reservoir  sites  above  Pittsburgh,  suggested  by  the  Pittsburgh  Flood 
Commission,  is  80,500  million  cubic  feet,  while  preliminary  surveys  made  by  the 
U.  S.  Geological  Survey  in  the  Kanawha  River  drainage  basin  showed  seventeen 
reservoir  sites  with  280,000  million  cubic  feet  capacity*  However,  there  are  other 
tributaries  of  the  Ohio  River,  and  to  control  these  would  require  a  very  large 
storage  capacity. 

1  Improvement  of  Rivers,  I,  p.  281,  1913. 

2  For  an  excellent  discussion  on  this  subject  see  Reservoir  Sites  hi  Wyoming  and 
Colorado,  by  H.  S.  Chittenden,  House  Doc.  141,  55th  Congress,  2nd  Session,  1898. 

3  U.  S.  Geol.  Survey  Wat.  Sup.  Paper,  334,  1913. 


284  ENGINEERING   GEOLOGY 

Ice  gorges.  —  In  some  streams  the  ice,  when  it  breaks  up,  becomes 
piled  against  some  obstruction  such  as  a  shoal  or  bar  and  forms  a  tem- 
porary dam.  Such  a  dam  may  obstruct  the  stream  flow  to  a  con- 
siderable extent,  so  that  when  the  pressure  of  the  water  behind  the  dam 
causes  it  to  burst,  a  serious  flood  may  result.  In  some  cases  the  ice 
dam  bursts  and  naturally  passes  down-stream,  only  to  become  lodged 
again  at  another  point  below.1 

It  seems  difficult  to  prevent  floods  due  to  ice  gorges  on  streams,  and 
it  is  sometimes  almost  impossible  to  keep  an  open  channel  in  winter. 
Explosives  are  occasionally  used,  but  the  stream  often  becomes  blocked 
for  many  miles.  About  the  only  remedy  to  be  applied  is  to  remove,  as 
far  as  possible,  the  causes  stopping  the  movement  of  ice. 

Dam  foundations.  —  Since  dams  are  constructed  for  the  purpose 
of  storing  up  river  waters,  it  is  not  out  of  place  here  to  discuss  briefly 
the  relation  of  geologic  structure  to  dam  foundations,  even  though  the 
subject  is  referred  to  in  Chapters  VI  and  X. 

In  dam  construction  it  is  essential  that  the  foundations  should  be 
sufficiently  strong  to  bear  the  weight  of  the  dam  and  also  sufficiently 
tight  to  prevent  seepage  under  or  around  the  structure. 

The  character  of  the  foundation  may  determine  the  height  of  dam 
which  it  is  practicable  to  construct,  and  the  amount  of  storage  capacity 
which  may  be  made  available.  Many  dam  failures  are  due  to  neglect 
to  thoroughly  investigate  the  character  of  the  foundations,  for  sound- 
ings and  borings  should  be  carefully  made  before  finally  locating  any 
dams  or  locks. 

Care  should  be  taken  not  to  mistake  boulders  for  bed  rock.  The 
need  of  these  precautions  is  not  only  to  insure  the  safety  of  the  dam, 
but  also  to  save  expense,  for  it  is  often  very  costly  to  patch  up  de- 
fective foundations  after  the  work  is  once  started. 

Bed-rock  foundations.  —  In  some  cases  the  bed  rock  outcrops  at 
the  surface,  or  has  but  slight  covering  over  it  on  the  stream  bottom. 

Care  should  be  taken  to  ascertain  its  tightness  and  continuity.  Lime- 
stones are  apt  to  have  solution  channels,  which  would  permit  under- 
flow, and  these  should  be  filled  up,  or  else,  if  of  shallow  nature,  the  bed 
rock  should  be  removed  until  it  is  solid. 

Sandstones  may  have  interbedded  shale  layers,  which  become  softened 
by  water  percolating  along  them  and  causing  the  foundation  rock  to 
slip,  unless  the  trench  for  the  dam  is  carried  sufficiently  deep. 

Some  stratified  rocks  are  so  seamed  by  joint  planes,  especially  near 

1  See  for  example  case  of  Susquehanna  River  Flood,  Pa.,  U.  S.  Geol.  Survey  Wat. 
Sup.  Paper  147,  p.  25,  1904. 


SURFACE  WATERS   (RIVERS)  285 

the  surface,  as  to  give  cause  for  concern  on  account  of  danger  from 
seepage.  Among  the  igneous  rocks,  the  porous  volcanics,  and  es- 
pecially tuffs  and  agglomerates,  are  sometimes  liable  to  be  very  porous 
and  need  grouting  (see  p.  59). 

It  must  not  be  assumed  from  what  has  been  said  above  that  the 
types  of  rock  mentioned  always  cause  trouble,  but  these  cases  are 
cited  simply  to  show  the  need  of  precaution. 

Where  solid  rock  is  struck,  it  should  be  bored  to  a  sufficient  depth  to 
prove  that  it  is  not  a  thin  layer,  such  as  a  lava  flow  resting  on  other 
material  (Plate  XLIII,  Fig.  2),  or  an  overhanging  ledge  of  a  buried 
stream  channel.  Moreover,  it  must  not  be  assumed  that  because  bed 
rock  is  found  at  a  given  level  on  one  side  of  a  river,  that  it  will  be 
found  at  a  similar  level  on  the  other  side. 

Valleys  are  sometimes  cut  along  the  contact  of  two  formations, 
which,  as  explained  on  page  211,  may  be  a  line  of  weakness  and 
solubility. 

Unconsolidated  material.  —  This  may  consist  of  gravel,  sand  or 
clay,  either  alone,  or  interbedded  or  intermixed.  These  materials  if 
found  in  the  valleys  may  represent  river  deposits,  lake  deposits  or 
glacial  deposits.  If  the  last,  the  material  might  be  either  modified 
drift  (Chapter  X)  consisting  of  indifferently  bedded  sand  or  gravel,  or  it 
may  be  till  (Chapter  X),  a  heterogeneous  mixture  of  boulders,  clay  and 
sand. 

Unconsolidated  materials  should  be  carefully  tested  for  dam  founda- 
tion work,  for  although  they  may  consist  of  dense,  water-tight  material 
on  top,  there  may  be  permeable  beds  or  lenses  below. 

Gravel  foundations  usually  permit  seepage.  With  sand,  or  clay  and 
sand  mixed,  there  is  danger  of  seepage  or  undermining  from  above, 
and  danger  of  erosion  on  the  down-stream  side  of  the  dam.  Sheet 
piling  is  commonly  used  to  protect  it  on  the  up-stream  side.  Coarse 
and  fine  sand  mixed  seem  to  have  a  greater  bearing  power  than  sand 
and  clay.  Clay  is  not  a  very  common  foundation  for  structures  in 
rivers,  but  when  present  may  vary  from  the  compacted  silt  of  abandoned 
river  channels  to  the  hard  clay  which  will  stand  a  strong  current  al- 
most unaffected.  This  last  variety  of  clay  is  rare  in  river  work,  but  is 
excellent  for  foundations,  as  it  is  water  tight  and  usually  of  high  bearing 
power.  With  the  softer  variety  of  clay  it  is  not  safe  to  trust  much  to  the 
bearing  power  of  the  material  unless  it  has  been  shown  by  tests  to  be 
reliable  in  this  respect.  Even  when  confined  by  sheet  piling  (as  should 
always  be  done  on  those  sides  of  the  structure  where  there  is  any  possi- 
bility of  the  material  spreading  under  concentrated  load),  such  clay  is 


286  ENGINEERING  GEOLOGY 

liable  to  flow  gradually  and  produce  displacement  of  the  masonry  un- 
der the  varying  pressure  of  the  water  thrust,  and  during  construction  the 
weight  of  the  banks  will  often  force  up  the  material  in  the  excavation  for 
the  floor,  as  the  weight  becomes  unbalanced  by  the  operations.  (Thomas 
and  Watt.) 

COMPOSITION  OF  RIVER  WATER 

In  addition  to  suspended  and  also  colloidal  matter,  which  may  consist 
of  silica,  iron  oxide  or  alumina,  river  water  may  contain  dissolved  gases 
such  as  carbon  dioxide,  and  various  dissolved  solids.  These  expressed  in 
ionic  form  include  iron,  calcium,  sodium,  aluminum,  magnesium,  potas- 
sium, hydrogen,  carbonate  radicle  (CO3),  bicarbonate  radicle  (HCO3), 
sulphate  radicle  (SO3),  nitrate  radicle  (N03),  and  chlorine. 

This  dissolved  mineral  matter  may  be  derived  from:  (1)  Spring  water, 
which  is  the  chief  source;  (2)  solvent  action  of  the  river  water  on  its 
banks,  or  on  the  grains  of  sediment  which  it  carries;  (3)  rain  wash; 
and  (4)  artificial  sources  as  factories,  sewers,  etc.  The  last  may  cause 
considerable  contamination  by  discharging  both  mineral  and  organic 
substances  into  the  river. 

The  composition  of  river  water  is  of  considerable  importance  for  several  reasons, 
which  follow: 

1.  To  be  used  for  drinking  purposes  the  water  should  be  hygienically  pure  and 
free  from  contamination,  and  for  this  reason  much  attention  is  now  given  to  the 
condition  of  watersheds  whose  drainage  is  drawn  upon  for  municipal  supplies. 

2.  For  different  manufacturing  purposes  the  water  should  be  free  from  certain 
deleterious  substances. 

3.  If  desired  for  steaming  purposes,  any  substances  present  in  sufficient  quantity  to 
cause  scale,  foaming,  or  other  troubles  are  not  desired.    Railroads  must  need  give  con- 
siderable attention  to  the  composition  of  the  water  for  engines  used  along  the  route. 

4.  In  the  West  waters  with  a  high  percentage  of  soluble  salts  have  sometimes 
caused  damage  to  bridges  and  other  piers  or  abutments.     Where  a  porous  stone  was 
used,  the  water  soaked  into  its  pores  during  high  water.     As  the  submerged  rocks 
dried  out  when  the  river  fell,  the  crystals  of  soluble  salts  formed  in  the  pores.     Repe- 
titions of  this  sometimes  cause  a  disintegration  of  the  rock,  similar  in  action  to 
the  sulphate  of  soda  test,  described  under  Building  Stone  in  Chapter  XI. 

The  waters  of  the  arid  region  contain  a  much  larger  quantity  of  salts  in  solution 
than  those  of  the  more  humid  regions. 

Water  used  for  irrigation  should  not  contain  any  considerable  quantity  of  solu- 
ble salts,  as  these  are  injurious  to  growing  crops.  The  total  quantity  of  soluble 
salts  or  alkali  permissible  cannot  be  stated,  as  it  depends  on  the  character  of  the 
salts,  natural  condition  of  soil,  amount  of  water  used  for  irrigation,  and  efficiency 
of  underground  drainage  to  prevent  alkali  crusts. 

Statement  of  analyses.1  —  The  usual  statement  of  water  analyses 
is  a  somewhat  firmly  established  though  incorrect  mode  of  procedure. 
If,  for  example,  a  water  is  found  to  contain  sodium,  potassium,  calcium, 

1  This  topic  is  a  condensation  of  a  statement  by  F.  W.  Clarke,  U.  S.  Geol.  Survey 
Bull.  491,  p.  57,  1912. 


SURFACE   WATERS   (RIVERS)  287 

magnesium,  chlorine,  and  the  radicles  of  sulphuric  and  carbonic  acids, 
and  these  are  combined  into  salts,  at  least  twelve  such  compounds  must 
be  assumed,  and  there  is  no  definite  law  by  which  their  relative  pro- 
portions can  be  calculated.  A  combination,  however,  is  usually  as- 
sumed, and  each  chemist  allots  the  several  acids  to  the  several  bases 
according  to  his  individual  judgment.  The  twelve  possible  salts  rarely 
appear  in  the  final  statement;  all  the  chlorine  may  be  assigned  to  the 
sodium,  and  all  the  sulphuric  acid  to  the  lime.  We  cannot  be  sure  that 
the  chosen  combinations  are  correct. 

With  regard  to  whether  the  radicles  are  combined  or  not,  the  prev- 
alent opinion,  among  physical  chemists  at  least,  is,  that  in  dilute  solutions 
the  salts  are  dissociated  into  their  ions,  and  that  with  the  latter  only 
we  can  legitimately  deal.  On  this  basis  all  water  analyses  can  be 
rationally  compared.  There  are,  however,  still  some  difficulties,  such 
as  whether  silica  is  present  in  colloidal  form  or  as  the  silicic  ion  SiO3; 
and  whether  ferric  oxide  and  alumina  are  present  as  such,  or  in  the  ions 
of  their  salts.  The  iron  may  represent  ferrous  carbonate,  alumina  may 
be  the  equivalent  of  alum;  but  as  a  rule  the  quantities  are  small,  and 
for  convenience  these  substances  are  regarded  as  colloidal  oxides  and  so 
tabulated.  If  we  consider  an  analysis  as  representing  the  composition 
of  the  anhydrous  inorganic  matter  which  is  left  when  a  water  has  been 
evaporated  to  dryness,  the  difficulty  as  regards  iron  disappears,  for 
ferrous  carbonate  is  then  oxidized  and  ferric  oxide  remains.  The  same 
is  true  of  bicarbonates  of  calcium  and  magnesium  which  can  only  exist 
in  solution  and  not  in  the  anhydrous  residue.  If,  in  a  given  water, 
notable  quantities  of  lime,  magnesia,  and  carbonic  acid  are  found, 
bicarbonic  ions  must  be  present,  for  without  them  the  bases  could  not 
be  dissolved;  but  after  evaporation  only  the  normal  salts  remain. 
Sodium  and  potassium  bicarbonates  are  not  so  readily  broken  down; 
but  even  with  them  it  is  better  to  compare  the  monocarbonates  so  as 
to  secure  uniformity  of  statement. 

Another  variable  requiring  consideration  is  that  due  to  solution.  A 
given  solution  may  be  very  dilute  at  one  time,  and  much  more  con- 
centrated at  another,  but  the  mineral  content  of  the  water  may  be  the 
same  in  both  cases.  The  ocean  water  for  example  has  3.5  per  cent 
saline  matter,  while  the  Black  Sea  has  a  little  more  than  half  as  much, 
but  the  salts  yielded  by  each  on  evaporation  are  almost  identical. 
Occasionally  it  may  be  desirable  to  compare  waters  directly,  but  in 
other  cases  it  is  more  convenient  to  study  the  composition  of  the  solid 
residues  in  percentage  terms.  The  following  case  illustrates  the  various 
methods  of  statement.  In  the  first  column  the  results  are  given  in 


288 


ENGINEERING   GEOLOGY 


oxides,  etc.,  as  in  a  mineral  analysis,  and  in  grains  to  the  imperial  gal- 
lon. In  the  second  column  they  are  stated  in  terms  of  salts,  and  in 
grains  to  the  imperial  gallon.  In  the  second  column  they  are  stated  in 
terms  of  salts,  and  in  parts  per  million  of  the  water  taken.  In  the 
third  column  the  composition  of  the  residue  is  given  in  radicles  or  ions, 
and  in  percentages  of  the  total  anhydrous  inorganic  solids. 

ANALYSIS  OF  WATER  STATED  IN  DIFFERENT  FORMS 


Oxides. 

Grains 

.    Per.  . 
imperial 
gallon. 

Salt, 

Parts 
per 
million. 

Radicles 
or  ions. 

Per  cent. 

SiO2 

0.891 
32.601 
4.554 
2.681 
11.463 
0.355 
13.117 
5.530 
0.189 

0.189 
2.397 

CaSO4  . 

457.7 
236.0 
9.4 
62.5 
63.2 
156.9 
21.9 
2.7 
2.7 

34.2 
1.3 

SiO2 

1.26 
55.28 
8.78 
3.79 
12.02 
0.41 
13.24 
4.69 
0.53 

SO3 

MgSO4  

SO4 

CO2            

K2SO4  
Na2SO4  

CO3 

Cl  

Cl  

Na2O  

NaCl  

Na  

K?O  

Na2CO3  

K  

CaO  
MgO 

Na2SiO3  
(FeAl)2O3  
Mn2O3 

Ca  
Mg  
R>O3 

(FeAl)2O3 

MnoOs  

Ignition  

100.00 
Ignition  omitted. 

Salinity,  1014  parts 
per  million. 

Ignition.  .  .  .  :  

Excess  SiO2  

Less  O  =  Cl 

73.967 
0.604 

1048.5 

73.363 

The  salinity  in  this  case  means  that  one  million  parts  of  this  water 
contain  in  solution  1014  parts  of  anhydrous,  inorganic,  solid  matter. 

Relation  of  river  water  to  rock  formation.  —  The  amount  of  dis- 
solved mineral  matter  in  the  natural  surface  waters  will  depend  chiefly 
on  the  nature  and  texture  of  the  rock  formations  in  contact  with  the 
water,  on  climatic  conditions,  and  on  the  amount  of  vegetation. 

It  is  a  well-known  fact  that  two  streams  flowing  over  different  kinds 
of  rocks  may  show  a  difference  in  composition,  and  also  that  a  stream, 
for  example,  flowing  for  a  part  of  its  course  over  a  limestone  formation, 
and  then  later  receiving,  let  us  say,  tributaries  which  rise  in  and  flow 
from  a  schist  area,  will  show  a  difference  in  composition  in  different 
parts  of  its  course  (Ref.  11). 

Small  streams  are  most  affected  by  local  conditions,  and  illustrate 
the  greatest  differences  in  composition,  while  large  rivers  usually  show 
closer  resemblance  to  each  other. 

A  most  interesting  case  of  variation  is  seen  in  that  of  the  Cache  la 


SURFACE  WATERS   (RIVERS) 


289 


Poudre  River  in  Colorado.1  This  flows  first  through  a  rocky  canyon 
over  schist  and  granite  boulders,  and  thence  over  the  plains.  It  is 
then  diverted  into  ditches  and  reservoirs  for  irrigation,  and  finally 
empties  into  the  Platte. 

The  analyses  reduced  to  ionic  form  and  expressed  in  percentages  of 
the  anhydrous  residue  are  as  follows: 

ANALYSES  OF  WATER  FROM  CACHE  LA  POUDRE  RIVER 


I. 

II. 

III. 

IV. 

V. 

CO3 

31.91 

33.68 

7  34 

10  34 

8  78 

SO4 

9.07 

23.36 

54  33 

55  28 

Cl  

4.03 

1.10 

2.52 

3  19 

3  79 

Ca  

14.53 

22.58 

12.31 

15.00 

13  24 

Sr 

0.19 

Me 

2.93 

5.53 

6  65 

5  00 

4  69 

Na 

10.80 

5.12 

9.84 

10  00 

12  02 

K  

2.72 

1.66 

0.34 

0  46 

0  41 

8K>j 

23  50 

6  49 

0  94 

1  42 

1  26 

RoO3 

0  51 

0  29 

0  07 

0  17 

0  53 

Salinity,  parts  per  million  

100.00 
37 

100.00 
137 

100.00 
1571 

100.00 
958 

100.00 
1011 

I.   Cache  la  Poudre  River  above  the  north  fork;   II.    Same,  water  from  faucet  in  laboratory  at  Fort  Col- 
lins;  III.   Same,  2  miles  above  Greeley;   V.   Platte  River  below  mouth  of  the  Cache  la  Poudre. 

The  first  represents  pure  mountain  water,  relatively  high  in  car- 
bonates and  rich  in  silica.  At  the  end  of  the  series  the  waters  are  rich 
in  sulphates  and  low  in  silica.  The  change  is  due  to  use  of  water  for 
irrigation  and  dissolving  of  constituents  from  an  originally  arid  soil. 

In  the  California  rivers  it  was  found  that  those  in  the  eastern  portion 
of  the  state  receive  but  little  mineral  matter  from  the  resistant  granite 
formations  of  the  Sierras,  but  that  the  coastal  rivers,  draining  areas 
underlain  chiefly  by  loose  sedimentary  deposits,  have  a  much  higher 
mineral  content. 

The  climatic  factor  is  important  as  affecting  the  mode  of  weathering. 
In  arid  and  semi-arid  regions,  disintegration  processes  predominate, 
while  in  humid  regions  decomposition  is  usually  the  dominant  process, 
so  that  the  soluble  constituents  formed  are  rapidly  removed.  However, 
in  arid  regions,  there  may  be  an  accumulation  of  soluble  matter  in  the 
soil  so  that  when  rainfall  comes,  the  streams  carry  a  high  amount  of 
dissolved  matter. 

In  the  case  of  the  California  rivers  it  was  found  that  the  average 
mineral  content  of  those  in  the  semi-arid  regions  is  roughly  four  times 
that  of  the  humid  regions.  Differences  in  percentage  composition  of 

1  Headden,  Bull.  Colo.  Agric.  Exper.  Sta.  No.  82,  1903,  p.  56,  and  Clarke,  U.  S. 
Geol.  Survey  Bull.  491,  p.  60,  1912. 


290 


ENGINEERING   GEOLOGY 


the  anhydrous  residues  show  that  the  waters  in  semi-arid  regions  con- 
tain about  two-thirds  the  proportionate  amount  of  silica,  less  calcium, 
four-fifths  as  much  carbonates,  and  twice  as  much  sulphates,  as  the 
waters  of  the  humid  regions.1 

In  arid  regions,  where  the  rainfall  is  low,  and  the  streams  are  more 
or  less  concentrated  by  evaporation,  the  water  may  contain  so  much 
dissolved  salts  as  to  be  undrinkable. 

River  waters  of  United  States.  —  The  following  table  gives  the 
composition  of  a  number  of  river  waters,  and  one  is  struck  by  their 
variation  in  dissolved  matter.  This  difference  is  not  surprising  when 
we  consider  the  source  of  the  dissolved  materials. 


ANALYSES  OF  SOLID  MATTER  IN  RIVER  WATERS 


I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

IX. 

X. 

XI. 

CO, 

41.66 
5.19 
1.51 

44.43 
11.17 
2.41 

'20'67 
6.44 

4.87 

'io'oi 

28.15 
12.78 
8.78 

'ii.'u 

4.18 
6.16 
tr 
18.14 
1.34 
3.33 

35.45 
15.84 
3.96 
0.79 
20.79 
3.76 
6.53 
1.78 
10.9 

V6!20 
108 

13.69 
44.85 
4.95 
0.70 
18.58 
3.56 
6.11 
1.08 
6.35 

"0.15 
130 

47.22 
4.43 
2.14 
1.86 
22.85 
5.86 
3.86 
1.00 
10.71 

"6!67 
140 

36.02 
8.67 
2.81 
0.37 
17.10 
3.66 
7.20 
1.34 
21.98 

"ti.85 
89 

24.93 
4.90 
6.34 
0.43 
8.50 
2.59 
10.09 
1.87 
37.47 

"2'88 
73 

32.53 
11.18 
2.79 
0.76 
16.52 
3.17 
8.78 
3.18 
20.33 

"6!76 
76 

25.29 
10.31 
5.48 
1.12 
11.43 
1.77 
11.59 
3.22 
28.99 

"o'so 

59 

51.65 
1.05 
0.48 

'22:94 
4.09 
5.14 
1.75 
9.40 
2.01 
1.49 

195 

so. 

Cl 

NO3            

Ca                          

20.08 
4.52 
3.2 
0.72 
23.12 

Mg 

Na                  

K                                        ... 

SiO2 

A12O3              

Fe2O3 

160 

148 

Salinity,  parts  per  million  

XII. 

XIII. 

XIV. 

XV. 

XVI. 

XVII. 

XVIII. 

XIX. 

XX. 

XXI. 

CO3  .. 

30.23 
20.50 
4.10 
0.81 
17.16 
5.72 
8.09\ 
1.52J 
11.44 

38.42 
16.30 
5.82 
1.67 
18.24 
7.76 

6.98 
4.65 

21.51 
19.55 
16.10 
0.82 
16.10 
3.46 
fll.04 
\  2.09 
9.09 

11.47 
42.59 
4.12 
2.32 
15.47 
2.84 
8.12\ 
1.42J 
10.82 

24.13 
32.77 
3.15 
0.53 
15.05 
4.37 

10.68 

8.98 
...1 

13.70 
16.94 
34.61 

"5.62 
1.88 
J26.07 

'6.'98' 
0.20 

2323 

37.55 
14.62 
3.77 

"26!24" 
5.13 
9.57 
0.60 
8.19 

0.33 
148 

2.65 
60.69 
4.89 

'12.78 

3.76 
14.50 
0.28 
0.451 

2134 

1.54 
43.73 
22.56 

'  13^43 
3.62 
14.02 
0.77 

0.33 
2384 

30.14 
12.21 
5.79 
0.48 
11.45 
5.59 
9.78 
1.68 
f  19.12 
3.35 
[    0.41 

118.5 

SO* 

Cl 

NO3.  . 

Ca 

Me 

Na 

K 

SiO2     

AUO, 

Fe203  

0.43 

202 

O.lfi 
267 

0.24 

87 

0.90 
81 

0.34J 
426 

Salinity  parts,  per  million  

I.  St.  Lawrence  at  Pointe  des  Cascades,  near  Vaudreuil,  above  Montreal;  II.  St.  Lawrence  opposite 
Montreal;  III.  Merrimac  River  above  Concord,  N.  H.;  IV.  Hudson  River  at  Hudson,  N.  Y.  Mean 
of  36  weekly  composites;  V.  Potomac  River  at  Cumberland,  Md.  Shows  effect  of  drainage  from 
coal  mines;  VI.  Shenandoah  River,  at  Millville,  W.  Va.  Shows  influence  of  limestone  country; 
VII.  James  River,  Richmond,  Va.;  VIII.  Neuse  River  at  Raleigh,  N.  C.;  IX.  Cahaba  River, 
near  Birmingham,  Ala.;  X.  Pearl  River,  near  Jackson,  Miss.;  XI.  Mississippi  River,  Brainerd, 
Minn.  Low  in  sulphates  and  chlorides;  XII.  Mississippi  River,  Memphis,  Tenn.  Shows  higher 
sulphates  and  chlorides.  These  come  chiefly  from  western  tributaries,  although  some  of  the  chlo- 
rides may  be  due  to  contamination;  XIII.  Illinois  River,  near  Kampsville,  111.;  XIV.  Allegheny 
River  at  Kittanning,  Pa.;  XV.  Mono ngahela  at  Elizabeth,  Pa.;  XVI.  N9rth  Platte  River  at  North 
Platte,  Neb.;  XVII.  Saline  River  above  New  Cambria,  Kaas.  Poor  in  carbonates  but  rich  in 
sodium  and  chlorine;  XVIII.  Arkansas  River,  Canon  City,  Colo.;  XIX.  Arkansas  River,  Rocky- 
ford,  Colo.  No.  XVIII  is  flowing  over  crystalline  rocks,  but  XIX  has  been  flowing  over  shales  that 
are  both  pyritic  and  gypsiferous.  Note  the  higher  salinity  also;  XX.  Pecqs  River,  New  Mexico. 
High  salinity,  predominance  of  alkaline  sulphates  and  chlorides,  and  deficiency  of  carbonates  of 
lime;  XXI.  Sacramento  River,  above  Sacramento,  Cal. 

i  Clarke,  I.e. 


SURFACE  WATERS   (RIVERS)  291 

The  preceding  table  will  serve  to  show  how  the  composition  of  natural 
waters  may  vary. 

Chlorine.  —  Chlorine,  which  is  a  constituent  of  common  salt,  is 
present  in  nearly  all  natural  waters. 

It  is  derived  originally  from  mineral-salt  deposits  and  finely-divided 
salt  spray  from  the  sea,  the  latter  being  carried  with  dust  particles  by 
the  wind  and  precipitated  with  the  rain;  or,  if  not  thus  derived,  it  may 
represent  the  leaching  by  spring  waters  of  saliferous  soils  or  rocks. 

Salt  found  in  waters  not  coming  from  these  sources  comes  from 
domestic  drainage  and  shows  that  the  water  is  either  polluted  now  or 
was  polluted  and  has  since  been  purified. 

A  comparison  therefore  of  the  salt  content  of  any  water  under  ex- 
amination with  the  normal  chlorine  content  for  that  region  will  indicate 
the  extent  of  past  or  present  pollution. 

Jackson1  states  that  the  "  amount  of  salt  in  a  water  is  a  valuable  indication  of 
pollution  because  of  the  following  facts:  (1)  The  animal  body  expels  the  same  amount 
of  salt  that  it  absorbs;  (2)  this  salt  is  unchangeable  in  the  soil  and  is  very  soluble 
in  water;  (3)  it  must  eventually  form  a  part  of  the  drainage  and  become  mixed  with 
the  general  run-off  of  the  region  in  which  it  is  expelled.  The  average  amount  of 
salt  entering  the  drainage  of  any  particular  district  is  so  constant  for  each  inhabi- 
tant that  it  has  been  claimed  that  the  number  of  people  living  on  a  drainage  area 
may  be  determined  with  a  fair  degree  of  accuracy  from  the  average  run-off  and  the 
excess  of  chlorine  over  the  normal." 

All  salt  in  natural  unpolluted  waters  farther  inland  than  Ohio  comes  from 
mineral  deposits.  The  salt  winds  from  the  sea  have  no  effect  beyond  this  state, 
but  unfortunately  west  of  this  state  a  large  proportion  of  the  natural  waters  are 
more  or  less  affected  by  the  salt  deposits.  The  underground  salt  seems  to  spread 
over  a  broad  area  and  exerts  not  only  a  wide  but  a  variable  influence  over  most  of 
the  waters.  In  these  inland  states,  while  the  '  normal  chlorine '  would  be  practi- 
cally zero  the  value  of  the  determination  of  chlorine  is  in  most  cases  vitiated  by 
the  variable  quantity  of  salt  from  mineral  sources. 

Determinations  of  chlorine  in  samples  of  water  taken  above  and  below  a  city 
which  runs  its  drainage  into  the  stream  examined  may  give  the  extent  of  pollution 
due  to  the  city  sewage,  but  the  waters  so  far  analyzed  in  the  inland  states  give  indi- 
cations that  the  question  of  normal  chlorine  does  not  to  any  great  extent  enter  into 
sanitary  problems. 

Clarke  (Ref.  3)  in  referring  to  A.  W.  Palmer's2  work  mentions  the  Chicago 
drainage  canal  as  a  good  example  of  pollution.  This  empties  into  the  Desplaines 
River  and  thence  passes  through  the  Illinois  River  into  the  Mississippi.  The  an- 
nual averages  for  1900,  representing  the  Illinois  River,  are  given  below  and  show  a 
decrease  in  chlorine  as  we  go  down-stream. 

1  W.  S.  Pap.  144,  p.  9,  1905. 

2  Chemical  Survey  of  the  Waters  of  Illinois,  1897  to  1902,  Univ.  of  111.,  1903. 


292 


ENGINEERING  GEOLOGY 


Total  dissolved 
solids, 
parts  per 
million. 

Chlorine. 

Parts  per 
million. 

Per  cent. 

Illinois  River: 
At  Morris       

235.3 
269.4 
245.4 
245.2 
236.3 
234.3 
232.6 
150.1 

23.1 
21.4 

18.7 
17.5 
14.8 
14.0 
13.1 
3.1 

9.82 
7.94 
7.62 
7.14 
6.27 
5.98 
5.63 
2.06 

At  Ottawa  

At  La  Salle  

At  Averyville 

At  Havana 

At  Kampsville 

At  Grafton   .      

Mississippi  River  at  Grafton  

Carbonates  in  water.  —  The  effect  of  carbonates  in  water  is  dis- 
cussed on  p.  338,  and  need  not  be  repeated  here;  suffice  it  to  state  that 
the  degree  of  hardness  may  vary  with  the  volume  of  discharge  of  the 


65,000 


ry    March        April         May 


July        August    September  October  November  December 


FIG.  133.  —  Chart  showing  variation  in  hardness  of  Allegheny  River  water  during  a 
year  (Pittsburgh  Flood  Commission  report). 

stream.  Thus  in  Fig.  133  it  will  be  seen  that  the  hardness  expressed  in 
parts  per  million  is  considerably  less  during  high  water  than  during  low 
water. 

Sulphuric  acid  waters.  —  Rivers  in  coal-mining  districts,  for  ex- 
ample, may  be  distinctly  acid  in  character,  due  to  sulphuric  acid  brought 
from  decomposing  pyrite,  by  drainage  from  the  mines. 


SURFACE  WATERS   (RIVERS) 


293 


Thus  the  Allegheny  and.  Monongahela  rivers  of  western  Pennsyl- 
vania receive  a  large  amount  of  coal-mine  drainage.  It  is  stated 
(Ref .  6)  that  there  are  450  mines  in  the  Allegheny  basin,  and  560  in  the 
Monongahela  basin,  150  of  which  are  in  West  Virginia. 

Although  considerable  mine  drainage  empties  into  the  Allegheny, 
especially  from  the  Kiskimetas,  the  water  of  the  main  stream  in  its 


Jan.     f.eb.  Maxell  April    May 


Oct.      MOT.      Dec. 


FIG.  134.  —  Chart  showing  variation  in  acidity  of  Monongahela  River 
water  during  different  years  (Pittsburgh  Flood  Commission  report) . 

lower  course  has  been  practically  always  alkaline.  The  Monongahela, 
on  the  other  hand,  owing  to  its  smaller  discharge  (only  about  one-third 
the  Allegheny  at  low  water),  but  with  greater  mining  developments  in 
its  basin,  is  highly  acid  (Fig.  134). 


294  ENGINEERING  GEOLOGY 

tThe  river  men  are  familiar  with  this  fact  and  bring  the  water  for 
their  boilers  on  flat  boats  from  the  Allegheny  River.  The  greatest 
contribution  to  the  acidity  of  the  Monongahela,  is  the  Youghiogheny, 
as  is  shown  by  the  following  figures. 

Monongahela  at  Clairton. 0.45  grains  SO3  per  U.  S.  gallon. 

Youghiogheny  at  Versailles 7.91  grains  SO3  per  U.  S.  gallon. 

Monongahela  at  McKeesport 2.03  grains  SO3  per  U.  S.  gallon. 

The  acidity  of  the  river  is  so  strong  as  to  exert  a  corrosive  action  on 
boilers,  and  to  shorten  the  life  of  exposed  iron  and  steel  parts  of  boats, 
and  canal  locks;  indeed  it  is  said  that  three-eighths  inch  plates  have  been 
eaten  to  a  knife  edge  in  one  year's  time. 


References  on  Rivers 

1.  Chamberlin  and  Salisbury,  Geology,  2  ed.,  Vol.  I,  p.  54,  1905. 
(Holt  and  Co.)  2.  Chittenden,  Amer.  Soc.  Civ.  Engrs.,  Trans.,  1908, 
and  Eng.  News,  Oct.,  1908.  (Relation  of  forests  to  stream  flow.) 
3.  Clarke,  U.  S.  Geol.  Survey,  Bull.  491,  pp.  56-108,  1912.  (Many 
analyses.)  4.  Glenn,  U.  S.  Geol.  Survey,  Prof.  Pap.  72,  1911.  (De- 
nudation and  erosion  in  Southern  Appalachians.)  5.  Greeley,  Jour. 
W.  Soc.  Engrs.,  XVIII,  No.  7,  p.  662,  1913.  (Rainfall  and  run-off 
with  reference  to  sewers.)  6.  Heinz  and  others,  Report  of  Flood  Com- 
mission of  Pittsburgh,  Pa.,  1912.  (Contains  excellent  bibliography.) 
7.  Jackson,  U.  S.  Geol.  Survey,  Water  Sup.  Pap.  144,  1905.  (Normal 
chlorine  in  N.  Y.  and  New  Eng.  rivers.)  8.  Leighton,  U.  S.  Geol. 
Survey,  Wat.  Sup.  Pap.  79,  1903.  (Relation  between  normal  and  pol- 
luted waters,  eastern  U.  S.)  9.  McGee,  U.  S.  Dept.  Agric.,  Bur.  Soils, 
Bull.  71,  1911.  (Soil  erosion.)  10.  Newell  and  Murphy,  Principles  of 
Irrigation  Engineering,  1913,  New  York.  (McGraw  Hill  Pub.  Co.) 
11.  Palmer,  U.  S.  Geol.  Survey,  Bull.  479,  1911.  (Geochemical  in- 
terpretation of  water  analyses.)  12.  Rafter,  U.  S.  Geol.  Survey,  Wat. 
Sup.  Pap.  80,  1903.  (Relation  of  rainfall  to  run-off.)  13.  Russell, 
Rivers  of  North  America,  1898,  New  York.  (G.  P.  Putnam's  Sons.) 
14.  Stabler,  Eng.  News,  LX,  p.  356,  1908.  (Interpretation  of  mineral 
analysis  of  water.)  15.  Thomas  and  Watt,  Improvement  of  Rivers, 
2  vols.,  2  ed.,  1913,  New  York.  (Wiley  and  Sons.) 


CHAPTER  VI 
UNDERGROUND  WATERS 

Introduction.  —  It  is  a  well-known  fact  that  the  rocks  of  the  earth's 
crust,  as  determined  by  borings  and  mining  operations,  contain  more 
or  less  water.  The  occurrence,  distribution,  and  movement  of  this 
water  are  of  interest  to  the  engineer  for  several  reasons:  (1)  Under- 
ground water  often  serves  as  a  source  of  water  supply,  and  (2)  it 
frequently  affects  engineering  operations,  such  as  tunneling,  dam 
foundations,  stability  of  embankments,  etc. 

Sources  of  underground  water.  —  The  water  found  in  the  rocks  may 
be  of  three  different  kinds,  viz.,  magmatic,  connate,  and  meteoric.  The 
first  and  third  sometimes  reach  the  surface  as  springs,  but  only  the 
third  (meteoric)  is  of  great  importance  as  a  source  of  underground 
water  supply,  and,  therefore,  the  other  two  can  be  briefly  disposed 
of  first. 

i/  Magmatic  water  is  that  which  is  given  off  by  igneous  rock  during  the 
process  of  cooling  and  consolidation  (Chapter  XVII).  It  comes  from 
unknown,  variable  depths,  and  is  important  because  it  has  played  an 
active  role  as  a  transporting  and  depositing  agent  of  ore  minerals. 
Such  water  is  occasionally  encountered  in  mine  and  tunnel  workings, 
and  may  reach  the  surface  as  hot  springs.  It  is  not  to  be  regarded 
as  a  source  of  underground-water  supply,  but  sometimes  on  account 
of  its  high  mineral  content  is  of  medicinal  value. 

Connate  water  is  water  which  is  indigenous  to  the  rocks  containing  it, 
such  as  original  sea  water  in  a  sedimentary  rock  or  magmatic  water  in 
an  igneous  rock.  It  is  occasionally  tapped  by  bored  wells. 

Meteoric  water  represents  that  part  of  the  rain  water  including 
melting  snow  which  has  soaked  into  the  rocks.  It  is  vastly  more 
important  than  the  other  two  kinds. 

Quantity  of  rainfall.  —  Few  areas  of  the  United  States  are  entirely 
free  from  rainfall,  and  in  some  regions  it  is  considerable,  averaging 
from  20  to  70  inches  in  that  portion  lying  east  of  the  Mississippi 
(Fig.  135). 

To  state  it  in  more  detail:  On  the  Mississippi  Delta  below  New 
Orleans,  and  along  the  Gulf  Coast  to  Tallahassee,  Fla.,  the  precipita- 

295 


296 


ENGINEERING  GEOLOGY 


tion  is  60  inches  or  even  more  annually,  while  a  similar  amount  falls 
on  the  coast  and  in  the  higher  mountains  of  North  Carolina,  as  well 
as  in  the  Adirondacks  and  White  Mountains. 

In  the  Gulf  and  South  Atlantic  states  it  is  between  50  and  60  inches  ; 
in  New  England,  the  Central-Atlantic  States,  and  Ohio,  between  40 


FIG.  135.  —  Map  showing  mean  annual  rainfall  of  the  United  States.    (After  Fuller, 

Domestic  Water  Supplies.) 


and  50  inches;  in  the  Upper  Mississippi  Valley  and  Great  Lakes,  30 
to  40  inches;  in  northwestern  Iowa  and  most  of  Minnesota,  20  to  30 
inches.  A  belt  extending  north  and  south  through  eastern  Kansas 
shows  20  inches  precipitation,  while  in  the  Black  Hills  and  higher 
mountains  of  the  Rockies  it  is  20  to  30  inches.  The  Great  Basin 
shows  only  2  to  3  inches,  but  the  Pacific  Coast  70  to  150  inches. 

Disposal  of  meteoric  water.  —  The  rain  water  falling  upon  the 
surface  may  be  disposed  of  by  evaporation,  run-off,  or  seepage.  The 
average  precipitation  of  the  land  is  estimated  to  be  about  40  inches 
per  year. 

The  proportion  of  rainfall  at  any  given  locality,  which  is  disposed 
of  in  the  ways  mentioned  above,  depends  on:  (1)  Topography;  (2) 
rate  of  rainfall;  (3)  porosity  of  the  soil  or  rock;  (4)  amount  of  water 
already  held  in  soil  at  time  of  precipitation  of  the  rain  or  snow;  (5) 


UNDERGROUND  WATERS  297 

the  amount  of  vegetation  on  the  surface;  and  (6)  dry  ness  of  the 
atmosphere. 

Evaporation.  —  This  is  small  while  precipitation  is  going  on,  but 
if  water  or  snow  remains  on  the  surface,  much  of  it  may  evaporate 
especially  in  clear,  dry  weather.  However,  the  proportion  of  rainfall 
that  returns  to  the  air  by  evaporation  varies  greatly  under  different 
conditions,  and  will  be  affected  by  temperature,  wind  velocity,  char- 
acter of  vegetation,  and  nature  of  soil. 

It  is  less  in  a  cool  climate  with  light  breezes  than  in  a  hot  one  with 
strong  winds.  It  goes  on  more  rapidly  in  cleared  areas  than  in  forested 
ones,  and  is  greater  from  clayey  than  from  sandy  soils. 

In  the  Virginia  Coastal  Plain,  for  example,  evaporation  amounts  to 
more  than  50  per  cent  of  the  rainfall  (see  also  Ref .  9  for  experiments) . 

Run-off.  —  Only  a  small  portion  of  the  rain  is  directly  disposed  of 
in  this  manner,  for  even  though  the  volume  of  a  stream  is  large,  much 
of  the  water  in  it  may  have  first  soaked  into  the  ground,  and  then 
rejoined  the  river  by  seepage  from  its  banks. 

Vegetation  and  temperature  seem  to  be  the  chief  factors  controlling 
run-off.  This  has  been  shown  by  Hoyt1  who  demonstrates  that 
the  winter  run-off  in  Vermont  is  92  per  cent  of  the  rainfall,  and  in  Vir- 
ginia 63  per  cent,  but  that  the  summer  run-off  is  practically  the  same 
for  the  two  states. 

A  high  run-off  is  to  be  looked  for  if  the  ground  is  saturated  with 
water  or  frozen,  or  if  the  downpour  of  rain  is  sudden. 

Absorption.  —  The  greater  part  of  the  rainfall  may  be  absorbed 
by  the  ground,  the  quantity  thus  taken  up  being  sometimes  as  much 
as  80  per  cent  in  the  East  and  90  or  95  per  cent  in  the  West  (Fuller). 

This  absorption  may  be  directly  from  the  rainfall,  or  indirectly 
from  the  rivers,  although  in  most  cases  the  water  in  the  ground  moves 
towards  the  streams. 

Movement  in  both  directions  may  take  place  at  different  points  in 
a  river's  course,  for  in  a  region  of  heavy  rainfall  the  water  will  move 
towards  the  river,  while  in  an  arid  country  in  another  part  of  a  stream's 
course,  it  is  more  likely  to  seep  from  the  river  into  the  ground.  The 
seepage  of  water  from  drainage  or  irrigation  canals  into  the  bordering 
fields  is  an  illustration  of  this.2 

The  chief  factors  which  regulate  the  absorption  of  water  by  the 
ground  are:  (1)  Surface  slope;  (2)  rate  of  precipitation;  (3)  air  tem- 
perature; and  (4)  soil  texture. 

1  Trans.  Amer.  Soc.  Civ.  Eng.,  LIX,  p.  431. 

2  See  also  Thomas  and  Watt,  Improvement  of  Rivers,  II,  p.  389,  1913. 


298  ENGINEERING  GEOLOGY 

If  the  slope  is  steep  the  water  drains  off  before  the  soil  can  absorb 
it.  Less  may  be  absorbed  during  a  heavy  shower  than  during  a  gentle 
rainfall,  because  each  type  of  soil  has  a  certain  rate  of  absorption,  and 
if  the  water  is  supplied  more  rapidly  than  it  can  be  taken  up,  the  excess 
runs  off. 

A  high  temperature  decreases  the  surface  tension  of  water,  and 
hence  it  can  be  more  rapidly  taken  into  the  soil  pores.  Sandy  soils 
soak  up  water  more  rapidly  than  clayey  ones  because  they  have 
larger  pores. 

Groundwater 

When  the  water  is  absorbed  by  the  ground  (Ref.  5)  some  of  it  is 
held  in  the  pores  of  the  soil  near  the  surface,  but  most  of  it  moves 
downward  into  the  deeper  layers  of  the  regolith1  which  it  saturates 
and  some  of  it  percolates  still  further  into  the  pores,  joints,  fissures,  or 
other  openings  of  the  bed  rock,  wherever  it  can  penetrate. 

This  body  of  water  is  known  as  the  groundwater,  and  forms  a  great 
reservoir  of  supply  for  many  lakes,  springs,  and  wells.  All  dug  wells 
and  many  shallow  driven  wells  obtain  their  supply  from  the  ground- 
water  which  saturates  all  except  the  upper  part  of  the  regolith. 

Water  table.  —  The  upper  limit  of  the  groundwater  is  known  as  the 
water  table  (Fig.  136)  and  agrees  somewhat  closely  with  the  configuration 
of  the  land  surface,  but  is  farther  from  it  under  the  hills  and  nearer  to  it 


FIG.  136.  —  Section  showing  relation  of  water  table  to  surface  irregularities.    (After 
Slichter.    From  Fuller,  Domestic  Water  Supplies.) 

under  the  valleys,  indeed  it  may  even  reach  the  surface  under  some 
depressions  giving  rise  to  springs  and  swampy  conditions  (Fig.  136). 

The  water  table  will  show  the  least  slope  in  porous  sands,  and  the 
steepest  slope  in  clays,  so  that  in  the  latter  it  may  follow  the  contour  of 
the  surface  very  closely.  Under  a  flat  expanse  on  a  high  -terrace,  for 
example,  the  water  table  may  lie  close  to  the  surface,  whereas  near  the 
scarp  or  front  of  that  terrace  it  may  be  50  feet  below  the  surface. 

1  The  term  regolith  is  applied  to  the  mantle  of  unconsolidated  material,  which 
covers  the  bed  rock  in  most  regions. 


UNDERGROUND  WATERS  299 

Its  depth  below  the  surface  is  quite  variable,  being  but  a  few  feet 
in  moist  climates,  and  often  several  hundred  feet  in  arid  regions,  but 
in  any  given  area  the  water  table  may  fluctuate  due  to  different  causes 
mentioned  later. 

In  solid  rocks  there  is  no  continuous  zone  of  groundwater  such  as 
is  found  in  the  regolith,  but  the  water  filling  joint  fissures  may  rise 
to  the  same  general  level. 

Movement  of  groundwater.  —  The  groundwater  tends  to  move  in 
the  direction  of  the  steepest  slope  of  the  water  table,  consequently 
its  flow  will  roughly  parallel  that  of  the  surface  drainage  (Fig.  137). 


FIG.  137.  —  Map  showing  position  of  water  table  by  contours  (continuous  lines), 
lines  of  motion  of  groundwater  (arrows),  and  surface  streams.  (After  Slichter, 
from  Fuller,  Domestic  Water  Supplies.) 

It  thus  flows  down  towards  the  valleys,  where  it  often  seeps  into  the 
channel  of  the  stream  occupying  the  depression,  thereby  augmenting 
its  volume. 

In  some  valleys  carved  in  bed  rock  the  surface  stream  flows  in  a 
channel  cut  in  a  filling  of  glacial  drift  or  stream  deposits,  and  then 
some  of  the  groundwater  may  form  an  underflow  in  this  porous  ma- 
terial, beneath  the  stream  channel,  but  not  always  exactly  coincident 
with  it. 

Instances  are  known  where  this  underflow  is  separated  from  the 
surface  stream  by  more  or  less  impermeable  clay  or  silt  layers  which 
prevent  the  groundwater  from  uniting  with  the  river  water.  We  thus 


300 


ENGINEERING  GEOLOGY 


have  at  times  the  case  of  a  surface  stream  of  impure  water,  and 
below  it  an  underflow  of  very  good  water.  The  latter  can  be 
drawn  upon  for  a  water  supply,  while  the  former  is  unsafe  to  use 
(Ref.  5).1 

The  magnitude  of  the  underflow  depends  on  (Ref  5):     (1)   The 

average  gradient  of  the  river 
valley;  (2)  the  depth,  width, 
and  composition  of  the  beds 
underlying  the  stream;  and 
(3)  the  fineness  of  the 
material. 

Another  type  of  underflow  is 
found  in  some  regions,  where 
the  valley  floor  along  the  foot 
of  a  mountain  range  is  under- 
lain by  open  gravel  and  sand 
deposited  by  swiftly-moving 
streams  from  the  canyons  or 
valleys  in  the  foothills  (Fig. 
138).  As  these  streams  emerge 
from  the  hills,  they  flow  over 
the  surface  for  a  short  dis- 
tance, and  then  sink  rapidly 
into  the  sand  and  gravel, 
through  which  they  travel  as 
underground  water. 

Instances   of  the  disappear- 
ance of  mountain  streams  are 
common    in    the    arid    regions 
of  the  West.     Many   other 
FIG.  138.  —  Map  showing  the  deltas  or  fans      cases    are    found    along   the 
of  disappearing  streams  as  they  leave  their      Coagt  Range  in  California,  and 

indeed  they  are  sometimes  no- 
ticed in  other  regions. 
Causes  of  fluctuation  of  water  table.  —  These  may  be  of  natural 
or  artificial   character.     Natural   causes  are  rainfall,  floods,  sympa- 
thetic tides,  thermometric  and  barometric  changes.    Artificial  causes 
are  dams  and  pumping. 

1  See  Final  Report  of  Chief  Engineer,  E.  S.  Nettleton:  Ex.  Doc.  41,  Pt.  II, 
Fifty-second  Congress,  first  session,  p.  35;  also  Trans.  Am.  Soc.  Civ.  Eng.,  Vol. 
XXX,  1893,  pp.  293-329. 


SCALE  OF  MILES 


6         12        18        24 


mountain  canyons.    (After  Slichter,  U.  S. 
Geol.  Survey,  Water  Supply  Paper,  67.) 


UNDERGROUND  WATERS  301 

Natural  causes.  —  It  is  a  well-known  fact  that  the  level  of  the 
water  table  rises  during  periods  of  rainfall  -and  sinks  during  time 
of  drought,  the  reason  for  this  being  self  evident,  but  these  changes 
are  not  sudden,  for  it  takes  the  soil  a  sensible  period  to  absorb  the 
rainfall  and  transmit  it.  Consequently  the  period  of  lowest  or 
highest  groundwater  may  lag  behind  that  'of  maximum  or  minimum 
rainfall. 

The  water  table  on  either  side  of  a  river  normally  slopes  towards 
the  stream,  but  if  the  river  rises  during  flood,  the  level  of  the  water 
table  may  be  changed. 

"  If  the  normal  level  of  a  river  is  raised  until  it  stands  only  a  short  distance  below 
the  surface  of  adjoining  fields,  it  tends  to  change  the  groundwater  level  to  an  extent 
that  may  affect  the  neighboring  fields.  Experience  has  shown  that  in  light  soils  the 
surface  of  the  pool  can  stand  about  1\  feet  and  in  heavy  soils  about  3  feet  below  the 
general  level  of  the  fields  without  causing  injury.  Small  floods,  provided  they  do 
not  overtop  the  banks,  do  not  affect  this  limit,  as  they  usually  pass  off  without 
affecting  the  groundwater  level  for  more  than  a  short  tune."1 

The  effect  of  sympathetic  tides  is  perhaps  less  easily  understood, 
although  the  action  has  frequently  been  noticed.  Thus  the  water 
level  in  some  wells  in  the  neighborhood  of  the  seashore  seems  to 
oscillate  in  harmony  with  the  tides,  rising  with  high  tide  and  falling 
with  low  tide.2 

That  this  vibration  is  in  sympathy  with  the  tides  there  can  be  no 
doubt,  because  of  the  facts  just  mentioned,  and  the  effect  has  been 
noticed  in  wells  from  200  to  300  feet  deep,  but  is  usually  more 
noticeable  close  to  the  shore  than  some  distance  from  it. 

It  is  explained  by  supposing  that  there  is  probably  a  yielding  clay 
layer,  which  acts  as  a  diaphragm,  and  responds  to  the  loading  and 
unloading  caused  by  flood  and  ebb  tides. 

Along  Chesapeake  Bay  and  its  tributaries  there  are  many  wells 
which  show  tidal  sympathy,  some  flowing  only  at  and  just  after  high 
water  (Ref.  54). 

While  a  clay  bed  often  separates  the  salt  from  the  fresh  water,  there 
are  cases  where  the  two  are  connected,  and  strong  pumping  on  a  well 
near  shore  may  draw  in  some  salt  water. 

The  changes  in  a  well  level  due  to  varying  thermometric  and 
barometric  conditions  have  been  noted  at  many  points.  Indeed, 
the  air  pressure  shows  a  strong  influence,  permitting  some  wells 

1  Thomas  and  Watt,  Improvement  of  Rivers,  II,  p.  389,  1913. 

2  Veatch,  U.  S.  Prof.  Pap.  44,  p.  72,  1906. 


302  ENGINEERING  GEOLOGY 

to  flow  during  low  barometer,  but  halting  the  current  with  high 
barometer. 

In  very  shallow  wells  changes  in  air  temperature  affect  the  surface 
tension  of  the  water.  Cold  increases  the  surface  tension;  hence  if 
some  of  the  groundwater  is  near  enough  to  the  surface  (within  a  few 
feet)  to  feel  the  change,  it  rises  into  the  partly  saturated  soil  above  the 
water  table  under  the  capillary  attraction  of  the  soil  particles,  thus 
lowering  the  level  of  the  water  in  the  wells  (Sanford). 

Artificial  causes.  —  It  has  been  previously  stated  that  the  water 
table  slopes  towards  the  valleys,  and  that  the  groundwater  flows 
towards  them,  seeping  into  the  stream  channel. 

If  now  a  dam  is  erected  across  the  stream  channel,  thus  ponding  the 
water,  the  water  table  will  not  sink  lower  than  the  surface  of  the  pond 
or  reservoir,  and  the  spring  discharge  from  the  groundwater  may  be 
lessened,  due  to  decreased  gradient  of  the  water  table  (Fig.  139). 


FIG.  139.  —  Section  illustrating  conditions  governing  movement  of  water  away  from 
streams  or  lakes.  N,  Normal  position  of  water  table;  F,  position  of  water  table 
during  floods.  (From  Fuller,  Domestic  Water  Supplies.) 

With  such  conditions  the  crest  flow  of  the  dam  may  be  less  than  the 
normal  flow  of  the  stream  before  the  dam  was  erected.  Indeed, 
the  dam  may  be  raised  to  a  sufficient  height  to  cause  a  flow  from 
the  reservoir  into  the  groundwater  zone. 

An  interesting  case  of  this  was  discovered  by  the  engineers  of  the 
Brooklyn,  N.  Y.,  waterworks  at  the  Hempstead  reservoir.  Here  it 
was  found  that  the  discharge  was  5,600,000  gallons  per  day  when  the 
water  was  maintained  at  a  depth  of  14.35  feet,  and  8,000,000  gallons 
when  it  stood  at  4  feet.1 

Strong  pumping  will  lower  the  level  of  the  water  table  in  the  ground 
surrounding  a  well  (Fig.  140),  and  if  the  latter  is  near  the  sea,  brackish 
or  salt  water  is  sometimes  drawn  in.     Pumping  water  from  mines  also 
often  affects  the  level  of  the  water  table  in  the  surrounding  ground. 
1  Veatch,  U.  S.  Geol.  Survey,  Prof.  Pap.  44,  p.  59,  1906. 


UNDERGROUND  WATERS 


303 


Digging  ditches  for  drainage  and  the  construction  of  artificial  cuts 
for  railways  and  highways  will  cause  a  local  deepening  of  the  water 
table,  if  they  are  cut  below  the  top  of  the  groundwater  zone. 


Pumping 


FIG.  140.  —  Section  showing  lowering  of  water  table  by  pumping.     (After  Veatch, 
U.  S.  Geol.  Survey,  Prof.  Pap.  44,  p.  72,  1906.) 

Perched  water  tables.  —  Above  the  main  water  table  small  bodies 
of  water  are  sometimes  found,  which  owe  their  presence  to  local  beds, 
or  basins  of  clay,  or  other  impervious  material.  These  then  hold  a 
supply  of  water,  and  their  upper  limit  is  referred  to  as  a  perched  water 
table  (Fig.  147).  They  occasionally  serve  as  sources  of  supply  for 
shallow  wells  in  a  district  where  the  main  water  table  lies  so  deep  as 
to  be  reached  only  by  driven  wells  (Ref.  54). 

Springs 

A  spring  has  been  defined  as  a  natural  outflow  of  water  from  the 
ground  at  a  single  point  or  within  a  restricted  area,  but  the  distinction 
between  springs  and  general  seepage  is  not  always  very  sharp.  The 
former  may  be  considered  to  have  a  visible  current,  while  the  latter 
does  not. 

Many  springs  emerge  in  the  beds  or  banks  of  streams  or  ponds,  but 
their  outflow  is  not  very  conspicuous.  Others  issue  from  the  bottom 
of  lakes,  at  the  base  of  bluffs,  from  the  mountain-side,  or  even  on  the 
flat  plains.  Their  volume  of  flow,  moreover,  is  as  variable  as  their 
location. 

Cold  spring  waters  are  of  meteoric  character.  They  represent  rain 
water  which  has  filtered  into  the  soil  or  rock  and,  flowing  along  below 
ground  through  pores,  fissures,  or  other  cavities  in  obedience  to  the 
laws  of  gravity,  emerge  at  some  other  points. 


304 


ENGINEERING  GEOLOGY 


Hot  springs  may  be  of  magmatic  origin,  or  they  may  represent 
surface  waters  which  have  percolated  downward  and  become  heated 
by  contact  with  uncooled  igneous  rock,  after  which  they  have  risen 
towards  the  surface  again.  The  hot  springs  of  the  Yellowstone  Park 
are  an  example  of  this. 

Springs  can  be  classified  according  to  their  mode  of  origin  as:  (1) 
Gravity  springs,  and  (2)  artesian  springs  (Ref.  1).  In  the  former  the 
water  is  not  confined  by  impervious  beds,  while  in  the  latter  it  is 
confined,  and,  therefore,  accumulates  under  some  pressure.  Grouping 
springs  according  to  the  nature  of  the  water-conducting  passages  we 
have:  (1)  Seepage,  (2)  tubular,  and  (3)  fissure  springs. 

Seepage  springs.  —  In  this  class  the  water  seeps  out  from  sand  and 
gravel,  within  a  restricted  area  (Fig.  141),  and  represents  a  common 


':Wat"erLe? 


FIG.  141.— 


spring  fed  from  unconfined  waters  in  porous  sands.     (From 
Fuller,  Water  Sup.  Pap.  255,  1910.) 


type.  The  existence  of  the  spring  is  often  indicated  by  an  abundance 
of  vegetation,  and  the  waters  which,  as  a  rule,  come  from  no  very 
distant  source  are  not  very  cold.  The  outflow  is  caused  either  by 
the  groundwater  following  the  top  of  an  impervious  layer  towards 
the  surface  (Fig.  141),  or  by  a  valley  extending  below  the  level  of  the 
water  table.  In  some  cases  a  line  of  springs  may  indicate  the  existence 
and  extent  of  a  water-tight  bed  whose  presence  might  not  otherwise 
be  expected,  such  evidence  being  at  times  of  value  in  geological 
mapping. 

Under  favorable  conditions  a  large  daily  flow  is  sometimes  obtained 
from  these  seepages,  and  some  municipalities  obtain  their  water 
supply  from  them. 

Springs,  especially  seepage  springs,  do  not  always  flow  steadily 
throughout  the  year,  but  dry  up  in  periods  of  drought  or  little  rainfall. 

Sanford  (Ref.  54)  in  referring  to  the  Virginia  Coastal  Plain  says  that  "  many  of 
the  springs  fail  in  every  dry  summer,  and  many  yield  less  water  after  several  months 


UNDERGROUND  WATERS 


305 


of  drought,  and  many  show  slight  difference  in  volume.    These  differences  represent 
differences  in  the  magnitude  of  the  fluctuations  of  the  water  table. 

"  Near  the  edges  of  high  terraces  wells  go  deep  for  water,  and  the  height  of  the 
water  in  the  wells  changes  but  little  during  the  year;  springs  flowing  from  the  scarps 
of  these  terraces  have  much  more  uniform  flow  than  those  in  hollows  on  terraces 
away  from  scarps,  where  wells  are  shallow  and  are  full  hi  the  spring  and  dry  in  the 
fall.  Still  there  are  springs  having  immediate  shallow  sources  that  flow  the  year 
through  with  little  reported  change  in  volume.  Some  such  springs  evidently  are 
supplied  by  water  that  comes  through  a  confined  channel  so  small  in  proportion  to 
its  length  that  fluctuations  of  groundwater  level  are  minimized.  Springs  flowing 
from  crevices  in  granite  in  hollows  of  high  terraces  are  of  this  class;  other  springs 
which  show  little  change  in  volume  though  having  apparently  shallow  covers  are 
fed  by  the  water  that  comes  from  under  a  terrace  above  the  one  that  seems  to  supply 
them/' 

Tubular  springs.  —  These  (Ref.  1)  are  formed  by  tubular  passages 
in  drift,  or  in  easily  soluble  rocks  (Fig.  142).  Those  formed  in  lime- 

Cesspool  "Well 


FIG.  142.  —  Diagram  showing  possibility  of  pollution  of  wells  and  springs  by  ma- 
terial conducted  from  cesspool  through  tubular  water  passages  in  till.  (After 
Fuller,  Water  Sup.  Pap.  255,  1910.) 

stones  are  the  most  important,  and  in  these  the  solution  passages 
formed  along  joint  and  bedding  planes  may  be  many  miles  long. 

In  some  cases  these  channelways  are  traversed  by  streams  of  con- 
siderable volume,  which  here  and  there  are  sometimes  dammed  due 


FIG.  143.  —  Tubular  springs  in  limestone,  the  passages  connecting  with  sink  holes. 
(After  Fuller,  Water  Sup.  Pap.  255,  1910.) 

to  the  choking  up  of  the  channelways.     Streams  of  appreciable  size 
often  issue  from  limestone  formations. 

The  waters  of  tubular  springs  though  of  variable  composition  are 
mostly  hard  (see  p.  338)  and  although  they  are  usually  clear,  they  may 


306  ENGINEERING  GEOLOGY 

be  muddy  after  storms,  because  unlike  seepage  springs,  the  water  flows 
through  open  channelways  and  is  not  filtered  by  percolation  through 
sand.  Tubular  springs  from  limestone  serve  some  municipalities,  as 
Roanoke,  Va. 

Fissure  springs.  —  This  term  is  applied  to  those  springs  found  is- 
suing along  bedding,   joints,  cleavage,  or  fault  planes.      They  usu- 


FIG.  144.  —  Section  showing  occurrence  of  a  fissure  spring.     (After  Fuller.) 

ally  emanate  from  a  deeper  source  than  the  preceding  type  and 
hence  are  colder;  moreover,  the  water  is  rarely  contaminated,  although 
it  may  be  highly  mineralized. 

A  characteristic  feature  is  the  occurrence  of  a  group  of  such  springs 
along  a  straight  line,  as  when  all  issue  along  the  same  line  of  fissuring. 

Value  of  springs  for  water  supply.  —  Springs  usually  serve  as  an 
excellent  source  of  water  supply,  but  the  volume  of  the  individual 
ones  is  not  necessarily  large. 

If  the  water  filters  from  sand  or  gravel  it  is  usually  free  from  pollu- 
tion, unless  cesspools  or  buildings  are  located  on  the  hillsides  above 
them.  The  water  is  rarely  strongly  mineralized. 

In  tubular  springs,  on  the  other  hand,  the  water  does  not  pass 
through  any  filtering  medium  and  pollution  may  be  carried  for  a  long 
distance. 

Miscellaneous  Effect  of  Underground  Waters 

Underground  water  is  thought  of  primarily  as  a  source  of  water 
supply,  and  while  this  is  not  unnatural,  still  it  often  does  other  work 
which  may  be  a  source  of  considerable  trouble  to  the  engineer. 

Among  these  we  may  mention  the  relation  of  groundwater  to  land- 
slides, tunneling  operations,  dam  foundations,  reservoir  sites,  railway 
embankments,  and  limestone  caves  and  sinks.  Some  of  these  will  be 
taken  up  and  cases  cited. 

Clay  slides.  —  Clay  shows  a  great  tendency  to  slide  when  it  be- 
comes water-soaked,  the  whole  mass  slaking  down  and  flowing  like  so 
much  tar.  Large  masses  along  river  banks  or  in  the  face  of  artificial 
cuts  are  thus  sometimes  set  loose  and  flow  down  to  a  lower  level. 


UNDERGROUND  WATERS  307 

The  trouble  is  sometimes  precipitated  in  excavations,  by  working  the 
clay  with  a  steep  face  instead  of  removing  it  in  steps. 

This  subject  is  treated  in  more  detail  in  Chapter  VII. 

Dam  and  reservoir  foundations.1  —  In  the  construction  of  dams 
for  reservoirs,  it  is  essential  that  the  foundations  shall  not  only  be 
solid  but  also  water-tight  hi  order  to  prevent  the  flow  of  water  around 
the  ends  of  the  dam  or  underneath  it. 

In  some  cases  bed  rock  lies  so  deep  that  the  dam  must  be  built 
on  unconsolidated  material  like  clay  or  sand.  If  this  is  not  water- 
tight —  and  sand  or  gravel  are  apt  to  be  permeable  —  the  water  either 
from  the  reservoir  or  ground  may  filter  through  at  the  sides  of,  or 
beneath,  the  dam,  in  gradually  increasing  quantities,  so  that  even- 
tually the  structure  is  liable  to  give  way,  if  proper  precautions  have 
not  been  taken.  Dam  failures  due  to  this  cause  are  not  so  uncommon.2 

The  breaking  of  a  dam  or  reservoir  wall  is  sometimes  caused  by 
the  giving  way  of  the  foundation  rock.  This  may  happen  if  shale  or 
clay  layers  are  present  in  the  rock  on  which  the  dam  rests. 

In  the  case  of  a  dam  in  Pennsylvania  it  is  said  that  the  rock  con- 
sisted mainly  of  sandstone  beds  from  1  to  3  feet  thick  which  dipped 
down  stream.  Between  these  were  some  shaly  layers  2  to  4  inches  thick 
into  which  the  water  percolated,  causing  them  to  soften  and  slake. 
This  permitted  some  movement  of  the  foundation  rock,  which  brought 
about  the  breaking  of  the  dam. 

Another  case  is  that  of  the  reservoir  at  Nashville,  Tenn. 

The  hill  on  which  the  reservoir  stands  is  composed  of  thinly-bedded 
and  much-jointed  limestone  between  which  are  layers  of  shale  from 
one-half  to  several  inches  in  thickness.  The  rocks  of  the  hill  dip  quite 
uniformly  3  to  4  degrees,  the  dip  being  about  north  25  degrees  west 
(Fig.  145).  At  the  point  where  the  first  break  occurred  there  is  a 
small  fold  in  the  rock,  causing  a  dip  of  8  degrees  in  the  opposite  di- 
rection. The  wall  was  built  on  this  clipping  rock.  Lying  between 
the  rock  beds  on  which  the  wall  stands  are  several  beds  of  clay,  the 
thickest  of  which  is  10  inches,  and  is  4  feet  below  the  base  of  the  wall. 
This  and  the  other  clay  layers  had  become  soft  due  to  seepage,  and 
under  the  weight  of  the  wall  and  the  pressure  of  the  water  the  rock 
beds  broke  loose  along  the  joints  of  the  limestone  and  slipped  off  over 
the  slickened  surface  of  the  clay  layers. 

Leakage  around  or  under  a  dam  might  be  due  to  several  causes, 

1  See  further  under  this  topic  in  Chapter  V. 

2  See  Eng.  Record,  Apr.  14,  1912  (Oswego,  N.  Y.);   Jan.  13,  1912  (Janesvffle, 
Wis.);  Nov.  30,  1912  (Port  Angeles,  Wash.). 


308 


ENGINEERING  GEOLOGY 


such  as  porous  beds  in  glacial  drift,  porous  rock,  solution  cavities 
and  joint  fissures. 

It  is  well  known  that  deposits  of  glacial  drift  are  rarely  homoge- 
neous, and  that  in  masses  of  comparatively  tight  till  there  may  be 


mestone  Beds, 
with  clay  Layers 
between  them 


VERTICAL  SECTION  OF  BREAK 
D  indicates  point  of  failure  and  SS  the  plane  of  slippage 


LAY  OF  LIMESTONE  STRATA 
Failure  occurred  at  C 


PLAN  SHOWING  LOCATION  OF  BREAK 

FIG.  145.  —  Plan  and  section  of  Nashville  reservoir,  showing  cause  of  break.    (After 
Purdue,  Eng.  Rec.,  LXVI,  p.  539.) 

lenses  or  beds  of  permeable  sand  or  gravel.  If  then  the  masonry  of 
a  dam  rests  in  solid  compact  till,  there  may  be  little  danger  of  leakage, 
but  if  sand  deposits  are  present  below  or  at  the  side  of  the  dam, 
seepage  of  water  is  possible.  This  fact  was  given  serious  considera- 
tion by  the  engineers  in  locating  reservoir  sites  for  the  Catskill  water- 
supply  system.1 

In  many  western  states  where  volcanic  rocks  are  abundant  the 
porosity  of  the  rocks  is  a  feature  that  has  to  be  given  serious  considera- 
tion. The  porosity  may  be  due  to  either  amygdaloidal  cavities  in 
lavas,  or  it  may  be  due  to  the  spaces  between  the  grains  and  fragments 
of  the  rock. 

A  specially  interesting  case  was  encountered  on  the  Clagamas  River 
in  Oregon  where  the  rock  was  a  very  porous  volcanic  agglomerate, 
and  it  was  found  necessary  to  close  it  up  in  some  way.  Some  idea 
of  its  porosity  may  be  gained  from  the  fact,  that  when  grout  was  forced 
down  a  50-foot  pipe  under  200  pounds  pressure,  it  flowed  across  a 
6-foot  interval  to  another  borehole,  rushed  up  this  and  spurted  30 
feet  into  the  air.2 

No  less  serious  sometimes  is  the  construction  of  a  dam  in  a  limestone 
formation,  for  in  some  of  these  the  rock  is  literally  honeycombed  by 
solution  channels  formed  by  underground  waters.3 

1  Berkey,  N.  Y.  State  Museum,  BuU.  146. 

2  See  also  case  of  Zuni  River  dam,  Eng.  News,  LXIV,  p.  203,  1910. 

3  See  case  of  Hale's  Bar  on  Tennessee  River,  Res.  of  Tenn.,  II,  No.  3,  Mar.  1912. 


UNDERGROUND  WATERS  309 

At  Johnson  City,  Tenn.,  a  reservoir  was  constructed  on  a  hill  of 
limestone,  capped  with  residual  clay.  As  usual  the  underlying  lime- 
stone surface  was  very  uneven,  and  under  one  corner  of  the  reservoir 
there  was  a  deep  cavern  in  the  bedrock  filled  with  clay.  "  As  the 
reservoir  filled  with  water,  the  clay  of  the  cavern  settled,  causing  a 
rent  in  the  floor  on  one  side  of  the  reservoir.  The  escaping  water 
did  not  flow  over  the  surface  of  the  hill  slope,  but  through  a  cavern 
and  out  into  a  railroad  cut  in  the  limestone  on  the  hillside."1 

Limestone  sink  holes  and  caverns.  —  Water  percolating  into  lime- 
stones along  joints  and  bedding  planes  often  enlarge  these  by  solution 
of  the  calcium  carbonate.  The  point  of  entrance  sometimes  becomes 
expanded  to  an  opening  of  considerable  size  (sink  hole)  into  which 
surface  drainage  and  occasionally  streams  disappear.  So  too  the 
underground  passages  become  enlarged  by  solution  so  that  the  lime- 
stone may  contain  a  network  of  tunnels  and  caverns. 

If  these  underground  channel  ways  become  obstructed  the  water 
maj'  stand  in  them,  and  is  occasionally  tapped  by  wells  (p.  319).  At 
other  times  they  serve  as  drainage  ways  for  surface  refuse  (p.  314). 
Occasionally  their  presence  may  be  little  thought  of  until  the  roof 
collapses. 

A  case  of  the  damage  caused  by  these  solution  channels  was  observed 
at  Staunton,  Va.  Here  a  steep  and  large  fissure  which  had  been  dis- 
solved in  the  limestone  extended  beneath  the  town,  the  top  of  it  being 
bridged  over  by  a  tightly-packed  mass  of  residual  clay.  The  fissure 
contained  water,  and  its  presence,  but  possibly  not  its  extent,  could 
have  been  known  from  the  fact  that  the  water  from  it  was  pumped 
up  through  a  well  and  used  for  making  ice. 

Suddenly  the  clay  bridge  caved  in  for  some  distance,  with  the  result 
that  portions  of  several  streets  and  other  objects  were  engulfed 
(Plates  XLVIII  and  XLIX). 

The  curious  but  absurd  theory  advanced  by  some  was  that  as  long 
as  the  fissure  remained  full  of  water,  the  latter  held  up  the  clay  bridge, 
but  that  the  removal  of  this  support  by  pumping  had  allowed  the  cover 
to  collapse. 

If  the  water  in  the  fissure  had  been  in  contact  with  the  clay,  it 
would  have  slaked  it  down  instead  of  supporting  it,  and  the  real  cause 
of  the  damage  was  the  breaking  of  a  sewer. 

The  water  from  the  latter  at  the  point  of  rupture  in  the  sewer 
softened  the  clay  so  that  it  no  longer  held  in  place.  The  clay  falling 
into  the  cavernous  opening  below  served  as  a  dam  to  the  subterranean 
1  Res.  of  Term.,  Ill,  No.  2,  Apr.,  1913. 


PLATE  XLVIII,  FIG.  1.  —  Street  in  Staunton,  Va.,  showing  sewer  pipe  whose 
break  started  the  caving,  and  holes  formed  in  pavement. 


FIG.  2.  —  Large  hole  formed  along  line  of  caving,  Staunton,  Va. 


(310) 


I    — 


. 


PLATE  XLIX.     View  looking  along  line  of  limestone  cavern,  Staunton,  Va.,  show- 
ing some  of  the  damage  caused  by  the  clay  roof  of  cavern  collapsing. 


(311) 


312  ENGINEERING  GEOLOGY 

stream  which  was  moreover  augmented  in  volume  by  the  water  from 
the  sewer.  This,  together  with  the  damming,  naturally  caused  the 
stream  in  the  fissure  to  rise.  -In  rising  more  soil  fell  in,  and  the  stream, 
being  more  or  less  completely  dammed,  rose  to  the  clay  cover  and 
caused  still  further  caving. 

Tunneling  operations.  —  In  the  construction  of  tunnels  strong 
flows  of  groundwater  are  sometimes  encountered.  These  may  enter 
along  joint  or  stratification  planes,  but  not  infrequently  they  follow 
fault  planes  more  or  less  directly  from  the  surface.  The  knowledge 
that  the  latter  cause  troubles  of  this  sort,  as  well  as  others  mentioned 
under  faulting,  should  be  borne  in  mind  by  engineers,  and  be  avoided 
if  possible.  (See  further  under  Faulting,  Chapter  III.) 

Railway  embankments.  —  Instability  of  bed  is  frequently  noticed 
where  the  road  is  laid  on  clay  formations,  and  is  often  caused  by  spring 
waters  which  soften  the  clay  and  cause  it  to  slide. 

Foundation  work.  —  Groundwater  is  often  encountered  in  excava- 
tions for  foundations,  especially  in  low-lying  land  where  the  water 
table  rises  close  to  the  surface.  At  other  times  subterranean  channel- 
ways  are  cut  into,  which  give  considerable  trouble,  until  confined. 
These  latter  are  not  by  any  means  to  be  looked  for  only  in  limestone 
formations. 

Drainage  by  Wells 

Types  of  drainage.  —  There  are  in  many  regions  land  areas,  some- 
times of  large  size,  which  are  so  poorly  drained,  that  they  cannot  be 
cultivated. 1 

Such  tracts  in  the  United  States,  for  example,  include  swamps  oc- 
cupying depressions  of  the  glacial  drift  in  many  of  our  northern  states; 
swampy,  flood-plain  areas  along  the  larger  rivers  and  the  Coastal 
Plain;  and  swampy  upland  areas  between  streams. 

Those  lying  close  to  sea  level  cannot  in  many  cases  be  made  self 
draining.  Others  can  be  freed  of  their  excess  of  water  by:  (1)  Ditches 
leading  into  some  stream;  (2)  tile  pipe  laid  below  the  surface;  and 
(3)  wells. 

Drainage  by  wells  consists  in  putting  down  a  hole  in  such  position, 
that  it  will  take  the  drainage  of  the  swamp  or  pond  and  conduct  it 
to  some  porous  bed  of  gravel  or  sand,  or  into  some  rock  cavern  below 
(Figs.  146,  147).  This  method  while  effective  when  applicable  cannot 
be  used  everywhere. 

Where  swampiness  or  ponds  are  due  to  the  water  table  of  the  region 
1  Fuller,  Water  Sup.  Pap.  258,  p.  6,  1911. 


UNDERGROUND  WATERS 


313 


extending  to  the  surface,  drainage  by  wells  is  hardly  feasible;  but 
where  the  conditions  are  due  to  an  impervious  layer  which  holds 
water  on  the  surface,  or  to  a  perched  water  table,  then  it  is  often  pos- 

Pond 


•  Porous  Bed 


FIG.  146.  —  Conditions  illustrating  the  drainage  of  wells  into  a  saturated  stratum  of 

lower  head.    (After  Fuller,  U.  S.  Geol.  Survey,  Water  Sup,  Pap.  258,  1911.) 
t  t 

sible  to  sink  a  well  down  to  a  porous  bed  or  to  the  main  water  table 
and  carry  off  the  surface  water. 

Of  the  unconsolidated  surface  deposits  sand  and  gravel  form  the 
most  efficient  drainage  material.     Clay  although  very  porous  is  so 


FIG.  147.  —  Conditions  encountered  by  wells  sunk  through  perched  water  tables. 
(After  Fuller,  U.  S.  Geol.  Survey,  Water  Sup.  Pap.  258,  1911.) 

fine-grained  that  the  water  passes  through  it  but  slowly.  Till,  being 
composed  of  a  heterogeneous  mixture  of  clay,  sand,  pebbles,  and  bowl- 
ders of  glacial  origin,  may  vary  in  its  porosity,  but  is  usually  fairly 
porous. 

Consolidated  rocks  vary  in  their  porosity.     Sand  and  gravel  are  said 
to  retain  porosities  of  from  10  to  15  per  cent  even  after  consolidation, 


FIG.  148.  —  Pond  held  hi  impervious  basin  above  the  water  table.     (After  Fuller, 
U.  S.  Geol.  Survey,  Water  Sup.  Pap.  258,  1911.) 

and  while  they  are  thus  capable  of  holding  considerable  water ,  it  does 
not  flow  into  them  as  readily  as  into  uncemented  materials.  Fuller 
states  that  drainage  into  sandstones  is  said  to  have  been  successful 


314  ENGINEERING  GEOLOGY 

in  Michigan,  and  that  several  wells  in  St.  Paul  and  Minneapolis  carry 
refuse  into  the  porous  St.  Peter  sandstone.  Limestones  will  take  up 
considerable  water  in  joint  and  stratification  planes,  and  if  they  con- 
tain solution  channels  their  drainage  capacity  is  still  greater. 

Application  of  drainage  by  wells.  —  In  the  drift-covered  area  of 
the  northern  states,  especially  in  Indiana,  Minnesota,  Wisconsin,  and 
Michigan,  there  are  numerous  marshes,  which  can  be  successfully 
drained  by  wells  if  situated  on  higher  ground.  In  Michigan  the  in- 
dividual tracts  thus  drained  vary  from  10  to  60  acres.  Those  undrained 
areas  lying  on  low  ground  are  not,  as  a  rule,  amenable  to  drainage  by 
wells. 

Many  of  the  ponds  in  the  states  mentioned  above  can,  however, 
be  drained  by  wells.  This  has  been  done  by  driving  the  well  either 
into  beds  of  sand  and  gravel  in  the  drift  or  into  the  St.  Peter  sand- 
stone. In  Georgia  and  Florida  the  limestone  often  serves  as  a  drain- 
age sump.  At  Quitman,  Ga.,  a  well  is  said  to  have  drained  off  1,500, 
000  gallons  from  a  pond  in  a  few  hours.  The  same  use  is  made  of 
limestone  in  parts  of  Virginia,  Kentucky,  Tennessee,  Indiana,  and 
other  states.  Cellars  have  been  drained  by  wells  at  Minneapolis,  St. 
Paul,  Hampton,  Blooming  Prairie,  Bricelyn,  Geneva,  and  other  places 
in  Minnesota.  Industrial  wastes  are  also  disposed  of  in  this  way  at 
some  localities.  In  Kentucky,  Georgia,  Florida,  and  other  states 
sewage  is  occasionally  poured  into  them,  and  Fuller  states  that  pub- 
lic or  private  sewage  wells  are  in  operation  at  Georgetown,  Ky.,  and 
at  Orlando,  Ocala,  Live  Oak,  Gainesville,  and  Lake  City,  Fla. 

Pollution  by  drainage  wells.  —  Polluted  water  flowing  into  sands 
and  gravels  will  probably  not  do  any  harm  beyond  a  few  hundred 
feet,  but  in  limestone  passages  the  contaminating  materials  may  be 
carried  a  long  distance. 

The  use  therefore  of  drainage  wells  for  carrying  off  sewage  or  in- 
dustrial wastes  is  often  exceedingly  dangerous,  and  should  in  the  opin- 
ion of  many  be  prevented  by  legislation,  especially  in  those  areas 
where  it  is  likely  to  contaminate  water  supplies. 

Artesian  Water 

Definition.  —  The  term  artesian  water  is  unfortunately  not  used 
always  to  designate  the  same  type  of  underground  water  accumula- 
tion. It  may  be  well  therefore  to  state  exactly  what  is  meant  by  the 
term. 

Fuller  has  suggested  that  the  term  artesian  be  used  to  designate 
the  hydrostatic  principle,  the  tendency  of  water  to  seek  its  own  level. 


UNDERGROUND  WATERS  315 

Hence  artesian  waters  are  those  which  rise  when  beds  or  deposits  con- 
taining them  are  tapped.  An  artesian  slope  is  a  slope  with  artesian 
water  below  it.  An  artesian  well  is  one  that  taps  artesian  water.  A 
flowing  well  is  one  in  which  the  water  rises  above  the  groundwater 
level. 

The  artesian  water  contained  in  the  rocks  may  gather  in  cavities 
of  diverse  size,  origin,  and  shape  (Ref.  2);  the  cavities  may  be  pores 
between  the  grains,  joint  cracks  (Fig.  157),  bedding  planes,  solution 
cavities  (Fig.  152),  breccia  cavities,  gas  cavities  in  lavas  (Fig.  159),  etc. 

Water  capacity  of  rocks.  —  In  view  of  the  variable  character  of  the 
water-holding  cavities  it  is  somewhat  difficult  to  estimate  accurately 
the  water  capacity  of  a  rock.  Moreover,  any  one  kind  of  rock,  such 
as  a  sandstone,  may  vary  in  its  porosity. 

The  following  figures  of  porosity  are  given  by  Fuller  (Ref.  3). 

Per  cent 

Soil  and  loam 55 

Clay 50 

Sand 30 

Chalk 50 

Sandstone 10+ 

Slate  and  shale 4 

Limestone  and  marble 4. 5± 

Granite 1 

Quartzite .5 

Now  if  the  surface  water  finds  its  way  into  the  open  spaces  of  a 
rock  and  is  held  there  by  some  confining  agent,  as  a  barrier  of  more 
or  less  impermeable  rock,  it  will  be  under  pressure,  so  that  if  some 
avenue  of  escape  is  opened  up  the  water  tends  to  rise  towards  the 
surface. 

Amount  of  groundwater.  —  The  amount  of  water  in  the  earth's  crust  is  of  great 
interest  to  those  seeking  deep  supplies,  as  well  as  to  those  interested  in  the  circula- 
tion of  underground  water  in  its  relation  to  mining.  Probably  no  other  question  is 
so  frequently  asked  in  the  field  as  that  in  regard  to  the  water  zone  which  most  people 
suppose  to  exist  somewhere  below  the  surface  and  which  they  invariably  believe 
will  always  be  found  if  a  well  only  "goes  deep  enough"  (Fuller). 

Distinction  should  be  made  between  free  water  which  occupies  fractures,  pores, 
and  other  openings  in  rocks,  and  chemically  combined  water  which  forms  a  part  of 
some  minerals.  Also  free  water  should  be  distinguished  from  available  water,  for 
certain  materials  like  clay  are  capable  of  holding  much  water  but  give  up  only  a 
small  quantity  of  it.  As  Fuller  remarks  a  rock  may  hold  35  or  40  per  cent  of  water 
and  yet  yield  almost  none  to  a  pump. 

In  estimating  the  total  amount  of  free  water  in  the  earth's  crust,  Fuller  empha- 
sizes the  most  important  factors  in  the  problem  to  be  (a)  porosity,  (6)  thickness  of 
sediments,  and  (c)  saturation. 

The  porosity  of  rocks  varies  widely  as  shown  in  the  following  table. 


316 


ENGINEERING  GEOLOGY 
POROSITY  OF  ROCKS  l 


Rock. 

Authority. 

Num- 
ber of 
tests. 

Mini- 
mum. 

Maxi- 
mum. 

Aver- 
age or 
mean. 

Remarks. 

Granite,  schist,  and  gneiss 
Granite,  schist,  and  gneiss 
Gabbro 
Diabase 
Obsidian 

Buckley 
Merrill 
Merrill 
Merrill 
Delesse 

14 
22 

1 
2 
1 

0.019 
0.37 

'6!90' 

0.56 
1.85 

"i!is 

0.16 
1.2 
0.84 
1.01 
0  52 

Wisconsin  rocks   only. 
Specific     gravity     not 

Sandstone 
Sandstone 
Quartzite 

Buckley 
Merrill 
Merrill 
Geikie 

16 

'"i" 

4.81 
3.46 

28.28 
22.8 

15.89 
10.22 
0.8 
0.21 

given. 
Mainly  brownstones. 

Specific     gravity     not 

Slate  and  shale 
Limestone,  marble,  and  dolo- 
mite. 
Chalk 

Delesse 
Buckley 

Geikie 

2 
11 

0.49 
0.53 

7.55 
13.36 

3.95 
4.85 

53 

given. 
Wisconsin  rocks  only. 
Specific     gravity     not 

Oolite 

Merrill 
Geikie 

8 

3.28 
1  32 

12.44 
3  96 

7.18 
2  64 

given. 
Indiana  stone  only. 
Specific     gravity     not 

Sand  (uniform) 

Sand  (mixture) 
Clay 
Clay 

King 

King 
King 
Geikie 

Many 

Many 
Many 

26 

35 

44 

447 

40 

47 

35 

38 
45 
53 

given. 
Theoretical    porosity; 
actual  results  similar. 

Specific     gravity     not 

Soila 

U.  S.  Dept. 
Agr. 

Many 

45 

65 

55 

given. 
Common  range. 

i  Fuller,  Water  Supply  and  Irrigation  Paper  No.  160,  U.  S.  Geol.  Survey,  1906,  p.  61. 

Summarizing  the  porosity,  Fuller  gives  the  following  values  to  the  different  kinds 
of  rock.  Sandstones,  15  per  cent;  shales,  4  per  cent;  limestones,  5  per  cent;  and 
crystalline  rocks,  0.2  per  cent. 

After  a  discussion  of  the  saturation  factors,  the  same  author  sums  up  the  results 
as  follows: 

Average  percentage  of  the  theoretical  capacity  of  stratified  rocks  actu- 
ally taken  up  by  water 37 

Average  percentage,  etc,,  of  igneous  rocks  actually  taken  up  by  water .     50 
The  average  thickness  of  the  sedimentary  rocks  is  taken  as  2600  feet,  and  that 
portion  of  the  crystalline  rocks  in  which  water  can  occur  as  15,375  feet. 

These  various  factors  affecting  the  computation  of  the  volume  of  underground 
water  are  tabulated  below: 

FACTORS  IN  COMPUTATION  OF  VOLUME  OP  UNDERGROUND  WATERS 


Rocks. 

Thickness, 
feet. 

Porosity, 
per  cent. 

Saturation 
factor, 
per  cent. 

Volume  occu- 
pied by  water, 
per  cent. 

1,040 

15  0) 

t  5.25 

Shale  

1,300 

4.  Of 

37 

I  1.48 

Limestone                ... 

260 

5.0) 

(  1.75 

Crystalline  rocks  1. 

15,375 

0  2 

50 

1 

1  Average  per  cent  of  rock  occupied  by  water,  0.52. 

On  the  basis  of  these  factors  Fuller  estimates  the  total  free  water  held  in  the 
earth's  crust  to  be  "equivalent  to  a  uniform  sheet  over  the  entire  surface  with  a 
depth  of  little  less  than  100  feet  (96  feet)."  l 

1  Fuller,  Water  Supply  and  Irrigation  Paper  No.  160,  U.  S.  Geol.  Survey,  1906. 


UNDERGROUND  WATERS  317 

Previous  estimates  of  the  amount  of  underground  water  by  others  are:  Delesse,1 
1,175,089  million  million  cubic  meters  or  1,530,000  million  million  cubic  yards, 
equivalent  to  a  sheet  of  water  over  7500  feet  thick  surrounding  the  earth.  Slichter 2 
computed  the  amount  to  be  equivalent  to  a  uniform  sheet  of  from  3000  to  3500  feet 
in  thickness.  Van  Hise3  estimates  the  amount  to  be  sufficient  to  an  amount  to 
cover  the  earth's  surface  to  a  depth  of  69  meters  or  226  feet.  Chamberlin  and 
Salisburv,4  assuming  the  average  porosity  to  be  2-|  per  cent,  estimate  the  amount  of 
underground  water  to  be  equivalent  to  a  layer  800  feet  deep  over  its  entire  surface, 
and  of  an  assumed  porosity  of  5  per  cent,  a  layer  1600  feet  deep. 

Rate  of  movement  of  underground  water.  —  The  rate  of  movement 
is  influenced  by  the  size  and  arrangement  of  the  grains  of  the  rock. 
Thus  in  the  case  of  fine  rounded  grains  the  pores  are  of  capillary  size, 
and  the  frictional  resistance  to  the  movement  of  the  water  is  great. 

In  coarse  sands,  the  spaces  being  larger,  the  water  can  move  more 
freely.  With  mixed  grains  we  have  intermediate  conditions.  A 
coarse  sand  is  said  to  transmit  water  one  hundred  tunes  more  freely 
than  a  fine  sand,  and  clay,  although  having  a  high  absorptive  capacity, 
has  a  transmission  rate  of  practically  zero. 

The  rate  of  movement  is  controlled  by  frictional  resistance  and 
difference  in  elevation  between  two  given  points  in  the  course  which 
the  water  is  following. 

On  the  south  shore  of  Long  Island  measured  velocities  in  the  Coastal 
Plain  deposits  range  from  15  inches  to  12  feet  per  day.5 

Requisite  conditions  of  artesian  flow.  —  The  requisite  conditions 
of  artesian  flow  may  be  stated  as  follows:  (1)  Adequate  source  of 
water  supply;  (2)  a  retaining  agent  offering  more  resistance  to  the 
passage  of  water  than  the  well  opening;  and  (3)  an  adequate  source 
of  pressure. 

That  portion  of  the  surface  where  the  water-bearing  bed  receives 
its  supply  is  known  as  the  collecting  area.  It  may  be  near  to,  or  far 
from,  the  well,  and  of  either  small  or  great  extent.  The  water-bearing 
rock  is  termed  the  aquifer. 

An  area  within  which  the  artesian  conditions  are  similar  is  termed 
an  artesian  province. 

The  old  idea  was  that  the  conditions  necessary  for  the  accumulation 
of  a  supply  of  artesian  water  were  those  shown  in  Fig.  149,  and  while 
it  may  gather  under  these  conditions  they  do  not  by  any  means 
represent  the  only  favorable  type  of  structure. 

1  Delesse,  Bull.  Geol.  Soc.,  France,  2d  ser.,  XIX,  1861-62. 

*  Slichter,  Water  Supply  and  Irrigation  Paper  No.  67,  U.  S.  Geol.  Survey,  1902. 

3  Van  Hise,  Mono.  47,  U.  S.  Geol.  Survey,  1904. 

4  Chamberlin  and  Salisbury,  Geology,  Vol.  I,  pp.  206-207. 

6  Slichter,  U.  S.  Geol.  Survey,  Wat.  Sup.  Pap.  140,  p.  67,  1905. 


318 


ENGINEERING  GEOLOGY 


Artesian  Water  in  Stratified  Rocks 

The  simplest  and  most  favorable  structure  for  artesian  accumulation 
is  that  which  is  sometimes  found  in  stratified  rocks.     Thus  we  may 


FIG.  149.  —  Section  of  an  artesian  basin.  A,  porous  stratum;  B,  C,  impervious 
beds  below  and  above  A,  acting  as  confining  strata;  F,  height  of  water  level 
in  porous  beds  A,  or,  in  other  words,  height  in  reservoir  or  fountain  head;  D,  E, 
flowing  wells  springing  from  the  porous  water-filled  bed  A.  (From  Fuller,  U.  S. 
Geol.  Survey,  Bull.  319,  1908.) 

have  inclined  beds  of  permeable  rock  capped  by  a  bed  of  impermeable 
or  but  slightly  permeable  character  (Fig.  149.) 

Water  seeping  into  the  outcrop 
of  the  water-bearing  layer  on  the 
surface  may  flow  down  them  either 
in  pores,  or  in  the  pores  and  joints 
together,  and  accumulate  in  this 
underground  reservoir  in  sufficient 
quantities  to  yield  an  abundant 
supply. 

Several  simple  cases  of  this  type 
of  accumulation  are  shown  in  Figs. 
149,   151,   153,  and  154.     In  all  of 
these  the  water  follows  the  water- 
bearing bed  and  accumulates  in  it  under  pressure. 

If  now  a  well  is  sunk  to  the  aquifer,  the  water  rises  in  the  tube  and 
flows  out  at  the  surface,  provided  there  is  enough  head,  the  latter  being 
governed  primarily  by  the  difference  in  elevation  between  point  of 
intake  and  mouth  of  well. 

Sands  and  sandstones.  —  These  form  our  great  source  of  artesian 
supply.  They  are  sometimes  of  considerable  thickness,  underlie  many 
hundreds  of  square  miles,  and  yield  water  under  strong  head. 

Among  the  artesian  systems  of  this  class  may  be  mentioned  the 
Atlantic  Coastal  Plain,  the  region  of  the  High  Plains  east  of  the  Rocky 
Mountains  and  the  upper  Mississippi  Valley. 


FIG.  150.  —  Section  showing  relation 
of  tide  to  level  of  water  table. 
(After  Ellis.) 


UNDERGROUND  WATERS 


319 


FIG.  151.  —  Section  in  water-bearing  gravel  with  intake  too  low  to  cause  water  to 
rise  to  surface.     (After  Ellis.) 


Limestones.  Limestones  are  not  such  important  sources  of  artesian 
water  as  sandstones,  but  may  yield  a  supply  under  two  different  sets 
of  conditions. 

1.  When  limestone  beds  are  included  between  shales  or  other  im- 
pervious rocks,  the  water  may  accumulate  in  them  along  the  joint  and 
stratification  planes.  This  type  of  occurrence  is  known,  for  example, 
in  southwestern  Ohio,  Indiana,  Iowa,  and  parts  of  Texas.  2.  A  modi- 
fication of  this  is  the  occurrence  of  jointed  limestone  under  a  capping 
of  glacial  drift,  so  that  the  water  absorbed  by  the  latter  percolates  into 
the  limestone. 

If  a  series  of  solution  channels  extend  through  a  limestone  from  a 
higher  to  a  lower  level,  the  water  will  follow  them.  If,  however,  these 


FIG.  152.  —  Section  illustrating  conditions  of  flow  from  solution  passages  in  lime- 
stone. A,  brecciated  zone  (due  to  caving  of  roof),  serving  as  confining  agent  to 
waters  reached  by  well  1 ;  B,  silt  deposit  filling  passage  and  acting  as  confining 
agent  to  waters  reached  by  well  2;  C,  surface  debris  clogging  channel  and  confining 
waters  reached  by  well  3;  D,  pinching  out  of  solution  crevice  resulting  in  con- 
finement of  waters  reached  by  well  4.  (After  Fuller,  U.  S.  Geol.  Survey,  Bull. 
319,  1908.) 

channels  become  clogged  at  some  point,  as  by  silt,  or  by  a  collapse  of 
the  roof  the  water  backs  up  behind  the  obstruction,  and  a  well  driven 
down  into  the  cavity  filled  with  water  may  yield  a  flow  (Fig.  152). 


320 


ENGINEERING  GEOLOGY 


At  Lawrenceburg,  Ky.,  a  supply  of  water  is  obtained  from  channels 
and  caverns  in  the  Lexington  limestone,  the  daily  supply  from  four 
wells  being  given  as  400,000  gallons  (Ref.  21). 

Factors  Affecting  Artesian  Water  Supplies 

Several  aquifers  in  same  section.  —  In  any  extensive  series  of 
stratified  rocks  the  same  kind  of  rock  at  different  depths  below  the 
surface  may  be  found  occurring  more  than  once,  and  so  it  happens 
that  in  an  artesian  province  we  may  find  more  than  one  water-bearing 
sandstone,  or  both  sandstones  and  limestones  may  be  found  in  the 
section,  all  of  them  yielding  water. 

It  should  be  remembered,  however,  that  the  water  from  these 
different  beds  is  by  no  means  always  of  the  same  quality.  One  may 
yield  good  water,  while  that  from  another  bed  above  or  below  may  be 
highly  mineralized  and  unfit  for  use. 

Thus  at  Cedar  Rapids  and  McGregor,  la.,  the  first  wells  drilled 
encountered  salty  and  corrosive  waters  in  the  Cambrian  sandstones, 
consequently,  wells  drilled  later  in  these  towns  were  stopped  before 
they  reached  the  horizons  at  which  the  poor  waters  were  obtained.1 

If  a  well  is  not  properly  cased,  or  the  casing  becomes  pitted  by 
corrosion,  water  from  several  different  beds  will  flow  into  the  same 


FIG.  153.  —  Section  illustrating  the  thinning  out  of  a  porous  water-bearing  bed. 
A,  inclosed  between  impervious  beds,  B  and  C,  thus  furnishing  the  necessary 
conditions  for  an  artesian  fountain  D.  (After  Chamberlin.) 


FIG.  154.  —  Section  illustrating  the  transition  of  a  porous  water-bearing  bed,  A, 
into  a  close-textured,  impervious  one.  Being  inclosed  between  the  impervious 
beds,  B  and  C,  it  furnishes  the  conditions  for  an  artesian  fountain  at  D.  (After 
Chamberlin.) 

well.     This  sometimes  accounts  for  a  good  water  turning  bad  after 
the  well  has  been  in  operation  for  a  time. 

Irregularities  of  artesian  supply.  —  The  pressure  of  a  well  will 
depend  on  the  difference  in  level  between  the  point  of  intake  and  the 
1  la.  Geol.  Survey,  XXI,  1912,  p.  150. 


UNDERGROUND  WATERS  321 

mouth  of  the  well,  the  friction  between  water  and  rock,  and  porosity. 
The  volume  of  flow  will  depend  on  pressure,  quantity  of  supply,  and 
freedom  of  movement  of  the  water  through  the  rock  pores. 

In  any  aquifer  there  may  be  dry  areas,  because  of  locally,  dense 
spots,  and  hence  a  well  drilled  to  these  will  be  a  failure.  Or,  a  porous 
sandstone  may  grade  into  an  impervious  shale,  so  that  if  two  wells  are 
sunk  to  the  same  bed,  the  one  striking  the  sandy  portion  would  yield 
a  flow,  while  that  penetrating  the  shaly  part  would  give  none. 

The  exhaustion  of  wells  may  be  caused  by:  (1)  Exhaustion  of  water 
in  reservoir,  because  it  is  drawn  up  faster  than  it  is  replenished;  (2) 
clogging  of  the  pores  of  the  rock  by  silt  or  clay;  (3)  interference  by 
neighboring  wells;  and  (4)  improper  casing,  which  either  allows  the 
well  to  cave  in  or  permits  the  water  to  leak  away  into  porous  strata 
nearer  the  surface. 

The  artesian  wells  of  Denver,  Colo.,  are  often  referred  to  as  an 
interesting  case  of  exhaustion.  A  few  years  after  the  discovery  of 
this  basin  in  1884  there  were  about  400  wells  sunk  in  an  area  about 
40  by  5  miles.  No  general  decrease  was  noted  up  to  1886,  but  between 
1888  and  1890  there  was  a  continuous  decrease  in  the  flow  of  the  city 
wells,  and  by  the  end  of  the  latter  year  many  of  them  had  to  be  pumped 
while  others  in  the  area  were  abandoned. 

The  cause  of  the  exhaustion  was  not  considered  to  be  insufficient 
rainfall,  but  rather  the  low  porosity  and  consequent  low-transmission 
power  of  the  aquifer. 

Interference.  —  It  is  sometimes  noticed  that  the  drilling  of  additional 
wells  in  a  region  affects  the  head  or  yield  of  those  already  in 
operation.  This  is  very  likely  to  happen  if  the  water-bearing  bed  is 
thin,  and  if  the  water  does  not  flow  into  the  bed  fast  enough  to  replace 
that  drawn  out.  Sanford  in  describing  the  artesian  water  supply  of 
the  Virginia  Coastal  Plain  says:  "At  Colonial  Beach  the  first  artesian 
wells  found  water  at  a  depth  of  about  200  feet  that  rose  fully  20  feet 
above  tide  level,  or  above  the  surface  at  the  highest  points  in  town. 
Possibly  200  wells  have  been  drilled  in  an  area  1J  miles  long  and  half 
a  mile  wide.  No  restrictions  have  been  put  on  flow  and  a  few  of  the 
wells  are  pumped  heavily.  As  a  result  the  head  of  the  water  in  the 
200-foot  sand  has  been  so  reduced  that  most  of  the  wells  in  the  center 
of  the  town  do  not  flow  at  the  surface,  and  many  at  lower  elevations 
flow  only  at  high  tide.  The  sinking  of  one  well  on  the  water  front  has 
stopped  the  flow  of  a  neighboring  well  on  ground  a  few  feet  higher. 
Many  of  the  wells  were  poorly  cased  and  there  is  probably  much 
leakage  under  ground.  That  this  loss  of  head  is  purely  local  is  shown 


322  ENGINEERING  GEOLOGY 

by  the  high  heads  of  wells  tapping  essentially  the  same  horizon  at 
points  a  mile  or  two  from  town." 

Yield  of  wells.  —  No  general  statement  can  be  made  regarding  the 
yield  of  wells  in  stratified  rocks,  since  it  varies  so  for  different  wells 
tapping  the  same  formation.  It  has  been  noted,  however,  that  with 
beds  of  the  same  porosity  it  varies  with  the  pressure  at  the  point  of 
discharge. 

Thus  it  has  been  noticed  in  Iowa,  that  some  of  the  deep  wells  of  the 
valley  towns  have  a  relatively  larger  yield  than  those  of  the  upland 
towns,  due  to  the  difference  in  elevation  with  relation  to  the  intake. 
An  experiment  bearing  on  this  point  is  the  case  of  a  well  at  Hitchcock, 
Texas. 1  Here  the  discharge  was  8,022  gallons  when  the  point  of  dis- 
charge was  25.35  feet  above  the  curb,  and  95,000  gallons  when  it  was 
.76  feet  above. 

Pumps  and  air  lifts  cause  a  similar  increase  in  flow. 

Source  of  water  in  aquifers.  —  Most  of  the  water  obtained  from 
artesian  wells  in  stratified  rocks  is  of  surface  origin.  In  some,  how- 
ever, there  is  found  saline  water  which  may  have  become  imprisoned 
between  the  grains  of  sediment  when  these  were  deposited  on  the  sea 
bottom  (connate  water). 

Fuller  says  : 2  "If  marine  beds  are  lifted  above  sea  level  while  still 
in  an  unconsolidated  condition,  much  of  this  water  will  drain  out, 
except  when  the  beds  are  so  warped  in  the  process  as  to  form  troughs 
or  when  drainage  is  prevented  by  the  presence  of  overlying  impervious 
beds." 

Some  wells  near  Wilmington,  N.  C.,  afford  cases  of  included  water 
in  beds  not  yet  uplifted,  for  flowing  wells  yielding  salt  water  have 
been  obtained  at  a  number  of  points.  The  pressure  here  comes  from 
meteoric  waters  which  enter  at  the  outcrop  near  the  inner  edge  of 
the  Coastal  Plain  sediments,  and  as  the  salt  water  is  pumped  out, 
fresh  water  takes  its  place. 3 

Depth  of  aquifer.  —  A  water-bearing  stratum  dips  away  from  the 
outcrop  with  a  uniform  or  varying  dip.  In  some  districts  wells  pen- 
etrate the  aquifer  at  not  more  than  100  or  200  feet  depth,  while  in 
other  districts  drillers  sometimes  go  to  a  depth  of  2000  or  3000  feet 
to  obtain  a  supply  of  water. 

Artesian  water  in  glacial  drift.  —  Glacial  deposits  consist  of  sand, 
gravel,  silt,  clay,  or  a  mixture  of  these.  The  first  two  not  only  have 

1  U.  S.  Geol.  Survey,  Water  Sup.  and  Irr.  Pap.  293,  p.  126,  1912. 

2  U.  S.  Geol.  Survey,  Bull.  319,  p.  18. 

3  Water  Sup.  and  Irr.  Pap.,  160,  p.  96. 


UNDERGROUND  WATERS 


323 


-Lake 
Michigan 


" 


""•»•••-,.  Hudson  Blwr  •»4rS$!c-«-r"r* 
Trenton 

FIG.  155.  —  Section  across  Michigan,  showing  cover  of  glacial  drift  yielding  flowing 

wells.     (After  Lane.) 

a  high  water  capacity,  but  permit  a  rather  free  percolation  of  water, 
and  under  favorable  circumstances  may  yield  flowing  wells.  Clays 
and  silts  are  less  productive. 


FIG.  156.  —  Map  of  artesian  field  of  Wapsipinicon  River,  Iowa,  and  of  buried 
channel  of  Bremer  River.     (la.  Geol.  Survey.) 

When  artesian  water  is  found  in  glacial  drift  it  is  usually  because 
pockets  of  sand  or  gravel  are  surrounded  by  less  permeable  material 
as  clay,  but  owing  to  the  changeable  character  of  the  drift  when 
traced  from  point  to  point,  it  is  rare  to  find  the  individual  water- 


324  ENGINEERING  GEOLOGY 

bearing  materials  extending  for  any  great  distance.  Many  small 
artesian  basins  are,  however,  often  thickly  scattered  over  an  area, 
and  in  Michigan,  for  example,  there  are  hundreds  of  them. 

Wells  in  glacial  drift  are  often  shallow,  usually  50  to  150  feet,  and 
the  intake  is  often  not  far  from  the  well  and  but  slightly  elevated 
above  it.  Neighboring  wells  may  interfere  to  a  marked  degree. 

Some  communities  of  moderate  size  obtain  their  water  supply 
from  a  series  of  wells  driven  in  the  glacial  drift,  and  yet  it  is  not  safe 
to  assume  that  the  volume  of  flow  will  be  the  same  in  two  drift-cov- 
ered regions  of  equal  rainfall.  This  is  because  the  structure  of  the 
drift  in  the  two  areas  may  be  totally  unlike.  In  some  drift-covered 
regions  pre-glacial  river  valleys  (Fig.  156)  are  filled  with  drift,  and 
a  variable  but  good  supply  of  water  can  usually  be  obtained  from  this 
filling. 

Artesian  Water  in  Crystalline  Rocks 

It  is  believed  by  many  that  very  little  water  is  to  be  obtained  from 
granite  and  similar  rocks,  because  they  are  very  dense  and  solid,  and 
hence  appear  to  offer  few  cavities  for  the  accumulation  of  water. 

This  assumption  is  no  doubt  due  to  the  fact  that  but  little  drilling 
for  water  in  these  rocks  was  attempted  until  recent  years,  and  hence 
the  possibilities  of  subterranean  accumulation  in  them  were  not 
understood. 

Engineers,  however,  who  have  had  occasion  to  tunnel  through 
such  materials  and  encountered  strong  flows  of  water,  have  no  doubt 
come  to  the  conclusion  that  crystalline  rocks  are  far  from  dry.  But 
one  feature  that  has  no  doubt  impressed  itself  on  those  who  have 
sought  an  artesian  supply  in  the  crystalline  rocks,  is  that  one  well 
may  be  a  success  while  a  near-by  one  is  a  complete  failure. 

The  rocks  which  are  included  under  this  type  are  the  plutonic- 
igneous  ones  such  as  granite,  diabase,  etc.,  or  metamorphic  rocks  such 
as  gneiss  and  schist.  These  vary  mineralogically  and  even  structurally, 
but  they  agree  in  the  absence  of  definite  stratification  planes,  and  in 
having  low  porosity.  And  yet,  although  their  porosity  is  often  under 
one  per  cent,  they  are  commonly  traversed  by  numerous  joints  and 
it  is  in  these  that  practically  all  the  water  accumulates  (Fig.  157). 
The  joints  may  be  horizontal,  vertical,  or  irregular;  they  are,  more- 
over, more  abundant  near  the  surface. 

The  water  filters  in  usually  along  the  steep  joints,  and  may  collect 
in  these  or  the  horizontal  ones.  Since,  however,  most  joints  are  rather 
narrow,  the  amount  of  water  likely  to  be  held  in  joint  fissures  is  very 


UNDERGROUND  WATERS 

s 


325 


FIG.  157.  —  Section  illustrating  artesian  conditions  in  jointed  crystalline  rocks  with- 
out surface  covering.  A,  C,  flowing  wells  fed  by  joints;  B,  intermediate  well 
of  greater  depth  between  A  and  C,  but  with  no  water;  D,  deep  well  not  encoun- 
tering joints;  E,  pump  well  adjacent  to  D,  obtaining  water  at  shallow  depths; 
S,  dry  hole  adjacent  to  a  spring,  showing  why  wells  near  springs  may  fail  to 
obtain  water.  (From  Fuller,  U.  S.  Geol.  Survey,  Bull.  319.) 


LEGEND 
•  Flowing  wells 
O  Wells  with  water  rising 
within  10  feet  of  surface 


FIG.  158.  —  Location  of  flowing  or  nearly  flowing  wells  of  Maine.     (After  Bayley, 
U.  S.  Geol.  Survey,  Water  Sup.  Pap.  114,  1905.) 


326 


ENGINEERING  GEOLOGY 


moderate,  and  wells  yielding  as  much  as  90  gallons  per  minute  are 
the  exception  rather  than  the  rule. 

Joints  in  crystalline  rocks  are  usually  very  irregular,  and  hence  the 
success  of  a  well  is  largely  a  matter  of  chance  (see  Fig.  157).  That  is, 
it  depends  on  whether  the  drill  hole  strikes  a  water-bearing  joint. 

Some  wells  may  strike  several  water-bearing  joints  and  thus  get 
an  increased  flow,  but  this  may  be  lost  if  the  hole  is  driven  still  deeper 
and  strikes  an  open  crack  in  which  the  water  is  lost. 

F.  G.  Clapp  l  endeavored  to  obtain  some  data  on  the  success  of 
wells  in  crystalline  rocks.  He  found,  for  example,  that  in  the  case  of 
wells  drilled  in  Maine  granites,  87  per  cent  were  successful,  but  that 
out  of  72  producing  wells,  only  3  yielded  over  50  gallons  of  water  per 
minute.  His  figures  also  show  that  by  far  the  greater  number  of  wells 
drilled  in  granite  to  a  depth  of  over  50  feet  do  not  exceed  100  feet. 

The  data  also  show  that  out  of  40  wells  drilled  to  a  depth  of  between 
50  and  100  feet,  95  per  cent  were  successful,  but  the  percentage  of 
successful  wells  decreased  with  depth. 

Ellis  has  also  tabulated  the  records  of  a  number  of  wells  drilled  in 
different  kinds  of  crystalline  rocks  in  Connecticut,  the  results  of  which 
are  given  in  the  following  table: 

YIELD  OF  WELLS  IN  VARIOUS  TYPES  OF  CRYSTALLINE  ROCKS  IN  CONNECTICUT 


Material. 

Depth  of  sur- 
face covering. 

Depth  in 
rock. 

Total  depth. 

Yield. 

No.  of 

records. 

Feet. 

No.  of 

records. 

Feet. 

No.  of 

records. 

Feet. 

No.  of 

records. 

Gal.  per 
min. 

Granite.  ...       

45 
69 
3 

23 
15 
5 

20.6 

16.3 
32.5 

13.7 
24.1 
14.4 

45 
70 
3 

23 
16 
5 

100.5 

112.6 
411.0 

96.0 
138.5 
80.2 

54 
73 
3 

23 
19 
5 

122.5 

131.4 
443.5 

109.7 
156.6 
93.8 

35 
50 
3 

16 
13 
5 

13.0 
12.3 
7.25 

13.9 
33.0 
very 
poor 

Gneiss  

Quartzite-schist  

Schist  other  than  quart- 
zite  

Granodiorite 

Phyllite  (slate)  .  . 

While  this  table  shows  that  the  granodiorite  in  Connecticut  yields 
more  water  than  granite,  gneiss,  or  common  schist,  it  cannot  be 
assumed  that  the  same  kind  of  crystalline  rocks  will  be  the  most 
important  source  of  water  in  other  regions. 

Clapp  concludes  that  contrary  to  the  popular  belief  that  the  quan- 
tity of  water  will  increase  with  depth,  experience  has  shown  that  there 

1  U.  S.  Geol.  Survey,  Water  Sup.  Pap.  223. 


UNDERGROUND  WATERS  327 

is  a  far  greater  chance  for  success  in  wells  shallower  than  100  feet, 
while  below  200  feet  the  chance  for  success  decreases  rapidly. 

Sanford  (Ref.  54)  gives  the  data  for  33  wells  in  the  Richmond,  Va., 
area  of  which  six  have  a  depth  of  250  feet  or  less,  while  the  others 
range  from  250  to  900  feet. 

He  says:  "  (1)  Of  the  deep  wells  in  crystalline  rocks  2  were  dry  or  gave  too 
little  water  to  be  of  use;  7  gave,  estimated  or  measured,  5  to  25  gallons;  16,  from 
26  to  100;  4,  from  101  to  200;  and  2,  over  200  gallons  per  minute.  Or,  5  gave  5  gallons 
or  less,  making  the  proportion  of  commercially  successful  wells  over  80  per  cent. 
(2)  Of  the  22  more  successful  wells,  15  or  nearly  70  per  cent  went  less  than  500 
feet  into  'granite'  and  1  went  less  than  200  feet.  (3)  Of  the  17  wells  yielding 
50  gallons  per  minute,  or  over,  6  were  on  high  ground,  6  on  low  ground,  and  5  on  hill- 
sides, showing  that  yields  bear  little  relation  to  the  situation  of  wells." 

Many  wells  sunk  in  crystalline  rocks  are  not  flowing  at  the  surface, 
for  the  head  is  usually  slight.  The  water,  however,  in  most  cases  is 


FIG.  159.  —  Section  illustrating  conditions  of  flow  from  vesicular  trap.    A,  vesicular 
zone  feeding  well  1.    (From  Fuller,  U.  S.  Geol.  Survey,  BulU  319,  1908.) 

of  excellent  quality,  but  those  sunk  close  to  the  seashore  may  become 
contaminated  by  an  inflow  of  salt  water. 

Irregularities  in  the  Behavior  of  Wells 

Both  dug  and  deep-drilled  wells  often  show  variations  in  head,  flow 
and  clearness,  which  puzzle  many  persons,  although  they  are  easy  of 
explanation. 

Fluctuations  of  head.  —  The  fluctuation  in  head  of  wells  may  be 
due  to  rainfall,  melting  of  snow,  freezing  and  thawing,  and  atmos- 
pheric pressure.  All  of  these  causes  affect  the  supply  of  water 
penetrating  the  soil,  and  apply  to  dug  wells.  The  atmospheric 
pressure  will  also  affect  deep  wells,  and  some  that  require  pumping 
during  fair  weather  flow  freely  during  storms. 


328 


ENGINEERING  GEOLOGY 


Roiliness  of  well  water.  —  Well  water  is  usually  clear,  but  sometimes 
becomes  milky  on  the  approach  of  a  storm,  which  is  due  to  small 
amounts  of  silt  or  clay  or  iron  oxide  if  the  color  suspended  in  the  water 
is  yellow  or  red. 

Blowing  wells.  —  This  phenomena,  which  is  noticed  in  both  drilled 
and  dug  wells,  is  due  to  a  current  of  air  which  issues  from  them.  It  is 
sometimes  strong  and  very  noticeable. 

Breathing  wells.  —  Blowing  usually  alternates  with  sucking,  and 
wells  which  show  both  expulsion  and  drawing  in  of  air  are  called 
breathing  wells,  but  the  indraft  is  often  overlooked  because  it  is  not 
as  conspicuous  as  the  outdraft.  In  moist  climates  blowing  is  com- 
monly strongest  before  storms,  and  sucking  in  clearing  weather,  and 
thus  they  show  a  relation  to  barometric  pressure. 


FIG.  160.  —  Conditions 
governing  freezing  in  a 
cased  well  with  escape 
of  air  at  bottom. 
(After  Sanford.  From 
Fuller,  Domestic 
Water  Supplies.) 


"Water  Level 

FIG.  161.  —  Conditions 
governing  freezing  in 
wells  with  leaky  cas- 
ings and  porous  walls. 
(Fuller.) 


FIG.  162.  —  Conditions 
governing  freezing 
in  limestone  wells. 
(From  Fuller,  Domes- 
tic Water  Supplies.) 


Freezing  of  wells.  —  In  the  northern  states  especially,  much  trouble 
may  be  caused  by  the  freezing  of  both  dug  and  drilled  wells,  more 
particularly  the  deeper  drilled  ones.  Indeed  some  wells  in  the  North 
are  kept  from  freezing  only  with  great  difficulty.1 


1  Fuller,  Water  Sup.  Pap.  258,  1911,  p.  23. 


UNDERGROUND  WATERS  329 

In  open  wells  cold  air  can  enter  and  freezing  may  occur,  but  in 
covered  dug  wells  there  is  usually  little  trouble  unless  the  water  level 
is  near  the  surface,  and  the  same  is  true  of  the  simpler  type  of  driven 
wells  with  single  continuous  casing  or  double  tubes,  which  are  carried 
below  the  groundwater  level.  (Fuller.) 

Most  of  the  wells  subject  to  freezing  are  the  drilled  or  double-tube 
wells,  in  which  the  inner  pump  tube  is  carried  below  the  outer  casing, 
and  stops  in  some  porous  stratum,  and  the  wells  drilled  in  limestone 
or  other  rocks  containing  open  solution  passages. 

In  Fig.  160  the  cold  air  entering  at  E  can  flow  downward  and  enter 
the  dry  sand  at  F.  If  any  water  is  left  in  the  valves  at  C  it  is  frozen, 
and,  moreover,  the  entrance  of  cold  air  may  eventually  freeze  the 
water  in  the  sand  around  G  and  D. 

A  second  case  is  shown  in  Fig.  162.  Here  the  cold  air  can  enter  not 
only  at  E  but  from  some  other  point,  and  will  follow  along  the  solu- 
tion channel  D. 

According  to  Fuller  the  wells  of  Maine,  for  example,  many  of  which 
are  in  granites,  slates,  shales,  and  other  hard  rocks  free  from  openings, 
give  no  trouble  by  freezing.  On  the  other  hand,  in  Minnesota,  North 
Dakota,  and  Nebraska  many  wells  penetrate  porous  deposits  or 
cavernous  limestones  and  freeze  every  winter.  Even  in  Pennsylvania 
freezing  sometimes  occurs  in  oil  wells  at  a  depth  of  several  thousand 
feet. 

Cause  of  preceding  phenomena.  —  It  seems  quite  evident  that 
fluctuations  of  head  and  flow,  breathing,  freezing,  etc.,  are  all  referable 
to  a  single  cause,  i.e.,  barometric  pressure. 

Thus  freezing,  indraft,  depressed  water  level,  decreased  discharge, 
and  clear  water  appear  to  accompany  a  high  barometer;  in  other  words, 
increased  atmospheric  pressure. 

Thawing,  blowing,  increased  head,  and  milkiness  all  accompany  a 
low  barometer  or  decreased  atmospheric  pressure. 

To  illustrate :  If  the  barometric  pressure  is  low,  the  water  may  flow 
from  the  well  more  rapidly,  and  the  increased  velocity  of  flow  may 
carry  clay  or  silt  out  of  the  pores  of  the  rock  causing  roiliness  of  the 
water.  During  high  barometer  in  cold  weather  the  cold  air  is  forced 
down  the  well  hole  and  produces  freezing.  The  remedy  for  this 
is  to  seal  up  the  top  of  the  well  and  prevent  the  ingress  of  air  as 
much  as  possible.  In  limestone  where  solution  channels  afford  a 
by-pass  to  the  cold  air,  the  well  may  need  packing  from  top  to 
bottom. 


330 


ENGINEERING  GEOLOGY 


Groundwater  Provinces  of  the  United  States 

Groundwater  supplies  are  found  in  many  parts  of  the  United  States, 
but  owing  to  the  diversified  character  of  the  water-bearing  materials 


FIG.  163.  —  Geologic  and  water-supply  districts  in  eastern  United  States.     (After 
Fuller,  Water  Supply  Paper,  114,  1905.) 

and  variations  in  geologic  structure,  the  manner  of  occurrence  of  water 
is  not  the  same. 

There  are,  however,  a  number  of  individual  areas,  some  of  them  of 
large  size,  throughout  each  of  which  the  groundwater  conditions  are 
somewhat  similar  and  are  known  as  groundwater  provinces. 

In  the  United  States  the  following  important  provinces  at  least 
may  be  mentioned. 


UNDERGROUND  WATERS  331 

1.  Drift  Area.  6.  Mississippi  Basin. 

2.  Weathered  Rock  Area.  7.  High  Plains. 

3.  Coastal  Plain.  8.  Rocky  Mountains. 

4.  Piedmont  Plateau.  9.  Great  Basin. 

5.  Appalachian  Mountains.  10.  Pacific  Coast  Belt. 

The  underground  water  conditions  in  these  are  briefly  as  follows: 

Glacial  drift  province.  —  This,  as  its  name  implies,  includes  that 
portion  of  the  United  States  which  is  covered  by  glacial  drift,  that 
consists  of  two  main  types,  viz.,  till  and  modified  or  stratified  drift. 

The  former  is  usually  a  heterogeneous  mass  of  clay,  sand,  and 
boulders,1  sometimes  called  hardpan. 

The  latter  consists  of  gravels,  sands,  or  other  stratified  materials, 
deposited  mainly  by  streams  flowing  from  the  ice.  While  much  of  the 
modified  drift  has  been  deposited  in  the  valleys,  some  was  also  laid 
down  in  temporary  glacial  lakes,  or  on  broad  surfaces  which  extended 
away  from  the  margin  of  the  ice  sheet. 

The  glacial  drift  usually  holds  more  water  than  the  bedrock,  and 
much  of  it  is  easily  obtained  with  shallow  wells,  especially  those  sunk 
in  the  modified  drift. 

Artesian  flows  are  also  obtainable  in  some  localities  but  since  the 
structure  of  the  drift  varies  so  from  point  to  point,  areas  of  equal 
rainfall  will  not  necessarily  show  similar  structural  conditions,  and 
hence  similar  artesian  flows. 

There  are  hundreds  of  artesian  wells  drawing  water  from  the  glacial 
drift,  but  most  of  them  are  of  only  moderate  depth  and  for  private  use. 
(See  especially  Refs.  124,  132,  and  136.) 

Occasionally  a  sufficient  supply  is  obtained  for  municipal  purposes. 

Weathered  Rock  province.  —  South  of  the  glaciated  area  the  bed- 
rock, especially  in  moist  climates,  is  often  covered  by  a  mantle  of 
decayed  rock.  The  soil  is  more  or  less  clayey,  often  red  or  yellow  in 
color,  and  contains  fragments  of  disintegrated  rock.  While  the  ma- 
terial holds  considerable  water,  it  is  of  little  value,  except  as  a  source 
of  supply  for  shallow  wells. 

Atlantic  Coastal  Plain.  —  This  strip  of  territory  (Fig.  163)  which 
extends  from  Long  Island,  N.  Y.,  to  the  Gulf  States,  has  a  width  of 
only  a  few  miles  at  the  north  end,  but  several  hundred  miles  at  the 
south.  Its  surface  is  generally  flat,  and  does  not  rise  to  more  than 
from  100  to  500  feet  above  sea  level,  but  the  major  streams  have  cut 
fairly  deep  valleys. 

1  Emphasis  might  here  be  placed  on  the  fact  that  the  term  hardpan  is  very  loosely 
used. 


332  ENGINEERING  GEOLOGY 

The  materials  underlying  the  plain  are  clays,  sands,  and  gravels, 
with  occasional  porous  sandstones  and  limestones,  the  last  two  being 
more  abundant  in  the  southern  states.  The  whole  series  of  beds  dips 
gently  seaward. 

At  the  northern  end  of  the  plain  the  waters  are  chiefly  in  sands  and 
gravels,  especially  those  near  the  base  of  the  formation,  but  farther 
south,  and  more  particularly  in  the  Gulf  states,  the  sandstones  and 
porous  limestones  also  serve  as  aquifers. 

The  water  in  the  sands  and  gravels  at  the  north  is  said  to  be  gen- 
erally soft  and  good,  but  farther  south,  where  limestone  beds  occur, 
the  water  is  often  hard  and  charged  with  sulphur  and  iron. 

There  are  several  thousand  wells  scattered  over  the  Coastal  Plain, 
but  many  of  them  are  of  large  capacity  and  flow  without  pumping. 

They  are  used  chiefly  for  domestic  and  farm  supplies  and  also  man- 
ufacturing plants  but  some  towns  utilize  them  for  municipal  purposes. 

Piedmont  Plateau  province.  —  This  province  (Fig.  163),  which 
extends  along  the  eastern  front  of  the  Appalachian  Mountains  from 
southeastern  New  York  to  Alabama,  is  composed  chiefly  of  crystal- 
line rocks  with  a  few  small  areas  of  Triassic  sediments. 

The  plateau  joins  the  Coastal  Plain  along  the  Fall  Line  on  the  east, 
and  there  has  an  elevation  of  not  more  than  a  few  hundred  feet,  but 
gradually  rising  west  toward  the  rolling  surface  attains  a  height  of 
several  thousand  feet  in  western  North  Carolina. 

With  the  exception  of  sandstones  and  shales  in  the  Triassic  basins, 
the  rocks  are  mostly  schists,  gneisses,  and  granites.  This  being  the 
case,  we  can  expect  the  waters  to  be  relatively  uncertain  in  their  oc- 
currence, since  they  accumulate  mainly  in  the  joint  planes  of  the 
rocks.  However,  the  waters  are  fairly  good,  and  at  times  are  rather 
strongly  mineralized. 

The  waters  of  the  Piedmont  Province  are  used  mainly  for  domestic 
and  farm  purposes  and  in  small  industrial  establishments,  while  the 
towns  and  cities  depend  chiefly  upon  the  streams  for  their  needs. 

Underground  water  conditions  similar  to  those  in  the  Piedmont 
Plateau  are  found  in  the  crystalline  rock  areas  of  New  York,  New 
England,  Minnesota,  and  Wisconsin. 

Appalachian  Mountain  province.  —  This  province  (Fig.  163)  ex- 
tends from  eastern  Pennsylvania  to  Alabama,  and  might  also  be  said 
to  include  the  Berkshire  Hills  of  Connecticut  and  Massachusetts, 
and  the  Green  Mountains  of  Vermont. 

The  rocks,  which  consist  of  quartzites,  sandstones,  limestones, 
and  shales,  are  strongly  folded  and  faulted.  Both  the  limestones  and 


''    ./. 


UNDERGROUND  WATERS 


333 


sandstones  may  contain  much  water,  but  it  is  rarely  used,  even  though 
the  synclines  sometimes  give  flowing  wells. 

Deep  wells  are  rare,  as  there  are  few  large  cities  in  the  belt,  and 
the  main  reliance  is  placed  on  surface  streams  and  springs. 

Mississippi-Great  Lakes  basin.  —  This  area,  whose  surface  is 
moderately  low  and  contains  few  prominent  elevations,  is  underlain 
by  a  series  of  flat  or  slightly  folded  sandstones,  shales,  and  limestones, 
ranging  from  Cambrian  to  Carboniferous  in  age. 

Of  these  formations  the  Cambrian  and  other  of  the  older  sand- 
stones are  especially  important.  Two  of  these,  the  St.  Peter  and 


FIG.  164.  —  Wisconsin  outcrop  of  Potsdam  and  St.  Peter  sandstones.  Figures  in- 
dicate height  in  feet  above  mean  sea  level.  (After  Slichter,  U.  S.  Geol.  Survey, 
Water  Supply  Bulletin,  No.  67.) 

St.  Croix,  have  a  large  collecting  area  in  Wisconsin  (Fig.  164)  and 
dipping  to  the  southward  supply  wells  in  southern  Wisconsin,  Iowa, 


334 


ENGINEERING  GEOLOGY 


and  Illinois.  The  same  sandstone  is  an  important  aquifer  in  eastern 
Minnesota. 

In  this  same  province  the  Silurian  limestones  may  carry  consid- 
erable water,  but  its  occurrence  is  erratic.  The  shales,  limestones, 
and  sandstones  of  the  Devonian  and  Carboniferous  formations  carry 
waters,  but  their  occurrence  is  uncertain  and  they  are  sometimes 
highly  mineralized. 

High  Plains  province.  —  Within  this  province  is  included  a  great 
area,  which  extends  eastward  from  the  eastern  edge  of  the  Rocky 
Mountains,  and  includes  a  large  part  of  North  and  South  Dakota, 
Nebraska,  Kansas,  Oklahoma,  and  Texas. 

It  is  underlain  by  a  thick  series  of  Cretaceous  and  Tertiary  clays, 
sandstones,  and  limestones.  In  this  series  the  Dakota  sandstone 
which  outcrops  on  the  flanks  of  the  Rocky  Mountains  and  around 


FIG.  165.  —  Barton's  map  of  catchment  area  of  the  Dakota  sandstone  and  the 
Dakota  artesian  basin.  (After  Slichter,  U.  S.  Geol.  Survey,  Water  Supply  Paper, 
67.) 

the  Black  Hills  (Fig.  165)  forms  an  important  water-bearing  forma- 
tion under  the  western  part  of  the  High  Plains,  giving  flowing  wells 
often  in  the  valleys. 

Farther  east  the  Dakota  sandstone  lies  too  deep,  and  the  formations 
higher  up  in  the  series  have  to  be  drawn  upon.    Many  of  the  under- 


UNDERGROUND  WATERS 


335 


FIG.  166.  —  Section  from  Black  Hills  to  eastern  South  Dakota,  showing  structure 
of  artesian  basin.     (After  Darton.) 

flows  in  the  gravels  are  also  tapped.  Some  of  the  limestone  beds,  es- 
pecially in  Texas,  yield  good  supplies  of  water. 

Rocky  Mountain  province.  —  In  this  province  the  rocks  of  the 
different  ranges  are  much  disturbed  by  folding  and  faulting,  and  no 
important  artesian  systems  exist.  Many  good  springs,  however, 
are  found  in  the  mountains,  and  the  valley  gravels  may  also  yield  an 
excellent  supply. 

On  the  western  edge  of  the  Rocky  Mountains,  facing  the  Great 
Basin,  the  gravels  wrashed  out  by  the  mountain  streams  often  hold 
much  water. 

Great  Basin  province.  —  In  this  region  of  desert  character,  which 
lies  between  the  Rocky  Mountains  and  the  Sierra  Nevada,  we  have 
a  number  of  ranges  and  ridges,  consisting  of  folded  and  faulted  rocks, 
with  basins  between.  These  latter  are  often  filled  to  considerable 
depths  with  sands  and  silts,  which  are  partly  stream  deposits  and 
partly  wrash  from  the  valley  slopes. 

The  rainfall  is  small,  and  much  of  the  water  courses  down  the  bare 
slopes  to  soak  into  the  valley  troughs.  While  in  many  parts  of  the 
region  the  water  level  lies  far  below  the  surface,  still  in  some  locali- 
ties a  good  supply  is  encountered,  especially  in  Utah,  Arizona,  and 
southern  California.  Some  of  the  deeper  California  waters  are  said 
to  be  under  sufficient  head  to  yield  flowing  wells,  suitable  for  mu- 
nicipal, ranch,  and  irrigation  purposes. 

The  vast  lava  beds  of  eastern  Washington  and  Oregon,  as  well  as 
of  Idaho,  form  an  important  aquifer  in  this  province. 

A  typical  case  of  an  underground  reservoir  in  a  desert  region  is  to 
be  found  in  Owens  Valley  of  east  central  California.  This  is  a  closed 
rock  basin  about  120  miles  long,  which  is  bounded  on  the  west  by 
the  Sierra  Nevada,  and  has  practically  no  subterranean  outlet. 

The  porous  valley  fill  (Fig.  167),  which  consists  of  clay,  gravel, 


336 


ENGINEERING  GEOLOGY 


sand,  and  boulders,  has  in  places  a  depth  of  as  much  as  2500  feet,  and 
forms  an  immense  underground  storage  reservoir  which  absorbs 
much  of  the  water  that  flows  down  from  the  eastern  slopes  of  the 


CRE8T   OF  SIERRA   NEVADA 


9    10   11   12   13   14   15   16   17   18   19   20   21   22 


DISTANCE  IN   MILES 


FIG.  167.  —  Sections  across  Owens  Valley,  Calif.,  showing  unconsolidated  beds  in 
which  the  groundwater  accumulates.    (After  Lee,  Water  Sup.  Pap.  259,  1912.) 

Sierras,  while  Owens  River  carries  off  the  excess  that  is  not  absorbed, 
and  delivers  it  to  Owens  Lake. 

The  city  of  Los  Angeles  will  develop  a  water  supply  from  the  sur- 
plus surface  waters  reaching  the  lower  end  of  the  valley  and  from 
the  underground  sources. l 

Pacific  provinces.  —  This  includes  several  sub-provinces,  as  the 
Sierra-Cascade,  Central  Valley,  Coast  Range,  and  Pacific  Coastal 
Plain  provinces.  The  Sierra-Cascade  and  Coast  Range  are  similar 
to  the  Rocky  Mountain  province. 

Much  moisture,  which  is  condensed  by  the  peaks  of  the  Sierra  and 
Cascade  Mountains,  flows  down  the  slopes  to  the  gravels  at  the  base, 
and  from  these  into  the  alluvial  deposits  of  the  Central  Valley.  Here 
it  forms  an  important  supply  of  underground  water. 

1  U.  S.  Geol.  Survey,  Water  Supply  Pap.  294,  1912. 


UNDERGROUND  WATERS  337 

In  the  Pacific  Coastal  Plain  there  are  deposits  of  considerable 
thickness  which  are  strong  water  bearers  in  southern  California, 
around  Puget  Sound  and  at  several  other  points. 

Composition  of  Groundwaters l 

Introduction.  —  All  groundwaters  contain  a  greater  or  less  quantity 
of  suspended  or  dissolved  matter,  derived  in  part  from  rocks  and  soil. 
The  former  may  consist  of  clay,  leaves,  or  bacteria;  the  latter  of 
mineral  substances,  obtained  in  part  from  the  rocks  or  soils  through 
which  the  water  percolates,  its  solvent  power  being  increased  by  the 
presence  of  organic  acids  derived  from  the  soils  or  other  acids  obtained 
from  the  air. 

The  water  may  thus  obtain  soda  and  potash  from  feldspars;  calcium 
and  magnesium  from  limestones,  etc.;  or  iron  oxide,  alumina,  and 
silica  from  different  minerals  of  the  soils  or  rocks. 

But  the  quantity  of  mineral  matter  which  the  groundwater  dis- 
solves will  depend  also  on  the  grain  area  exposed,  the  underground 
pressure  and  the  rate  at  which  the  water  is  moving  through  the  rocks. 

As  a  result  we  find  that  groundwaters  differ  greatly  in  the  kind 
and  amount  of  mineral  matter  which  they  carry  in  solution,  and 
upon  this  depends  the  usefulness  of  the  water  for  one  purpose  or 
another. 

It  was  formerly  customary  to  state  the  water  analyses  in  terms  of 
hypothetical  compounds  that  were  thought  to  be  present  in  solution. 
But  at  present,  in  conformity  with  the  ionic  theory,  it  is  assumed  and 
known  that  the  mineral  matter  of  dilute  solutions  exists  mainly  as  free 
radicles,  with  the  exception  of  silica. 

The  amount  of  mineral  matter  in  solution  is  usually  expressed  in 
parts  per  million.2 

Relation  of  rock  material  to  dissolved  matter.  —  Since  many  sands 
and  gravels  consist  chiefly  of  silica,  they  may  show  only  a  few  parts 
per  million  of  dissolved  mineral  matter,  although  in  desert  sands  and 
gravels  the  amount  of  alkaline  and  calcareous  material  may  be  large. 
Some  sands  and  gravels  may  contain  soluble  mineral  grains  or  other 
soluble  impurities,  which  succumb  to  the  attacks  of  the  water  filtering 
through  them. 

Fine-grained  materials,   like  clay,   expose  considerable  surface  to 

1  See  further  regarding  composition  of  water  in  Chapter  V. 

2  One  liter  of  water  weighs  1,000,000  milligrams,  and  therefore  1  milligram  or 
.001  gram  of  solids  per  liter  of  water  is  equivalent  to  one  part  per  million.     To  get 
grains  per  United  States  gallon,  from  parts  per  million,  divide  by  17.1,  or  from  grama 
per  liter,  by  0.0171. 


338  ENGINEERING  GEOLOGY 

solution,  and  the  waters  in  them  may  be  much  more  strongly  mineral- 
ized than  those  in  sand  and  gravel,  indeed,  some  are  so  alkaline  or 
calcareous  as  to  be  unfit  for  boiler  use. 

Waters  in  both  sandstones  and  slates  are  somewhat  more  min- 
eralized than  those  materials  mentioned  above,  probably  because  they 
contain  more  cementing  material  than  sands  and  clays,  but  the  crystal- 
line rocks  contain  still  less,  because  the  water  circulates  mainly  in 
joint  planes,  and  hence  has  comparatively  little  solution  surface  to 
work  on. 

Limestones  give  more  soluble  matter  than  any  of  the  other  rocks, 
as  the  carbonate  of  lime  is  rather  easily  soluble,  and  the  waters  from 
the  softer  ones  often  carry  hydrogen  sulphide. 

Effect  of  mineral  ingredients.  —  It  is  not  within  the  province  of 
this  book  to  go  into  a  detailed  discussion  of  the  chemistry  of  ground- 
waters,  but  a  few  of  the  more  important  points  may  be  briefly  touched 
upon. 

Hardness.  —  This  is  due  to  sulphates  and  bicarbonates  of  calcium 
and  magnesium.  If  the  first  type  of  compound  predominates  the 
hardness  is  permanent,  but  if  the  latter,  it  is  temporary  and  can  be 
broken  up  by  boiling  the  water.  It  is  said  that  if  a  water  has  250 
parts  per  million  of  hardness  producing  constituents,  it  is  unfit  for 
washing,  but  even  harder  water  is  still  potable. 

Boiler  scale.  —  Many  waters  which  are  satisfactory  for  drinking 
purposes  are  unfit  for  boilers,  since  the  mineral  compounds  are 
deposited  as  the  water  evaporates.  Such  deposits  are  poor  heat 
conductors,  and  if  allowed  to  collect  may  cause  an  explosion.  The 
scale  includes  suspended  silica,  iron,  and  aluminum  oxides  or  hydroxides 
and  calcium  or  magnesium  sulphates  or  carbonates. 

The  requirements  of  water  for  boiler  use  cannot  be  the  same  in  all 
regions  as  waters  vary.  The  strictest  demands  are  found  in  New 
England  where  the  railroads  require  water  containing  less  than  4 
grains  of  mineral  matter  per  gallon,  and  grade  this  as  excellent. 
From  4  to  8  grains  per  gallon  is  considered  good;  from  8  to  12  grains 
per  gallon,  fair;  and  above  12  grains  per  gallon,  unfit  for  boilers.  In 
some  regions  the  waters  are  so  mineralized  that  the  last  would  be 
considered  good  or  usable. 

Corrosion.  —  Some  waters  corrode  or  pit  boiler  iron  due  to  the  free 
acids  which  they  contain.  This  is  especially  true  of  waters  draining 
from  coal  mines,  since  the  alteration  of  pyrite  in  the  coal  yields  sul- 
phuric acid.  Hydrogen  sulphide,  dissolved  oxygen,  and  free  carbon 
dioxide  also  exert  a  corrosive  effect. 


UNDERGROUND  WATERS  339 

In  some  cases  the  acids  may  be  freed  in  the  boiler  by  decomposition 
of  salts  which  were  in  solution. 

Potable  water.  —  The  ordinary  mineral  ingredients  of  water,  such 
as  calcium,  magnesium,  silica,  iron  oxide,  etc.,  are  usually  harmless 
in  the  quantities  commonly  present,  but  any  constituent  which  is 
abundant  enough  to  taste  is  bad. 

It  is  said  that  water  containing  two  parts  per  million  of  iron  oxide 
is  distasteful  to  some,  and  may  even  stain  bowls  and  cloths. 

Exposure  to  the  air,  or  decrease  of  pressure,  causes  precipitation  of 
the  iron  and  consequent  turbidity.  Such  waters  often  favor  the 
growth  of  Crenothrix  (a  low  form  of  plant  life).  This  forms  tufts  and 
layers  in  pipes  and  well  casings,  sometimes  clogging  them.  It  is  not 
of  itself  a  cause  of  disease,  but  gives  the  water  an  unsightly  appear- 
ance, and  causes  rusty  stains. 

Four  or  five  parts  of  hydrogen  sulphide  give  an  unpleasant  taste, 
and  corrode  well  strainers  and  metal  fittings.  About  250  parts  per 
million  of  chlorides  will  give  water  a  salty  taste. 

The  presence  of  abnormal  amounts  of  chlorine  in  waters  which  have 
travelled  but  a  short  distance  from  the  surface,  or  receive  drainage 
from  cesspools  or  barns,  etc.,  is  indicative  of  pollution,  but  the  test 
is  of  less  importance  in  deep  artesian  waters,  as  the  chlorides  may  be 
soluble  ingredients  of  the  rocks  traversed. 

In  regions  where  the  chloride  content  runs  as  low  as  5  or  10  parts 
in  normal  waters,  unaffected  by  animal  pollution,  the  chlorides  are 
frequently  taken  as  a  measure  of  contamination.  In  southwestern 
Ohio,  for  example,  the  chloride  content  of  the  artesian  water  is  natur- 
ally high,  and  wells  near  together  may  differ  200  or  300  per  cent, 
owing  to  difference  in  the  composition  of  the  materials  from  which 
they  draw  their  respective  supplies. 

Nitrites  indicate  the  presence  of  decomposing  organic  matter, 
and  nitrates,  of  such  material  already  decomposed. 

Suspended  matter.  —  The  suspended  matter  found  in  surface  waters 
may  be  of  animal,  vegetable,  or  mineral  character.  That  which  is 
very  fine-grained  can  be  carried  into  the  pores  of  the  soil  and  rocks, 
but  unless  these  openings  are  fairly  large,  the  suspended  matter  even 
if  fine  is  not  likely  to  be  carried  for  a  great  distance. 

Suspended  animal  and  vegetable  matter  is  not  so  common  in  well 
waters,  but  finely-divided  sand  and  clay  is  not  rare. 

For  industrial  purposes,  where  the  water  is  used  for  washing  or  comes 
in  contact  with  food  materials,  suspended  matter  is  objectionable,  for 
it  is  likely  to  stain  or  spot  the  product.  If  the  suspended  animal  or 


340  ENGINEERING  GEOLOGY 

vegetable  matter  is  liable  to  decomposition  or  partial  solution  it  is 
even  more  objectionable,  even  in  small  amounts  (10  to  20  parts  per 
million)  than  are  equal  quantities  of  mineral  matter. 

Color.  —  The  color  of  water  is  due  mainly  to  dissolved  vegetable 
matter,  and  where  it  is  to  be  used  for  bleaching,  dying  or  paper 
making,  any  discoloration  is  undesirable.  Color  causes  serious  objec- 
tion only  when  the  vegetable  matter  in  solution  exceeds  20  or  30  parts 
per  million. 


References  on  Underground  Waters 

General. —  1.  Fuller,  W.  S.  Paper  255,  1910.  (Underground 
waters  for  farm  use.)  2.  Fuller,  U.  S.  Geol.  Surv.,  Bull.  319,  1908. 
(Factors  controlling  artesian  flows.)  3.  Fuller,  Domestic  water  sup- 
plies for  the  farm,  New  York,  1912.  (Wiley  &  Sons.)  4.  Keilhack, 
Lehrbuch  der  Grundwasser  und  Quellenkunde,  Berlin,  1912.  (Ge- 
bruder  Borntrager.)  5.  Slichter,  W.  S.  Paper  67,  1902.  (Under- 
ground waters.) 

Areal.  —  Alabama.     6.  Smith,  Ala.  Geol.  Surv.,  1907.  —  Arizona. 

7.  Lee,  W.  S.  Paper  104,  1904.     (Gila  Valley,  Ariz.)  —  California. 

8.  Hamlin,  W.  S.  Paper  112,  1905.     (Underflow  Los  Angeles  River 
Basin.)      9.  Lee,  W.  S.  Paper  181,  1906.      (Owens  Valley.)  —  Colo- 
rado.    10.  Darton,  U.  S.  Geol.  Surv.,  Prof.  Paper  52,  1906.     (Ark. 
Valley.)  —  Connecticut.     11.  Gregory  and  Ellis,  W.  S.  Paper  232,  1909. 

-  Florida.  12.  Sellards,  Fla.  Geol.  Surv.,  Bull.  1,  1908.  (cent.  Fla.); 
also  Matson  and  Sanford,  W.  S.  Paper,  319,  1913.  —  Georgia.  13. 
McCallie,  Ga.  Geol.  Surv.,  Bull.  7,  1898.  —  Indiana.  14.  Capps  and 
Dole,  W.  S.  Paper  254,  1910.  (n.  cent.  Ind.)  15.  Leverett,  W.  S. 
Paper  26,  1899.  (s.  Ind.)  —  Idaho.  16.  Russell,  U.  S.  Geol.  Surv., 
Bull.  199,  1903.  (Snake  River  Plains.)  17.  Russell,  W.  S.  Paper  78, 
1903.  (Ido.  and  s.  e.  Ore.)  —  Iowa.  18.  Norton  and  others,  la.  Geol. 
Surv.,  XXI,  1912. —.Kansas.  19.  Haworth,  W.  S.  Paper  6,  1897. 
(s.  e.  Kas.)  —  Kentucky.  20.  Glenn,  W.  S.  Paper  164,  1906.  21. 
Matson,  W.  S.  Paper  233,  1909.  (Blue  Grass  Region.)  —  Louisiana. 
22.  Harris,  W.  S.  Paper  101,  1904.  (s.  La.)  —  Maine.  23.  Clapp; 
F.  G.,  W.  S.  Paper  258,  p.  32,  1911.  —  Michigan.  24.  Lane,  W.  S. 
Paper  30,  1899.  (Mich,  lower  peninsula).  25.  Leverett,  Ibid.,  183, 
1907.  —  Mississippi.  26.  Crider  and  Johnson,  W.  S.  Paper  159,  1906. 
—  Missouri.  27.  Shepard,  W.  S.  Paper  195,  1907.  —  Montana.  28. 
Fisher,  W.  S.  Paper  221,  1909.  (Great  Falls  Region.)  —  Nebraska. 
29.  Darton,  W.  S.  Paper  12,  1898.  (s.  e.  Neb.)  30.  Darton,  U.  S. 


UNDERGROUND  WATERS  341 

Geol.  Surv.,  Prof.  Paper  17K 1903.  (Neb.)  31.  Darton,  U.  S.  Geol. 
Surv.,  Prof.  Paper  32,  1905.  (Great  Plains.)  —  New  England.  32. 
Clapp,  F.  G.,  W.  S.  Paper  258,  p.  40,  1911.  —  New  Mexico.  33. 
Fisher,  W.  S.  Paper  158,  1906.  (Roswell  artesian  area.)  34.  Lee, 
W.  S.  Paper  188,  1906.  (Rio  Grande  Valley.)  —New  York.  35. 
Veatch,  U.  S.  Geol.  Surv.,  Prof.  Paper  44,  1906.  (Long  Island.)  — 
Ohio.  36.  Fuller  and  Clapp,  W.  S.  Paper  259,  1912.  (s.  w.  Ohio.)  — 
Oregon.  37.  Russell,  U.  S.  Geol.  Surv.,  Bull.  252,  1905.  (cent.  Ore.) 

-South  Dakota.  38.  Darton,  W.  S.  Paper  227,  1909.  (S.  Dak.) 
39.  Todd,  W.  S.  Paper  34,  1900.  (s.  e.  S.  Dak.)  —  Tennessee.  40. 
Glenn,  W.  S.  Paper  164,  1906.  —  Texas.  41.  Gould,  W.  S.  Paper  191, 
1907.  (Panhandle.)  42.  Taylor,  Ibid.,  190,  1907.  (Coastal  Plain.) 

-  United  States.  43.  Darton,  W.  S.  Paper  57  and  61,  1902.  (Deep 
borings.)  44.  Fuller  and  others,  W.  S.  Paper  114,  1905.  (e.  U.  S.) 
45.  Fuller,  W.  S.  Paper  102,  1903.  (Hydrology,  e.  U.  S.)  46.  Fuller, 
W.  S.  Paper  110,  1904.  47.  Fuller  and  others,  U.  S.  Geol.  Surv.,  Bull. 
264,  1905.  (Deep  borings.)  48.  Fuller,  W.  S.  Paper  145,  1905.  (e. 
U.  S.)  49.  Fuller,  W.  S.  Paper  120,  1905.  (Bibliography.)  50. 
Johnson,  W.  S.  Paper  112,  1905.  (Laws  of.)  —  Utah.  51.  Lee, 
W.  S.  Paper  217,  1908.  (Beaver  Valley.)  52.  Richardson,  W.  S. 
Paper  199,  1907.  (Sanpete  and  Sevier  Valleys.)  53.  Richardson, 
W.  S.  Paper  157,  1906.  (Valley  of  Utah  Lake,  and  Jordan  River.)  - 
Virginia.  54.  Sanford,  S.,  Bull.  IV,  Va.  Geol.  Survey,  1913.  55. 
Watson,  Mineral  Resources  of  Virginia,  1907.  —  Washington.  56. 
Calkins,  W.  S.  Paper  118,  1905.  (e.  cent.  Wash.)  57.  Landes, 
W.  S.  Paper  111,  1905.  58.  Smith,  G.  O.,  W.  S.  Paper  55,  1901. 
(Yakima  Co.)  —  Wisconsin.  59.  Kirchoffer,  Bull.  Univ.  Wis.,  106, 

1905.  —  Wyoming.     60.  Fisher,  U.  S.  Geol.  Surv.,  Prof.  Paper  53, 

1906.  (Bighorn  Basin.) 


CHAPTER  VII 
LANDSLIDES  AND  THEIR  EFFECTS 

Definition.  —  Under  this  term  are  included  all  downward  and  often 
sudden  movements  of  surface  clay,  sand,  gravel,  and  even  solid  rock. 

The  movement  is  in  response  to  gravity,  and  is  often  aided  by  the 
fact  that  the  material  has  become  water-soaked  and  is  very  mobile. 

Landslides  are  frequently  referred  to  in  a  casual  manner,  in  geological 
text  books,  and  their  destructive  effects  are  sometimes  commented  on, 
but  it  is  doubtful  if  their  full  importance  as  a  factor  in  applied  geology 
is  always  realized;  moreover,  in  the  minds  of  many  their  occurrence 
is  commonly  associated  with  mountain  districts. 

The  slow  creep  of  soil  down  the  hillside,  the  sudden  rush  of  rock  or 
unconsolidated  material  down  the  mountain  slope,  or  the  slide  of  soft 
mud  below  the  water  surface,  all  interfere  from  time  to  time  more  or 
less  seriously  with  engineering  operations,  and  consequently  it  is  of 
importance  for  the  engineer  to  know  something  about  them. 

Although  the  presence  of  water  in  the  rocks  and  soils  is  often  a 
powerful  factor  in  initiating  a  landslide,  still  in  some  cases  earthquake 
shocks  have  played  an  important  role  in  dislodging  the  masses  of 
moving  material. 


Classification  of  Landslides 

Professor  Heim  of  Switzerland  who  has  given  the  subject  of  land- 
slides most  careful  study  has  suggested  the  following  classification 
which  does  not  translate  very  satisfactorily  into  English. 


Landslides 
(Bergstiirze). 


Movements     involving 
detritus  (Schuttbewe- 
gungen). 


I.   Soil      slips      (Schuttrut- 

schungen). 

II.   Earth     slides    of    greater 
magnitude  than  I  (Schutt- 

stiirze). 
Rock  slips  (Felsschlipfe). 

Rock  falls  (Felsstiirze). 


III. 
IV. 


Movements  involving 
solid  rock  (Felsbewe- 
gungen). 

V.  Compound  slides,  with  respect  to  character  and  move- 
ments of  materials.  (Gemischte  und  zusammen- 
gesetzte.) 

VI.   Unclassified  and  special  cases. 
342 


LANDSLIDES  AND  THEIR  EFFECTS  343 

The  several  types  will  be  taken  up,  and  examples  of  each  given  as 
far  as  possible,  together  ^with  a  statement  of  the  trouble  they  have 
caused. 

Type  I.  —  This  type  includes  those  slow,  downward  movements  of 
soil  or  other  unconsolidated  material,  and  is  commonly  referred  to  as 
creep.  It  may  originate  on  any  slope  except  one  of  very  low  angle, 
and  involves  not  only  soft  clay  and  sand,  but  also  the  angular  rock 
fragments  of  talus  slopes. 

Where  steeply-dipping  rocks  crop  out  on  a  hillside,  the  upper 
portions  of  the  layers  are  sometimes  bent  over  by  the  general  down- 
slope  movement  of  surface  material,  so  as  to  give  the  impression  that 
the  dip  is  in  the  opposite  direction  from  what  it  really  is.1 

These  slow,  creeping  slides,  while  not  as  disastrous  in  causing  loss 
of  life,  as  rapid  ones,  nevertheless  often  give  much  trouble. 

Thus  if  a  railway  track  is  laid  across  them  it  has  to  be  re-aligned 
from  time  to  time  because  the  slow  movement  of  the  soil  or  talus 
material  displaces  it.  For  example,  near  Field,  B.  C.,  on  the  main 
line  of  the  Canadian  Pacific  Railway,  the  track  is  laid  across  the  lower 
edge  of  a  large  talus  heap  on  the  eastern  side  of  Mount  Stephen.  This 
slide  is  slowly  creeping  down  necessitating  more  or  less  frequent 
straightening  of  the  track.  The  same  thing  often  happens  where 
railroads  cross  clay  slopes,  but  here  the  case  is  sometimes  aggravated 
by  the  clay  swelling  when  it  absorbs  water. 

Tunnels  or  mine  shafts  penetrating  material  of  this  sort,  are  also 
likely  to  be  thrown  out  of  line,  or  even  squeezed  together. 

Drinker  (Ref .  3)  in  his  classic  work  on  Tunneling  states  that  there 
have  been  many  cases  of  landslides  by  which  parts  of  railroads  located 
along  mountain-slopes  have  been  displaced,  and  that  sometimes 
tunnels  have  been  affected,  one  of  the  most  noted  examples  being  that 
of  the  Miihlthal  tunnel  on  the  Brenner  Railroad  in  Europe  (Fig.  168). 
The  rock  was  an  argillaceous  schist,  requiring  blasting,  and  where  the 
slide  occurred  the  tunnel  was  very  near  the  surface.  "  During  the 
building  it  was  observed  that  the  hillside  had  been  shaken,  and  finally 
it  became  necessary  to  break  through  the  side  walls,  and  sink  shafts 
down  some  20  feet  to  solid  rock  all  along  the  damaged  section,  and  a 
heavy  retaining  wall  was  then  built  up." 

The  foundations  of  buildings  built  on  a  creeping  surface  may  be 
similarly  affected. 

Type  II.  —  The  slides  of  this  type  differ  from  Type  I  in  being  of 
greater  magnitude,  but  mainly  in  the  more  sudden  and  violent  char- 

1  See  U.  S.  Geol.  Survey,  Prof.  Paper  56,  Plate  VII,  p.  60,  1907,  for  a  good  case. 


344 


ENGINEERING  GEOLOGY 


acter  of  the  slide  which  may  be  either  rock  or  soil.  The  angle  of 
slope  is  not  necessarily  steep,  or  the  point  of  starting  necessarily  high 
above  the  surrounding  country. 

Common  examples  of  this  type  are  the  frequent  dirt  and  rock  slides 
that  move  down  the  slopes  in  some  mountain  regions,  cleaning  out  all 


FIG.   168.  —  Section  showing  position  of  Miihlthal  tunnel  and  creep  material  on 
Brenner  Railroad.     (After  Drinker,  Tunneling). 

the  vegetation  in  their  path  and  leaving  a  bare  scar  on  the  mountain 
side. 

Such  a  mass  may  cling  to  the  mountain  slope  for  a  long  time  until 
loosened  by  frost,  or  softened  and  soaked  with  rain  water,  when  it 
comes  down  suddenly  and  without  warning. 

Clay  slides  of  considerable  magnitude  are  not  uncommon.  The 
movement  here  is  due  to  certain  layers  in  a  clay  bank  becoming  wet 
and  slippery,  or  to  water  seeping  along  over  a  smooth  rock  surface,  on 
which  a  clay  formation  rests.  In  either  case  the  mass  lying  on  the 
lubricated  surface  yields  to  the  pull  of  gravity,  and  slides  to  a  lower 
level. 


LANDSLIDES  AND   THEIR  EFFECTS 


345 


It  should  be  remembered,  however,  that  it  is  not  necessary  for  the 
moving  material  to  rest  on  a  steeply-sloping  surface,  in  order  to  slip. 
On  the  contrary  large  areas  of  clay  land  have  sometimes  moved  down- 
ward with  almost  irresistable  force  over  comparatively  gentle  slopes. 

A  good  case,  is  that  of  a  slide  which  occurred  on  the  Lievre  River, 
north  of  Buckingham,  Quebec  (Fig.  169.)  Here  there  was  a  clay  ter- 


40  Chains 


FIG.  169.  —  Map  of  slide  on  Lievre  River,  Que.    (After  Ells,  Can.  Geol.  Survey,  XV, 

Pt.  AA,  1904.) 


race  resting  on  gneiss  and  granite.  The  clay  had  become  so  thoroughly 
water-soaked  after  several  days  rain,  that  an  area  of  about  100  acres 
slid  into  the  river.  But  so  great  was  the  pressure,  that  the  clay  was 
pushed  entirely  across  the  stream,  which  had  a  width  at  this  point 
of  six  chains,  and  masses  of  it  were  deposited  on  the  east  bank  to  a 
height  of  from  20  to  30  feet.1  In  addition  a  tongue  of  the  clay  moved 
up  stream  and  displaced  a  crib-work  dam,  pushing  it  at  least  100  feet. 
This  is  not  an  uncommon  phenomenon  in  valleys  where  clay  terraces 


1  Can.  Geol.  Survey,  Ann.  Rept.,  Vol.  XV,  Part  AA,  p.  136,  1904. 


PLATE  L,  FIG.  1.  —  Slide  of  clay  caused  partly  by  undermining  action  of  stream, 
and  partly  by  clay  becoming  water-soaked.    (H.  Ries,  photo.) 


FIG.  2.  —  View  of  Turtle  Mountain,  Frank,  Alberta,  showi  ig  place  from  which  rock 

fell,  and  a  portion  of  slide  in  foreground.     (H.  Ries,  photo.) 
(346) 


348  ENGINEERING  GEOLOGY 

rise  above  the  river  level,  and  many  of  them  have  occurred,  for  example, 
in  the  Hudson  River  Valley  of  New  York  State. 

While  slides  of  this  sort  are  likely  to  occur  when  the  clay  becomes 
water-soaked,  still  their  descent  is  sometimes  hastened  by  any  cause 
which  steepens  the  face  of  the  bank.  Thus  the  undercutting  of  a  clay 
deposit  by  a  stream,  or  any  artificial  excavation  which  gives  a  steep 
face,  leaves  the  bank  without  proper  support  and  invites  a  slide. 

Some  years  ago,  the  brick  pits  at  Haverstraw,  N.  Y.,  were  worked 
towards  the  city,  leaving  a  steep  and  high  face,  which  resulted  in  a 
portion  of  one  of  the  streets  and  a  number  of  houses  sliding  into  the 
excavation. 

Engineers  in  making  railway  cuts  through  clayey  material  some- 
times overlook  the  tendency  of  the  clay  to  slide,  which  is  sure  to  occur 
if  the  angle  of  the  embankment  is  too  steep. 

Shales  which  slake  down  easily  are  apt  to  slide  almost  as  readily 
as  clay,  and  where  towns  are  located  on  terraces  underlain  by  such 
materials,  some  means  should  be  taken  to  retard  the  slipping  of  the 
banks,  for  if  it  goes  on  unrestrictedly,  the  face  of  the  cliff  often  slowly 
but  surely  recedes. 

The  Panama  canal  has  furnished  fine  examples  of  clay  slides  (Ref. 
9)  some  of  which  are  influenced  by  the  rock  structure. 

Another  good  illustration  of  an  extensive  clay  slide  is  that  of  the 
Slumgullion  mud  flow  (Ref.  6)  which  dammed  Lake  Fork  of  the  Gun- 
nison  River  near  Lake  City,  Colo.,  and  formed  Lake  San  Cristobal. 
This  flow  started  at  an  elevation  of  11,500  feet,  and  the  mass  of  mud 
derived  from  cliffs  of  decomposed  volcanic  rock,  "  rushed  as  a  flow 
southwesterly  down  the  lateral  gulch  to  the  main  Slumgullion  and 
due  west  down  that  to  the  Lake  Fork  6  miles  from  the  place  of  start- 
ing. On  reaching  the  Lake  Fork,  whose  course  is  here  at  right  angles 
to  Slumgullion,  it  turned  north  and  ended  about  three-fourths  mile 
below  the  mouth  of  Slumgullion.  The  volume  was  sufficient  to  dam 
the  main  stream  and  to  cause  the  formation  of  Lake  San  Cristobal, 
which  now  extends  for  two  miles  up  the  Lake  Fork  valley.  The  end 
of  the  flow  is  at  about  8900  feet." 

The  landslide  barriers  forming  lakes,  as  above,  sometimes  give  way, 
the  rush  of  water  causing  devastation  in  the  valley  below. 

In  some  cases  a  slide  is  precipitated  by  a  soft,  porous  bed  at  the 
bottom  of  a  cliff  giving  way.  Thus  Russell  in  his  report  on  the  Cas- 
cade Mountains  in  northern  Washington  (Ref.  13)  states  that  "  the 
conditions  favorable  for  landslides  are  fulfilled  in  nearly  all  particu- 
lars in  places  where  the  Columbia  lava,  in  sheets  400  or  500  feet  or 


LANDSLIDES  AND   THEIR  EFFECTS  349 

more  thick,  rests  on  clays  and  sands,  or  on  deposits  of  volcanic  lapilli, 
and  the  series  has  been  eroded  so  as  to  form  steep  escarpments  .... 
Many  examples  of  these  conditions  are  furnished  along  the  great 
northward-facing  escarpment  of  Clealum  Ridge,  and  on  the  western 
margins  of  the  sloping  table-lands  known  as  Lookout  and  Table 
Mountains.  (Fig.  170).  Numerous  other  localities  along  the  western 

S.E. 


N.W. 

Wheat  Lauds 


FIG.  170.  —  Ideal  profile  of  landslides  on  the  northern  side  of  Lookout  Mountain, 
Wash.     (After  Russell,  U.  S.  Geol.  Survey,  20th  Ann.  Rept.,  Pt.  II.) 

border  of  the  Columbia  lava,  from  Table  Mountain  northward  to 
beyond  the  mouth  of  Okanogan  River,  mainly  on  the  east  side  of  the 
Columbia,  furnish  as  favorable  conditions  for  landslides  as  those  just 
referred  to.  Throughout  this  irregular  line  of  great  escarpments  the 
landslides  that  have  occurred  are  to  be  numbered  by  the  hundreds. 

The  fact  that  the  escarpments  referred  to  are  formed  of  the  edges 
of  nearly  horizontal  or  but  slightly  inclined  layers  of  hard  basalt  which 
are  traversed  by  joints  at  right  angles  to  the  planes  of  bedding,  and 
also  the  occurrence  of  layers  of  soft  rocks  beneath  the  hard  cliff  form- 
ing layers,  furnish  conditions  unusually  favorable  for  landslides." 

Type  III.  —  This,  according  to  Heim,  is  restricted  to  cases  where 
stratified  rocks  have  a  dip  in  the  direction  of  the  slope  of  the  hill  of 
which  they  form  a  part.  Slipping  is  therefore  initiated  along  the 
bedding  planes  of  a  rock.1  Cleavage  planes  might  produce  the  same 
type  of  rock  slip. 

Slips  of  this  type  are  likely  to  start  from  artificial  causes.  Thus, 
for  example,  if  the  stratification  or  cleavage  planes  dip  towards  the 
face  of  a  slope,  the  removal  of  stone  for  quarrying,  or  for  road  and 
railway  cuttings,  leaves  the  material  unsupported  (Fig.  170).  If  a 
slide  does  not  occur  at  once,  it  is  very  likely  to  take  place  later  when 
water  and  frost  get  into  the  mass. 

Belonging  to  this  type  also  are  the  interesting  Rock  Streams  which 
Howe  has  described  from  the  San  Juan  Mountains  of  Colorado  (Ref. 

1  See,  for  example,  slipping  of  bridge  piers  in  a  slippery  clay  over  coal  seam,  Eng. 
News,  XXXIX,  p.  278,  1898. 


350 


ENGINEERING  GEOLOGY 


6).  He  states  that  many  of  the  high  "  glacial  cirques  of  the  San  Juan 
Mountains  are  covered  by  enormous  masses  of  rock  de*bris  resembling 
in  its  general  appearance  ordinary  talus,  but  the  form  of  these  accu- 
mulations is  quite  unlike  that  of  the  long,  even  slopes  of  detritus  at 
the  base  of  cliffs.  In  many  respects  these  masses  closely  resemble 

those  of  landslide  origin  in  their 
general  form  and  in  their  rela- 
tion to  the  points  from  which 
the  material  has  been  derived.  " 
Indeed  they  remind  one  at  times 
of  small  glaciers  completely 
buried  under  a  covering  of  loose 
rock. 

One  of  the  largest  of  these 
found  in  Pierson  basin,  in  the 
San  Juan  Mountains  is  from  50 
to  100  feet  thick,  three-quarters 
of  a  mile  long,  one-third  of  a 
mile  average  width,  and  has  a 
minimum  estimated  mass  of 
nearly  13,000,000  cubic  yards. 
The  material  is  volcanic  rocks 
derived  from  the  neighboring 
cliffs. 

It  is  supposed  that  after  the 
time  of  maximum  glaciation,  the 
walls  of  the  cirques  were  left  in 
an  oversteepened  condition  by 
the  undermining  of  the  ice  at 
the  "  bergschrund,"  and  that 
after  the  disappearance  of  the  ice  the  walls  were  left  unsupported  and 
toppled  over. 

Type  IV.  —  Rock  falls  may  take  place  regardless  of  the  character 
or  attitude  of  the  rock  mass.  A  fine  example  of  this  type  was  the  rock 
fall  that  occurred  at  Frank,  Alberta,  (Ref.  2,  8)  in  1903  (Plate  L, 
Fig.  2).  This  was  due  to  the  breaking  loose  of  a  great  mass  of  rock, 
about  one-half  mile  square,  and  from  400  to  500  feet  thick,  from  the 
top  of  Turtle  Mountain.  The  latter  towers  about  3000  feet  above  the 
valley  of  Oldman  River  in  which  the  coal-mining  town  of  Frank  is 
situated. 

Turtle  Mountain  consists  of  westerly  dipping  limestones  in  its  up- 


S=  Stratification  planes 

J  =  Joint  planes 

A=  Area  liable  to  slip 


FIG.  171.  —  Section  showing  structural 
conditions  likely  to  produce  rock  slides 
along  joint  or  stratification  planes. 


LANDSLIDES  AND  THEIR  EFFECTS  351 

per  part  (Plate  LII)  and  sandstones  and  shales  in  its  lower  portion, 
the  former  being  thrust  over  the  latter  by  faulting.  The  rocks  are 
also  cut  by  numerous  fracture  and  joint  planes.  In  the  lower  beds 
there  is  moreover  a  coal  bed  which  is  being  mined. 

When  the  great  mass  of  rock  estimated  at  40,000,000  cubic  yards 
broke  loose  it  was  dashed  to  the  base  of  the  mountain,  plowed  its  way 
across  the  valley  and  400  feet  up  the  other  side.  The  slide  mate- 
rial covered  1.03  square  miles  in  the  valley  to  a  depth  of  from  5  to 
150  feet, 

The  slide  or  rather  rock  fall  was  due  to  a  combination  of  causes, 
as  follows:  (1)  The  form  and  structure  of  Turtle  Mountain,  which 
had  a  steep  face,  weak  base,  and  was  much  jointed;  (2)  earthquake 
tremors  in  1901  which  probably  loosened  the  rock  somewhat;  (3)  a 
period  of  heavy  precipitation  and  heavy  frost ;  (4)  the  removal  of 
coal  from  the  seam  along  the  foot  of  the  mountain  which  removed 
some  of  the  support.  Curiously  enough  the  width  of  the  slide  was 
about  the  same  as  that  of  the  mine  workings. 

When  the  rock  mass  fell  from  the  south  peak  it  buried  a  number  of 
ranches  in  the  valley  and  a  portion  of  the  town  of  Frank. 

Since  this  slide  occurred  a  widening  crack  which  has  appeared  on  top 
of  the  northern  peak  has  given  rise  to  the  fear  that  this  is  also  likely 
to  fall.  Accordingly  a  commission  was  appointed  to  investigate  the 
matter,  and  has  advised  moving  the  town  of  Frank  farther  up  the 
valley,  and  also  discontinuing  the  mining  of  coal  under  the  northern 
peak  (Refs.  2  and  8). 

Rock  and  clay  falls  are  often  caused  along  valleys  by  streams 
undermining  their  walls,  or  along  the  seacoast  by  waves  undercutting 
the  cliffs. 

Type  V.  —  This  includes  compound  slides. 

A  case  quoted  from  Heim  is  that  of  a  large  mass  of  limestone  which 
broke  off  across  the  bedding  and  became  detached  from  the  cliff  face. 
It  fell  down  onto  a  wet  clayey  mass  and  started  the  latter  sliding,  the 
whole  on  reaching  the  edge  of  the  ravine  being  precipitated  into  the 
stream  and  damming  it. 

Slides  of  this  type  are  not  necessarily  uncommon,  but  few  have  been 
recorded.  They  have  been  described  from  Canyon  Creek,  southwest 
of  Ouray,  where  the  rock  debris  from  Hay  den  Mountain  has  fallen 
on  glacial  gravels  (Ref.  6). 

Type  VI.  —  The  last  type  includes  special  cases  as,  for  example, 
soft  clays  squeezed  out  between  heavier  massive  beds  and  coastal 
slips.  In  the  latter  case  soft  beds  at  the  base  of  a  submerged  mass, 


LANDSLIDES  AND  THEIR  EFFECTS  353 

like  a  delta  deposit,  may  squeeze  out,  allowing  the  overlying  material 
to  settle  and  slide. 

Slips  along  roadways  or  railway  cuttings  and  cavings  due  to  mining 
operations  also  belong  in  this  group. 

Engineering  Considerations 

Knowing  that  landslides  frequently  follow  excavating  operations, 
it  becomes  important  for  the  engineer  to  know  if  possible  what  degree 
of  slope  is  safe  in  different  kinds  of  rocks. 

Before  explaining  this,  it  is  desirable  to  understand  the  meaning 
of  several  terms  that  are  sometimes  used,  such  as  angle  of  rest,  angle 
of  slide,  and  excavation  deformation. 

Angle  of  rest.  —  This  is  the  angle  (with  a  horizontal  plane)  at  which 
loose  material  will  stand  on  a  horizontal  base  without  sliding.  It  is 
often  between  30°  and  35°. 

Angle  of  slide.  —  This  may  be  defined  as  the  slope  (measured  in 
degrees  deviation  from  horizontality)  on  which  a  slide  wTill  start. 
It  is  perhaps  self-evident  that  it  may  vary  considerably  depending 
on  several  factors  such  as:  (1)  The  weight  of  the  overlying  mass 
above  the  slipping  plane;  (2)  the  character  of  the  slipping  surface, 
whether  flat  or  undulating,  and  whether  dry  or  wet.  Clay,  when 
wet,  makes  a  very  slippery  surface;  (3)  character  of  material  below 
slipping  surface,  and  whether  it  will  flow  under  pressure,  like  a  wet 
clay.  If  this  under  clay  squeezes  out,  a  slide  may  be  initiated  on  a 
slope  of  very  low  inclination. 

As  illustrative  of  the  second  point,  mention  may  be  made  of  an 
occurrence  along  the  West  Shore  Railroad  south  of  Newburgh.  Here 
considerable  broken  stone  was  dumped  along  the  river  bank  to  make  a 
fill  for  the  road.  Although  the  slope  of  the  mass  did  not  exceed  the 
angle  of  repose,  there  was  much  sliding.  It  was  finally  discovered 
that  the  river  bottom  on  wrhich  the  rock  was  dumped  consisted  of 
hard  mud  with  a  20-degree  slope,  running  down  to  300  feet  depth,  and 
formed  a  splendid  slipping  surface  (Ref.  12). 

Angle  of  pull.  —  This  term  is  used  by  some  to  indicate  the  angle 
between  the  vertical  and  an  inclined  plane  bounding  the  area  affected 
by  the  subsidence  beyond  the  vertical. 

Others  would  apply  it  to  the  effective  resultant  of  the  two  groups 
of  stresses  set  up  in  rocks  adjoining  an  excavation,  if  it  breaks  the 
rocks  down  to  that  angle.  In  the  rocks  contiguous  to  the  excavation 
these  stresses  may  be  of  two  kinds:  (1)  Crushing  or  direct  gravity 
stresses,  which  are  at  a  maximum  near  the  toe  of  the  steep  excava- 


354  ENGINEERING  GEOLOGY 

tion  slope;  and  (2)  tensional  or  flowage  stresses,  due  indirectly  to 
gravity,  and  exerting  a  horizontal  pull  towards  the  excavation,  but 
giving  a  maximum  deformation  near  the  surface. 

Excavation  deformation.1  —  It  has  been  suggested  that  the  special 
name  of  excavation  deformation  should  be  applied  to  the  zone  around 
any  excavation  within  which  a  structure  might  be  disturbed  by 
rock  movements  resulting  from  that  excavation. 

Factors  affecting  excavation  deformations.  —  The  strains  set  up  in 
the  rocks  adjoining  an  excavation  may  be  due  to:  (1)  Natural  proc- 
esses, as  stream  erosion,  solution,  fault  escarpments,  etc.;  and  (2) 
artificial  causes,  as  open  cuts,  underground,  and  submarine  excava- 
tions. The  extent  to  which  any  of  these  affect  the  rock  is  said  to 
depend  on  the  following  factors: 

1.  Crushing  strength  of  large  masses  of  the  material  involved. 

2.  Tensile  strength  of  large  masses  of  the  material  involved.     Va- 

riations in  1  and  2  are  due  to:  (a)  Strength  of  small  compo- 
nent masses;  (6)  character  of  jointing;  (c)  character  of  bed- 
ding; and  (d)  fault  conditions. 

3.  Physical  and  chemical  character  of  the  rock  units. 

4.  Amount  and  character  of  groundwater. 

5.  Earth  tremors  set  up  by  earthquakes,  blasts,  trains,  etc. 

6.  Other  factors,  as:    (a)  Heavy  structures  next  to  excavations; 

(6)  water  freezing  in  rock  openings,  and  wedging  off  rock 
masses;  (c)  variations  of  barometric  pressure;  and  (d)  earth 
strains  from  kneading  or  tidal  pull. 

These  factors  may  be  briefly  discussed: 

1  and  2.  A  rock  of  high  crushing  strength,  with  few  joint  or  other  planes  will 
stand  with  a  face  that  is  practically  vertical.  The  same  rock,  much  cut  by  fracture 
planes,  sloughs  off  masses  from  steep  slopes,  until  a  certain  angle  of  permanent  slope 
is  attained.  Any  fissures  inclining  towards  the  excavation  tend  to  cause  slides, 
especially  bedding  planes  with  shale,  lignite  or  other  greasy  rock  partings,  or  fault 
planes  with  talcose  partings.  Such  slides  may  occur  even  if  the  planes  slope  but 
gently,  and  have  relatively  slight  back  pressure. 

With  rock  of  low  crushing  strength,  but  relatively  high  tensile  strength,  slide 
movement  shows  sinking  near  excavations,  slight  advance  of  lower  slope  towards 
cut,  and  bulging  upward  of  the  excavation  floor. 

3.  Very  soft  rocks,  such  as  fine-grained  and  compact  argillites  and  clays,  may 
maintain  a  vertical  face  until  excavation  reaches  a  depth  of  45  to  120  feet,  or  until 
unbalanced  pressure  is  great  enough  to  cause  them  to  deform.  Such  deformation 
destroys  stability  of  the  clayey  cementing  materials,  loosens  them  up  so  that  surface 
water  can  enter,  and  causes  mobility  of  the  mass,  with  the  result  that  the  slope  may 
break  back  from  almost  perpendicular  to  1  on  10. 

Deformations  of  the  above  type  have  occurred  in  the  volcanic  clay  rocks  of  the 
Culebra  cut  of  the  Panama  Canal. 

1  Freely  abstracted  from  Ref.  10. 


LANDSLIDES  AND  THEIR  EFFECTS  355 

Excavations  which  change  the  water  table  level  may  weaken  surrounding  rocks 
by  dissolving  and  loosening  their  more  soluble  parts,  especially  in  regions  where  the 
groundwater  contains  much  carbon  dioxide  and  organic  acids. 

4.  Groundwater  in  rocks  exerts  a  weakening  influence,  increasing  their  tendency 
to  deformation  because:    (1)  It  adds  to  weight  of  the  rock  mass;     (2)  weakens  the 
rock  by  solution  and  softening;   and  (3)  increases  the  mobility  of  a  mass  of  rock 
material. 

If  a  porous  rock  rests  on  an  impervious  one,  the  water  descending  through  the 
former  will  not  only  be  deflected  by  the  latter,  but  the  wet  clay  particles  carried 
down  to  this  contact  surface  facilitate  slipping.  Even  capillary  water  in  a  weak 
rock  is  a  source  of  danger,  for  with  deformation  much  of  the  capillary  water  may  be 
crushed  into  the  larger  shear  planes,  thus  giving  them  increased  lubrication.  In 
estimating  sliding  or  deforming  tendencies  of  a  rock,  careful  determinations  of  its 
water  content  should  be  made  on  both  fresh  and  air-dried  samples. 

The  most  troublesome  slides  of  Culebra  cut  occurred  in  fine-grained  basic  volcanic 
clay  shales  of  fairly  massive  character,  which  show  from  6  to  17  per  cent  of  water. 

5.  Earthquakes  may  be  a  cause  of  deforming  movements  in  rock  masses,  but 
blasting  is  a  common  cause.     Surface  blasts  cause  less  subsurface  vibration  than 
deep  ones.     Two  large  blasts  in  Culebra  cut  gave  the  following  approximate  vibra- 
tion records.     A  blast  of  2250  pounds  of  dynamite,  exploded  in  14-,  24-  and  28-foot 
holes,  gave  a  maximum  amplitude  of  vibration  of  20  mm.  at  1100  feet  distance. 
Another  of  5370  pounds  of  dynamite  exploded  in  forty-eight  24-foot  holes  at  about 
the  same  distance  gave  an  amplitude  of  28  mm.  vibration  on  the  recording  instru- 
ment.    But  as  the  magnification  of  the  latter  was  10,  the  earthwaves  set  up  by  the 
blasts  were  about  2  and  2.8  mm.  respectively,  or  enough  to  damage  seriously  a 
steep  slope  of  brittle  rocks  already  heavily  strained.     Railway  trains  may  also  set 
up  sufficient  vibration  to  cause  damage. 

6.  Heavy  structures  near  excavations  increase  a  tendency  to  slide,  as  subway 
and  foundation  engineers  know.  Variations  in  barometric  pressure  and  the  kneading 
of  tidal  pull  are  not  to  be  overlooked.  The  maximum  variations  in  atmospheric 
pressure  near  sea-level  may  be  over  4,000,000  tons  per  square  mile. 

Slopes  to  minimize  sloughing  and  deformation.  Two  classes  of  rock  have  to  be 
considered:  I,  solid  rocks  which  will  not  deform  under  pressure;  and  II,  those 
which  will  show  deformation  under  pressure. 

I.  Rocks  of  this  class  will  slough  but  not  flow,  and  the  following  limiting  cases 
are  given  by  MacDonald. 

1.  Solid  rock  with  relatively  high  crushing  and  tensile  strength;   jointing,  fissur- 
ing  and  bedding  planes  a  minimum.     Permissible  slope  10  on  1.     Includes  rock  like 
granite,  trap,  quartzite,  solid  sandstone,  and  shale. 

2.  Same  rock  as  No.  1,  but  jointing  and  fissuring  increased  to  average  com- 
monly encountered  in  excavations.     Permissible  slope  7  on  1. 

3.  Same  rock  as  1  and  2,  but  jointing  and  fissuring  increased  to  maximum  encoun- 
tered in  such  rocks.     Permissible  slope  3  on  1. 

4.  Rock  same  as  1,  jointing  corresponding  to  2  or  3,  but  excavation  paralleling 
bedding  or  fault  planes  which  dip  towards  it.     Slope  likely  to  be  controlled  by  such 
planes  as  follows:    (a)  Individual  beds,  meter  or  more  thick,  with  no  clayey  or 
slaty  rock  along  them.     Permissible  slope  2  on  1;    (6)  rocks  thinly  bedded,  with 
shaly,  clayey,  or  slickensided  conditions  along  bedoling  or  fault  planes.     Safe  slope, 
2  on  3,  but  if  bedding  is  not  slippery,  a  1  on  1  slope  is  safe. 

II.  In  this  case  deformations  or  movements  may  extend  to  some  depth  below  exca- 
vation, and  some  distance  from  it  horizontally.  The  swelling  ground  in  some  tunnels, 
and  in  coal  mines  is  an  example  of  rock  deformation  of  this  type.  These  deformable 
rocks  may  stand  at  a  steep  angle  until  the  excavation  reaches  a  depth  of  perhaps 
60  or  90  feet,  then  deform,  and  later  slide  until  a  flat  angle  is  reached.  For  such 


356 


ENGINEERING   GEOLOGY 


rocks  the  slope  which  minimizes  danger  of  deformation  and  gives  maximum  steep- 
ness will  be  a  curved  one. 

The  following  table  suggested  by  MacDenald  gives  the  best  slopes  to  adapt  for 
excavation  in  the  materials  described. 


TABLE  OF  SLOPES  TO  ADOPT  AT  DIFFERENT  DEPTHS  FOR  DEFORMABLE  ROCKS 


Excavation 

depth. 

A 

B 

c 

D 

E 

F 

G 

Meters. 

Feet. 

10 

33 

50   on  10 

40  on  10 

30   on  10 

20.0  on  10 

12  on  10 

7  on  10 

5   on  10 

20 

66 

41     10 

33    10 

25     10 

17.0   10 

10.3   10 

6.1    10 

4.2    10 

30 

98 

36     10 

28    10 

21     10 

15.4    10 

9.3    10 

5.6    10 

3.6    10 

40 

131 

32     10 

25    10 

19     10 

14.4    10 

8.6    10 

5.2    10 

3.2    10 

50 

164 

29     10 

22    10 

16     10 

18.5    10 

8.0   10 

4.9    10 

2.8    10 

60 

197 

26     10 

19    10 

14     10 

12.7    10 

7.5    10 

4.6    10 

2.5    10 

70 

230 

24     10 

18    10 

13     10 

12.0    10 

7.2    10 

4.4    10 

2.2    10 

80 

262 

23     10 

16    10 

12     10 

11.4    10 

6.8    10 

4.2    10 

2.0    10 

90 

295 

21     10 

15    10 

11     10 

10.8    10 

6.5   10 

4.0    10 

1.9    10 

100 

328 

20     10 

14    10 

10     10 

10.2    10 

5.2    10 

3.9    10 

1.8    10 

150 

492 

14.5    10 

11    10 

8.6    10 

6.8    10 

5.5    10 

3.4    10 

1.4    10 

200 

656 

12.0    10 

10    10 

8.0    10 

6.0    10 

5.0    10 

3.0    10 

1.2    10 

The  kind  of  material  indicated  in  each  of  these  columns  is  as  follows : 

A.  Certain  fairly  soft  and  weak  sandstones,  shales,  some  limestones,  soft  tuffs, 
agglomerates  and  clay  rocks  —  all  deformable  under  great  pressure,  but  yielding 
slowly.     Includes  rocks  that  cause  swelling  ground  in  coal  mines,  and  other  excava- 
tions, but  stronger  than  the  clay  rocks  and  tuffs  of  Culebra  cut. 

B.  Same  as  A,  but  with  medium  shearing  and  jointing. 

C.  Same  as  A  and  B,  but  with  maximum  of  shearing,  jointing,  and  fissuring. 
Beds  may  dip  towards  excavation. 

D.  Soft  volcanic-clay  rocks,  bedded  friable  tuffs,  and  lignitic  shales.     This  type 
caused  the  large  slides  at  Culebra  cut.    They  have  much  water  and  chloritic  material, 
and  minimum  of  jointing,  fissuring,  and  bedding. 

E.  Same  as  D,  but  some  jointing  and  fissuring. 

F.  Same  as  D  and  E,  but  much  jointed. 

G.  Very  soft  and  crushed  rocks,  talcose  clays,  etc.,  rendered  slippery  by  ground- 
water. 

From  these  tables  a  theoretic  slope  should  be  first  determined  corresponding  to 
the  depth  of  excavation,  character  of  rock,  etc.  Then  plot  cross-section  of  slope  and 
bottom  planes  of  the  excavation  as  selected.  A  hyperbola  tangent  to  these  two. 
with  its  vertex  in  the  projection  of  the  bottom  plane  will  represent  about  the  proper 
slope  and  curvature  of  the  excavation.  In  all  excavations,  allowance  must  be  made 
for  the  fact  that  the  soft  decayed  rock  and  soil  material  near  the  top  will  tend  to 
erode  back  from  the  excavation  until  the  surface  approaches  logarithmic  curvature. 


References  on  Landslides 

1.  Dana,  J.  D.,  Manual  of  Geology,  Landslides,  4th  ed.,  1895, 
pp.  208,  232. 

2.  Daly,  Miller  and  Rice,  Can.  Geol.  Survey,  Memoir  27,  1912. 

3.  Drinker,  H.  S.,  Tunneling,  1878,  p.  733. 

4.  Geikie,  A.,  Textbook  of  Geology,  3rd  ed.,  1893,  Landslips,  or- 


LANDSLIDES  AND  THEIR  EFFECTS  357 

dinary  origin  of  370;  effects  of,  on  rivers,  382;   caused  by  earthquakes, 
280. 

5.  Heim,  A.,  Ueber  Bergsttirze,  Zurich,  1882. 

6.  Howe,  E.,  Landslides  in  San  Juan  Mountains,  Colo.,  U.  S.  Geol. 
Survey,  Prof.  Paper  67,  1909. 

7.  Lyell,   C.,   Principles  of  Geology,   llth  ed.,   1889,  Vol.   I,  in 
Dorsetshire,  p.  540;    on  the  Amazons,  p.  467,  floods  caused  by,  p.  346; 
Vol.  II,  during  Calabrian  earthquake,  p.  130;  imbedding  of  organic 
remains  by,  p.  520. 

8.  McConnell,   R.   G.   and  Brock,   R.  W.,  Report  on  the  great 
landslide  at  Frank,  Alta.,  1903;  Ann.  Reptv  Can.  Geol.  Survey,  Pt.  8, 
1903. 

9.  McDonald,  Landslides  of  Culebra  Cut,  Panama  Canal,  Min.  and 
Sci.  Press.,  Dec.  7,  1912. 

10.  MacDonald,  Excavation  Deformations,  Internal;.' Geol.  Congress, 
Toronto,  1913. 

11.  Patton,  H.  B.,  Rock  Streams  of  Veta  Peak,  Colo.,  Bull.,  Geol. 
Soc.  Amer.,  XXI,  1910,  p.  663. 

12.  Rice,  Jour.  W.  Soc.  Eng.,  XVIII,  No.  7, 1913.     Landslides,  many 
references. 

13.  Russell,  I.  C.,  U.  S.  Geol.  Survey,  20th  Ann.  Report,  Pt.  II, 
1900,  p.  193.     (Cascades  of  Northern  Washington.) 

14.  Turner,  H.  W.,  U.  S.  Geol.  Survey,  17th  Ann.  Rept.,  Pt.  I,  p.  553. 

15.  Miscellaneous,  Arrow  Lake,  B.  C.,  Can.  Geol.  Survey,  XV,  p.  55 
AA;  Blanche  River,  Que.,  Can.  Geol.  Survey,  XI,  p.  121  A. 

For  others  see  Ibid.  XVI,  p.  16F;    I,  p.  546;    IV,  p.  95D;    XI,  p. 
123A;  XI,  p.  11J;  I,  p.  606;  XVI,  p.  241A;  V,  p.  72E;  XI,  p.  70J. 


CHAPTER  VIII 

WAVE  ACTION  AND  SHORE   CURRENTS:   THEIR  RELATION 
TO   COASTS   AND   HARBORS 

Introductory.  —  Commercial  intercourse  between  nations  having 
coast  lines,  coastwise  traffic  on  the  ocean  or  inland  bodies  of  water, 
etc.,  demand  the  existence  and  maintenance  of  good  harbors,  as  well 
as  the  preservation  of  shore  lines.  Along  some  coasts  excellent  natural 
harbors  exist  while  along  others  some  of  the  harbors,  at  least,  require 
improvement  by  engineers.  In  either  case  the  harbor  is  sometimes 
closed  up  or  shallowed,  either  by  sedimentation  or  gradual  uplift  or 
both,  if  natural  forces  are  allowed  to  operate  undisturbed. 

There  are  cases,  of  course,  where  a  harbor  may  become  improved 
without  the  work  of  man.  Thus,  subsidence  of  the  land,  accompanied 
by  little  or  no  sedimentation,  or  in  excess  of  sedimentation,  will  result  in 
the  deepening  of  harbors  along  coast  lines  of  rugged  topography. 

The  southeast  Atlantic  Coast  harbors  of  the  United  States,  for  example,  belong 
to  the  troublesome  harbors  on  a  sandy  coast,  because  they  are  difficult  to  keep  open 
and  navigable.  They  represent  a  type  which  have  been  so  long  a  cause  of  worry 
to  engineers  and  governments,  and  which  so  often  obstinately  refuse  to  "stay  put" 
after  much  money  and  time  have  been  spent  on  their  improvement.  Maintained 
and  destroyed  by  the  same  power,  the  sea,  their  formation  and  maintenance  depend 
on  so  nice  an  adjustment  and  control  of  these  forces,  that  it  is  not  strange  that 
disappointment  has  frequently  followed  so  much  painful  effort  (Black). 

The  trouble  is  caused  primarily  by  wind  and  waves  acting  together 
in  breaking  down  the  shore  line,1  and  transporting  the  products  of 
attack  from  one  part  of  the  coast  to  another,  but  the  shore  topography 
and  the  sediment  brought  by  the  streams  from  the  land  are  also  factors 
that  enter  into  the  problem. 

The  engineer  who  is  engaged  in  harbor  improvements  or  maintenance 
should  familiarize  himself  with  the  manner  in  which  these  agents  work, 
so  that  he  can  if  possible  counteract  or  prepare  for  their  operations, 
or  even  utilize  their  power  to  aid  him. 

1  The  phenomena  of  wave  action  and  shore  currents  are  not  confined  to  the 
ocean,  but  have  full  play  on  lakes  and  inland  seas  as  well,  where  the  water  can  be 
agitated  sufficiently  by  the  wind. 

358 


WAVE  ACTIONS   AND  SHORE  CURRENTS,   ETC.  359 

Formation  of  Waves 

Cause  of  waves.  —  The  most  common  waves  are  generated  by  the 
wind.1  When  a  strong  wind  blows  across  the  surface  of  the  ocean  or 
lake,  it  starts  each  particle  of  water,  near  the  surface  at  least,  oscillating 
in  an  orbit,  which  is  approximately  circular  and  lies  in  a  vertical  plane. 
In  the  case  of  an  off-shore  wave  there  is  probably  little  advance  of  the 
water,  so  that  each  particle  returns  nearly  to  its  starting  point. 

In  waves  known  as  the  swell,  which  is  outside  of  the  area  directly 
affected  by  the  wind,2  the  particles  have  closed  orbits,  so  that  there  is 
no  permanent  advance  of  the  water.  But  in  the  wind  wave,  the  particle 
advances  slightly  more  than  it  recedes,  each  particle  describing  a  circle 
rather  than  an  ellipse,  which  develops  a  current,  that  is  slower  than 
the  wind. 

If  we  think  of  the  water  as  being  made  up  of  layers,  then  the  top 
layer  will  move  a  little  faster  than  the  one  next  below  and  so  on. 

Depth  of  wind  disturbance.  —  Theoretically  the  wave-motion  is 
propagated  indefinitely  downward,  but  the  effect  rapidly  diminishes 
with  depth.  This  fact  is  of  interest  to  engineers,  because  of  its  relation 
to  the  disturbance  of  submarine  structures. 

Engineering  operations  have  shown  that  submarine  structures  are 
little  disturbed  at  depths  of  five  meters  in  the  Mediterranean,  and 
eight  meters  in  the  Atlantic  Ocean. 

Debris  as  coarse  as  gravel,  which  is  transported  by  rolling  on  the 
bottom,  is  not  infrequently  carried  out  to  depths  of  50,  and  sometimes 
even  to  150  feet. 

Fine  sediment  like  silt,  is  disturbed  at  still  greater  depths,  for  ripple 
marks  which  are  usually  present  in  the  finest  sediments,  and  indicate 
agitation  of  the  water,  are  said  to  have  been  found  at  depths  of  100 
fathoms. 

In  deeper  water  the  waves  will  be  larger  and  currents  stronger,  as 
the  movements  are  not  retarded  by  friction  on  the  bottom.  There 
the  waves  may  be  50  feet  high,  and  as  much  as  1500  feet  long,  measured 
from  crest  to  crest. 

Theory  of  wave  motion.  —  The  crests  of  waves  are  deeper,  sharper, 
and  more  sharply  curved  than  the  trough.  This  is  because  the  wave 
assumes  the  form  of  a  trochoid  curve,  such  as  is  generated  by  any  point 
within  a  circle  rolling  on  a  horizontal  plane. 

1  Destructive  waves  of  great  size  are  sometimes  produced  by  earthquake  move- 
ments and  submarine  volcanic  eruptions. 

2  The  appearance  of  the  swell  sometimes  indicates  the  approach  of  a  gale  several 
hours  in  advance  of  its  arrival. 


360 


ENGINEERING  GEOLOGY 


A  shortening  of  the  wave  length  or  increase  of  its  height,  makes  the 
crest  sharper. 

The  following  illustrations  given  by  Fenneman  will  explain  the 
difference  in  waves. 

"  Thus  Fig.  172,  represents  a  series  of  oscillating  particles.  The  several 
particles  may  be  thought  of  as  rotating  in  the  direction  of  the  hands  of 

2OQQQOQQQQQQQOQ 

FIG.  172.  —  Series  of  particles  in  their  orbits.  The  circles  represent  the  orbits  of 
the  particles  which  revolve  from  left  to  right.  At  any  given  moment  each 
particle  is  advanced  in  its  orbit  90  degrees  more  than  its  neighbor  to  the  right. 
The  curved  line  representing  the  form  of  the  wave,  advancing  from  left  to  right, 
connects  the  simultaneous  position  of  all  particles. 


FIG.  173.  —  The  same  with  orbits  doubled  in  size,  phasal  difference  45  degrees. 
Absolute  amount  of  differential  movement  the  same  as  in  Fig.  172. 


FIG.  174.  —  The  same  as  Fig.  172,  with  phasal  difference  reduced  to  45  degrees. 


FIG.  175.  —  The  same  as  Fig.  173,  with  phasal  difference  increased  to  90  degrees. 
A  condition  for  breakers. 


FIG.  176.  —  The  same  as  Fig.  175,  with  orbits  sufficiently  reduced  in  size  to  pre- 

vent breaking. 


FIG.  177.  —  The  same  as  Fig.  176,  with  orbits  still  further  reduced. 
(FIGS.  172  to  176  after  Fenneman,  Wis.,  Geol.  Survey,  Bull.  VIII,  1903.) 

a  watch  while  the  wave  advance  is  from  left  to  right.  Each  one  is  more 
advanced  in  its  orbit  (or  has  a  more  advanced  phase)  than  the  one  in 
front  of  it.  By  connecting  the  several  points  a  curve  is  produced 


WAVE  ACTION  AND  SHORE  CURRENTS,   ETC.  361 

which  has  the  form  of  a  water  wave.  The  particle  at  the  crest  of  each 
wave  is  moving  forward  in  the  direction  in  which  the  wave  is  traveling, 
while  the  lowest  particle  in  the  trough  is  moving  backward  at  the  same 
rate.  Particles  in  front  of  the  crests  are  rising  in  their  orbits  while 
those  in  the  rear  are  descending. 

It  follows  that  a  particle  starting  at  the  crest  and  making  one  complete 
revolution  occupies  successively  all  positions  from  crest  to  trough  and 
back  to  crest  again,  hence  the  wave  form  moves  forward  a  distance 
equal  to  its  length  while  each  particle  is  making  one  revolution.  It 
has  been  found  that  for  waves  of  a  given  length  the  time  occupied  in 
one  revolution  is  the  same,  whatever  be  the  size  of  the  orbit  and  the 
consequent  height  of  the  wave. 

This  is  equivalent  to  saying  that  the  rate  at  which  waves  travel 
depends  upon  their  length  alone. 

If  each  one  of  a  series  of  particles  is  describing  its  own  orbit  and 
the  phases  of  successive  particles  differ  (as  shown  hi  Figs.  172  to  177)  it 
follows  that  each  particle  is  subject  to  a  gliding  between  its  neighbors. 

This  amount  is  called  the  differential  movement  of  particles.  For 
diagrammatic  purposes,  this  differential  can  be  considered  as  a  con- 
siderable arc  of  the  orbit,  hence  particles  are  chosen  which  are  removed 
from  one  another  by  a  considerable  fraction  of  the  length  of  the  diameter. 

Figs.  172  and  174  show  that  when  the  differential  movement  between 
particles  is  great  the  length  of  the  wave  is  correspondingly  small.  As 
the  differential  movement  between  particles  is  increased  and  the  waves 
shortened,  the  contrast  between  crests  and  troughs  becomes  increasingly 
apparent. 

The  limit  of  possible  steepness  of  waves  is  the  curve  known  as  the 
common  cycloid.  In  this  condition  the  crests  are  sharp  angles  or 
cusps,  and  Fig.  176  shows  a  series  of  particles  in  waves  of  this  shape.  In 
Fig.  175  the  differential  movement  of  particles  has  been  increased  beyond 
the  amount  which  results  hi  cusped  waves.  If  the  several  particles  of 
this  series  be  connected  the  resulting  curve  is  seen  to  be  looped  instead 
of  cusped. 

The  water  surface  cannot  assume  this  form,  hence  the  wave  breaks. 

Zone  of  breakers.  —  As  a  wave  approaches  a  shelving  shore,  its 
form  changes,  and  the  wave  becomes  both  higher  and  shorter,  the  crest 
becomes  steeper  and  sharper  with  the  velocity  of  the  advancing  particle 
of  water  increased,  and  the  front  steeper  than  the  back.  This  results 
finally  in  the  breaking  of  the  wave. 

Waves  of  a  given  height  will  break  in  the  same  depth  of  water,  and 
the  line  of  breakers  is  that  along  which  the  incoming  waves  collapse. 


362  ENGINEERING   GEOLOGY 

The  breaking  of  these  waves  of  oscillation  starts  new  waves  of  trans- 
lation. 

These  are  quite  efficient  in  sweeping  material  ashore,  during  their 
forward  dash.  The  return  wash  down  the  beach  meets  the  next  in- 
coming translatory  wave. 

There  is  usually  a  zone  between  the  water's  edge  and  the  breaker 
line,  where  material  is  being  washed  back  and  forth. 

Storm  waves.  —  During  a  gale  the  wave-height  is  reduced,  but 
the  surface  velocity  of  the  water  (and  its  erosive  action)  is  greatly 
increased.  After  the  gale  has  passed,  the  waves  are  again  higher,  and 
while  of  lower  velocity,  appear  to  have  greater  battering  power. 

According  to  Stevenson  (Ref.  13),  the  height  of  a  wave  crest  above 
mean  water  level  is  two-thirds  the  wave  height  from  trough  to  crest, 
and  waves  break  when  they  enter  water  whose  mean  depth  is  equal  to 
or  twice  the  wave  height,  breaking  earlier  when  the  bottom  shoals 
rapidly,  than  where  the  slope  is  gradual. 

The  velocity  of  the  longest  waves  on  the  Atlantic  Ocean  is  115  feet 
per  second,  and  of  ordinary  storm  waves  50  to  60  feet  per  second. 
(Stevenson.) 

However,  the  data  worked  out  for  a  rocky  coast  do  not  always  apply 
exactly  to  a  shoaling  bottom  like  that  along  the  coast  of  the  eastern 
states. 

Gaillard's  observations.  —  Gaillard1  in  studying  the  force  of  waves  on  the  Florida 
coast  gives  the  following  conclusions  (summarized  by  Black).  "Waves  with  the 
form  of  a  common  cycloid  have  an  energy  in  foot-pounds  for  each  foot  in  length 
measured  along  the  crest,  and  for  that  portion  above  a  horizontal  plane  tangent  to 
the  hollow,  equal  to  6.3  h3,  in  which  h  is  the  height  from  hollow  to  crest.  The 
application  of  this  formula  is  expressly  limited,  and  is  useful  mainly  in  comparing 
the  relative  exposure  of  two  places  where  shoal  water  extends  for  some  distance 
seawards  and  where  the  fetch  of  the  waves  is  practically  unlimited. 

In  shallow  water,  immediately  before  breaking,  for  waves  varying  in  height 
from  2  ft.  to  7  ft.  (hollow  to  crest),  the  observed  relation  between  wave  height 
and  velocity  is  expressed  approximately  by  the  formula  h  =  0.08242  zi,  in  which  h 
is  the  height,  and  x  the  velocity  in  feet  per  second. 

These  waves  broke  when  they  arrived  at  depths,  which,  when  water  was  un- 
disturbed, were  from  0.72  h  to  2  h.  For  the  great  majority  h  equalled  d  (depth  of 
undisturbed  water)  at  breaking.  For  a  given  locality  the  variation  in  the  ratio  of 
d  to  h  seemed  to  be  caused  by  the  direction  and  force  of  the  wind.  A  strong  wind 
in  the  direction  of  wave  motion  made  d  =  1.25  h.  A  strong  contrary  wind  made 
d  =  0.72  h.  With  no  wind  and  a  uniform  bottom  with  a  slope  of  1  on  100,  d 
equalled  h.  With  a  slope  of  1  on  12,  d  sometimes  equaled  2  h. 

Observations  on  waves  varying  in  height  from  2|  to  6  ft.  showed  that  the  height 
of  wave  crest  above  the  mean  (undisturbed)  water  surface  varied  between  0.67  h 

1  Rep.  Chf.  of  Engrs.,  1889,  II,  1319. 


WAVE  ACTION  AND  SHORE  CURRENTS,   ETC. 


363 


and  0.89  h,  with  a  mean  value  of  0.76  h.  A  gently-sloping  bottom  or  an  opposing 
wind  increased  the  height  of  crest;  a  steep  slope  or  a  favoring  wind  decreased  it. 
Considering  only  a  well-defined  wave,  breaking  in  water  of  a  depth  equal  to 
its  height  from  hollow  to  crest,  the  maximum  effect  (recorded  by  dynamometer 
readings)  was  found  at  a  distance  above  the  water  surface  equal  to  about  ^  of  the 
wave  height,  from  which  point  it  decreased  to  zero  at  a  distance  above  the  water 
surface  equal  to  about  one-fourth  of  the  wave  height." 1 

Undertow.  —  Since  the  water  is  being  piled  up  against  the  shore 
by  the  waves  some  provision  must  be  made  for  its  return,  and  this  is 
done  by  the  undertow.  This  is  a  permanent  out- 
ward current  normal  to  the  coast  line,  of  pulsating 
character. 

Another  function  of  the  undertow  is  to  dispose 
of  material  eroded  by  the  waves  by  conveying  it 
seaward,  and  it  also  helps  to  scour  the  submerged 
shelf  across  which  the  waves  are  eating  their  way 
into  the  land. 

If  a  wave  approaches  the  shore  at  an  angle, 
there  will  be  a  tendency  for  it  to  start  a  shore  cur- 
rent, and  the  drift  thus  set  up  is  a  strong  factor  in 
the  transportation  of  sediment  along  shore. 

Thus  in  Fig.  178,  the  line  ab  represents  the  di- 
rection of  the  incoming  wave,  bd  the  direction  of 
undertow,  and  be  direction  of  shore  current.  A 
particle  of  sediment  affected  by  both  shore  cur- 
rent and  undertow  would  tend  to  move  in  the 
direction  be,  which  represents  the  resultant  of  the 
two  forces.  But  the  next  incoming  wave  would 
move  it  in  the  direction  ab  again.  It  would 
therefore  migrate  in  the  direction  be,  but  follow 
a  zigzag  path  in  doing  so. 


FIG.  178.  —  Diagram 
showing  relative  di- 
rections of  wave  (ab), 
undertow  (bd),  and 
shore  current  (be). 
Particle  carried  hi  by 
wave  in  direction  ab, 
but  by  undertow  and 
shore  current  action 
on  it,  in  direction  be. 
(After  Chamberlin 
and  Salisbury,  Ge- 
ology, I.) 


Work  Performed  by  Waves 

The  work  accomplished  by  waves  in  general  may 
be  classified  under  (1)  erosion,  (2)  transportation,  and  (3)  deposition. 

Erosion.  —  Waves  beating  against  the  shore  perform  erosion, 
chiefly  by  the  impact  of  the  water  and  by  the  debris  which  the  water 
carries,  as  well  as  in  other  less  important  ways.  The  impact  of  the 
water  alone  may  cause  considerable  erosion  if  the  coast  line  is  of  weak 
or  unconsolidated  material,  or  if  the  rock  consists  of  alternating  weak 

1  Lt.  Gaillard,  Rep.  Chf.  of  Engrs.,  1891,  III,  1637. 


364  ENGINEERING  GEOLOGY 

and  strong  material,  the  removal  of  the  former  may  leave  the  latter 
unsupported,  and  cause  it  to  collapse. 

Forcing  of  the  water  into  joint  planes  and  other  similar  spaces  can 
produce  hydraulic  pressure,  sufficient  to  disrupt  the  rock  if  it  is  weak, 
especially  when  due  to  weathering.  Very  little  effect  is  accomplished 
by  waves  of  clear  water,  on  solid,  hard  and  fresh  rocks. 

Storm  waves  especially  strike  a  blow  of  tremendous  force.  Stevenson 
in  conducting  a  series  of  experiments  on  the  force  of  breakers,  found 
that  the  average  force  on  the  Atlantic  .coast  of  Britain  was  611  pounds 
per  square  foot,  while  in  winter  it  was  2086  pounds.  The  greatest 
efficiency  is  shown  in  bold  coasts,  bordered  by  a  broad  stretch  of  deep 
water. 

The  erosive  work  of  waves  is  greatly  augmented  by  the  debris  which 
the  waters  are  able  to  move.  Thus  sand,  pebbles,  and  stones  moved 
by  the  waves,  not  only  serve  as  weapons  of  attack  against  the  coast 
itself,  but  also  help  to  break  down  loose  rock  fragments  too  large  for 
the  waves  themselves  to  move. 

These  large  fragments  gradually  become  worn  down  by  the  detritus 
which  is  moved  back  and  forth  over  them,  until  they  are  finally  small 
enough  for  the  waves  to  handle  and  hurl  about,  using  them  in  turn  as 
cutting  tools. 

The  effectiveness  of  the  waves  will  depend  on  their  strength,  and 
also  on  the  concentration  of  their  blows.  The  former  is  dependent 
on:  (1)  Strength  of  wind,  (2)  depth  of  water,  and  (3)  expanse  of  water 
across  which  wind  can  sweep.  The  latter  is  dependent  on  the  slope 
of  the  surface  against  which  they  break. 

Stevenson  and  Harcourt  give  numerous  examples  of  the  power  of  waves  in  deep 
harbors  and  exposed  situations.  Examples  of  wave  action  on  our  own  more  pro- 
tected coasts  and  in  our  shallow  waters  are  not  so  numerous.  In  the  report  to  the 
Chief  of  Engineers  of  1890  on  the  Improvement  of  St.  Augustine  Harbor  is  the 
following: 

"  A  wave  may  act  on  the  jetty  directly,  by  a  blow,  or  a  push,  or  a  blow  and  a 
push  combined;  and,  indirectly,  by  a  pull,  by  compressing  the  air  in  the  voids  of 
the  masonry,  by  upward  pressure  due  to  the  difference  of  head  produced  on  the  two 
sides  of  the  jetty,  or  by  a  combination  of  these  actions.  The  direct  action  measured 
on  the  dynamometers  had  effects  equal  to  pressures  varying  between  190  and  753 
pounds  per  square  foot.  This  action  took  place  when  a  wave  broke  directly  on  or 
in  advance  of  the  jetty;  this  also  compressed  the  air  in  the  voids  of  the  jetty.  .  .  . 
Jets  of  water  and  sand  were  sometimes  projected  up  from  the  cracks  in  the  jetty  to 
some  height.  The  maximum  height  of  any  wave  observed  striking  the  work  was 
6  ft.  .  .  .  Up  to  a  height  of  about  2  feet  above  mean  low  water,  rip-rap  weighing 
40  to  50  Ibs.  was  but  little  disturbed.  Above  this  limit,  to  the  height  of  10  ft.,  the 
highest  point  observed,  rip-rap  varying  in  weight  from  40  to  200  Ibs.  could  not  be 


WAVE  ACTION   AND  SHORE  CURRENTS,   ETC.  365 

held  at  any  slope.  An  isolated  piece  of  concrete  weighing  350  Ibs.  and  resting  on 
its  flat  base  0.7  sq.  ft.  in  area,  with  its  center  of  gravity  7  ft.  above  mean  low  water, 
was  moved  several  feet  by  breakers  whose  crests  were  above  1\  ft.  above  mean  low 
water.  These  breakers  measured  3^  ft.  from  hollow  to  crest.  ...  All  that  portion 
of  a  mound  or  whig,  composed  of  rip-rap  (varying  in  weight  from  40  to  220  Ibs.) 
tightly  chinked  with  oyster  shells,  lying  between  4  and  6  ft.  above  mean  low  water, 
no  matter  what  side  slopes  the  rip-rap  was  given,  would  be  carried  away  in  a  single 
tide  whenever  breakers  greater  than  4  ft.  in  height  struck  it  fairly.  ...  A  block  of 
concrete  weighing  527  Ibs.  was  elevated  1.3  ft.  by  the  action  of  a  single  breaker. 
During  the  same  tide  it  was  moved  23  ft.  in-shore.  A  dynamometer  within  8  ft. 
of  its  original  position  recorded  a  maximum  pressure  of  575  Ibs.  per  square  foot 
during  this  tide.  A  piece  of  concrete  weighing  200  Ibs.  was  lifted  vertically  to  a 
higher  level  than  that  of  the  water  surface  by  a  wave  which  broke  just  in  front  of 
it.  Another  block  of  concrete  weighing  1,600  Ibs.  was  lifted  from  its  bed  verti- 
cally at  least  14  his.,  and  then  moved  several  yards." 

Later,  a  concrete  block  10  X  6  X  2|  ft.  and  weighing  dry  21,000  Ibs.,  lying  at 
about  the  mean  low  water  line  of  the  beach,  was  lifted  vertically  3  ins.,  and  there 
caught  and  held  fast.  The  maximum  wave  height  and  dynamometer  reading  dur- 
ing that  gale  were  5.5  ft.  and  633  Ibs.,  respectively. 

Another  familiar  effect  of  wave  action  was  shown  at  Sandy  Hook  where  a  line  of 
rip-rap  placed  at  the  ordinary  high  water  line,  composed  of  blocks  weighing  from 
300  Ibs.  to  3  tons,  was  undermined  and  sunk  into  the  sand  from  4  to  6  ft.  It  may 
be  stated  generally,  that  where  an  obstruction  is  placed  on  a  sand  beach  between 
high  and  low  water,  if  it  is  too  heavy  to  be  moved,  the  resultant  effect  will  be  to 
smooth  off  the  beach,  either  by  sinking  the  object,  or  by  building  the  beach  over  it. 

Vertical  range  of  wave  action.  —  The  range  of  wave  erosion  is  as 
restricted  vertically  as  it  is  horizontally,  but  it  may  be  extended  some- 
what by  the  rise  and  fall  of  the  tide.  The  efficient  impact  of  the  wave 
is  limited  by  the  crest  above  and  the  trough  of  the  wave  below.  The 
range  indirectly,  however,  is  often  great,  being  limited  by  the  height  of 
the  shore  only,  for  by  the  under-mining  of  a  cliff,  a  considerable  mass 
of  material  may  be  brought  down.  This  fallen  mass  will  temporarily 
protect  the  shore  against  wave  action,  until  it  is  broken  up  and  dis- 
posed of.  Frost  also  dislodges  more  or  less  rock  and  soil  from  the  face 
of  sea  cliffs. 

Recession  of  coast.  —  As  a  result  of  wave  attack,  the  sea  some- 
times encroaches  on  the  land,  and  protection  walls  are  necessary  in 
order  to  prevent  the  destruction  of  buildings,  roads,  railway  tracks,  etc. 

This  recession  may  be  especially  rapid  on  sandy  coasts,  such  as  that 
of  New  Jersey,  and  many  different  forms  of  walls,  bulkheads  and  jetties 
have  been  constructed  by  riparian  owners  with  varying  results.  In 
some  cases  failure  is  due  to  improper  type  of  protective  work,  in  other 
cases  it  may  be  due  to  lack  of  concerted  effort  at  different  points  along 
the  shore. 


366  ENGINEERING  GEOLOGY 

Incidentally,  it  has  been  found  that  conditions  at  a  given  point  some- 
times become  reversed,  so  that  erosion  stops  and  deposition  begins. 
Thus  Haupt1  describes  a  case  at  Hereford  Inlet  and  Five-mile  Beach, 
where  he  states  that  "a  substantial  bulkhead,  built  about  1890  at  Five- 
mile  Beach  to  protect  Wildwood  and  Holly  Beach,  was  carried  away, 
and  that  the  island  had  wasted  some  600  feet,  when,  without  apparent 
cause,  the  sea  began  to  deposit  and  the  beach  to  gain  its  former  position. 
Upon  investigation  as  to  the  cause,  it  was  found  that  Hereford  Inlet 
(Plate  LIII),  at  the  head  of  the  island,  had  drifted  to  the  south  1000 
feet  in  the  past  ten  years  and  the  shore  in  front  of  Anglesea  had  ad- 
vanced some  1500  feet  in  consequence.  This  deposit  projected  the 
ebb  tide  farther  out  and  caused  an  eddy  current  which  cut  away  the 
beach  to  the  southwest  of  the  shoal,  as  was  the  case  at  Absecon  Beach 
prior  to  1850,  when  the  shore  was  close  to  Pacific  Avenue." 

Wave  Cut  Topography 

Cliff  and  terrace.  —  Waves  cutting  into  and  undermining  the 
shore  at  the  water  level  develop  a  sea  cliff,  whose  slope  will  depend  on 


Sea  level 


FIG.  179.  —  Section  of  wave-cut  terrace  in  gentle  slope.     (After  Gilbert.) 

the  character  of  material  and  rate  of  cutting,  and  where  height  will 
depend  on  the  height  of  the  land  on  which  the  sea  advances. 

At  the  base  of  the  sea  cliff  there  is  a  submerged  shelf,  covered  by 
shallow  water,  the  wave-cut  terrace.  (Fig.  179.) 

In  some  cases  the  land  has  been  elevated  since  the  terrace  was  cut, 
so  that  it  is  now  preserved  as  a  bench  above  sea  level,  as  for  example  on 
the  coast  of  southern  California. 

Coast  outline.  —  The  outline  of  a  coast  as  developed  by  wave 
erosion  depends  on  the  character  of  the  rock,  its  structure,  and  original 
outline.  The  following  cases  may  be  cited: 

1.  A  regular  coast,  equally  exposed,  but  of  unequally  resistant  mate- 
rial, is  made  more  irregular  by  wave  action,  resulting  in  the  develop- 

1  N.  J.  Geol.  Surv.,  Ann.  Kept.,  1905,  p.  70,  1906.  The  same  report  contains 
many  other  examples. 


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368 


ENGINEERING  GEOLOGY 


ment  of  headlands  where  the  rock  is  hard,  and  indentations  or  bays 
where  the  materials  are  soft,  or  much  fractured,  so  as  to  be  easily  eroded. 


>  T>  _* x    I    ^- 

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g>'$'~J&       >,, 

^/I'^viSa  "-s 

i~^7  !_~^  ^- '  -  ^"^^  ^^-  Seajercl 


FIG.  180.  —  Section  of  wave-cut  terrace  on  steeply  sloping  coast.     (After  Gilbert, 
U.  S.  Geol.  Survey,  5th  Ann.  Kept.) 

2.  A  regular  coast,   unequally  exposed,   but  of  uniform  material, 
becomes  more  irregular. 

3.  An  irregular  coast,  of  uniform  or  homogenous  material,  becomes 
more  regular. 

Transportation  by  shore  currents.  —  The  incoming  waves  tend  to 
shift  material  toward  the  shore,  especially  inside  the  line  of  breakers, 
while  the  undertow  tends  to  carry  it  out  (seaward)  again.  If  the 
waves  strike  the  shore  obliquely,  the  particles  of  sediment  follow  a  zig- 
zag path  along  shore  —  the  direction  of  littoral  or  shore  current.  Coarse 
materials  accumulate  where  the  disturbance  of  the  water  is  greatest, 
while  finer  material  is  moved  even  when  the  water  is  less  agitated. 

The  coarse  material  covering  the  bottom,  in  shallow  water  along 
shore,  or  where  agitation  reaches  the  bottom,  is  termed  the  shore  drift. 
It  may  include  either  material  derived  by  wave  action  or  that  delivered 
to  the  sea  by  streams. 

Shore  Deposition  Topography 

Beach  and  Barrier.  —  The  beach  (Fig.  181)  is  the  belt  or  zone  occupied 
by  the  moving  shore  drift,  and  it  may  have  a  variable  width.  The 
upper  margin  is  the  level  reached  by  storm  waves;  its  lower  margin 
is  slightly  beyond  the  breaker  line  of  storm  waves.  While  the  beach 
follows  the  water  and  land  boundary  in  a  general  way,  still  it  does  not 
conform  to  all  the  minor  irregularities,  such  as  indentations  and  pro- 
jections. 

If  the  slope  of  the  coast  is  flat,  then  the  undertow  is  weaker  than 
the  shoreward  movement  of  the  waves,  and  the  material  is  shifted 


PLATE  LIV,  FIG.  1.  —  Cliffs  formed  by  wave  action,  Sydney,  C.  B.     (H.  Hies, 

photo.) 


FIG.  2.  —  Beach  and  sand  dunes  formed  by  wave  and  wind  across  harbor  of  Inver- 
ness, N.  S.     (H.  Ries,  photo.) 

(369) 


370 


ENGINEERING  GEOLOGY 


shoreward,  being  cast  up  near  the  waters  edge  and  forming  a  beach 
ridge  (Fig.  181). 

If  the  sea  or  lake  bottom  near  shore  has  a  very  gentle  slope,  the  waves 
break  some  distance  out  from  the  shore  line,  and  it  is  at  this  point  of 


FIG.  181.  —  Section  of  a  beach  ridge.     (After  Gilbert.) 

greatest  agitation  that  deposition  takes  place,  and  a  ridge  may  be  built 
up  known  as  a  barrier  beach  (Fig.  182).     If  now  the  storm  waves  build 

b  Sea  Level 


FIG.  182.  —  Section  of  a  barrier  beach;  b,  barrier;  I,  lagoon.     (After  Gilbert.) 

this  up  above  the  water  surface,  a  lagoon  is  formed  between  the  barrier 
and  the  main  land,  which  may  eventually  become  filled  by  sediment, 
(Plate  LV).  The  lagoon  at  one  stage  of  filling  becomes  a  marsh. 
Most  of  the  material  is  washed  in  from  the  land,  but  some  may  be 
brought  in  by  tidal  currents. 

Barrier  beaches  are  not  only  liable  to  shift  (Fig.  183),  but  are  some- 
times of   considerable  width.     At  Atlantic  City   (Plate  LV),  on  the 


Sea  Level 


FIG.  183.  —  Section  of  a  barrier  beach  which  has  moved  inland,  part  way  across  a 
marshy  lagoon.  6,  barrier;  m,  marsh;  p,  peat;  d,  dune.  (After  Goldthwait, 
111.  Geol.  Survey,  Bull.  1,  1908.) 

coast  of  New  Jersey,  a  barrier  one  mile  broad  has  formed  and  at  present 
is  growing  on  the  seaward  side,  although  formerly  it  was  eroded  at 
different  periods. 

The  sand  which  is  piled  high  on  either  a  beach  or  barrier  is  not  allowed 
to  rest,  but  is  carried  by  the  wind  and  heaped  up  to  form  sand  dunes 


I 

3 
02  C? 


o 


fcfi 

I 
I 


372 


ENGINEERING   GEOLOGY 


(Chapter  II).  An  artificial  barrier  will  sometimes  cause  a  dune  10  feet 
high  to  build  up  in  one  season. 

Spits,  hooks,  and  bars.  —  The  littoral  or  shore  current  does  not 
follow  the  larger  indentations  of  the  coast.  In  maintaining  its  course 
across  the  mouth  of  a  bay,  the  current  may  pass  into  deeper  water. 
This  would  result  in  checking  the  velocity  of  the  current,  and  the  dep- 
osition of  a  part  at  least  of  the  sediment  it  was  carrying. 

The  deposited  material  assumes  the  form  of  a  submerged  ridge, 
usually  narrow,  across  the  mouth  of  the  bay,  and  is  termed  a  spit  (Fig. 


FIG.  184.  —  Sketch  map  showing  the  development  of  a  hooked  spit. 
thwait,  111.  Geol.  Survey,  Bull.  7,  1908.) 


(After  Gold- 


FIG.  185  —  Sketch  map  showing  a  bay  enclosed  by  a  pair  of  overlapping  spits.  The 
arrows  indicate  the  direction  of  the  wind-driven  currents.  (After  Goldthwait, 
111.  Geol.  Survey,  Bull.  7,  1908.) 

184  and  Plate  LVI),  so  long  as  it  is  free.  As  the  level  of  the  ridge  is 
built  up  towards  the  water  surface,  it  comes  within  the  zone  of  agitation 
of  the  waves,  and  by  these  it  may  be  built  up  above  the  surface  of  the 


374  ENGINEERING  GEOLOGY 


PLATE  LVII.  —  Shows  simplification  of  shore  line  by  deposition  (and  subordinately 
by  erosion),  especially  on  southern  shore  of  Martha's  Vineyard  west  pf  Katama 
Bay,  where  the  coastal  bars  deposited  by  waves  and  shore  currents  have  closed 
in  a  series  of  bays,  converting  them  into  ponds. 

Little  water  enters  these  ponds,  but  what  does,  finds  its  way  into  the  sea  by 
seepage  through  the  sand  and  gravel  of  the  beach.  The  only  permanent  stream 
is  that  entering  Tisbury  Great  Pond,  and  the  inflowing  water  seems  to  be 
sufficient  here  to  keep  an  outlet  across  the  beach.  Bars  or  beaches  seem 
to  be  in  process  of  development  south  of  Katama  Bay,  and  may  in  time  connect 
with  each  other  unless  the  tidal  flow  between  Edgartown  Harbor  and  Katama 
Bay  is  sufficiently  strong  to  keep  the  passage  open.  The  material  for  build- 
ing the  bars  was  probably  cut  from  points  of  land  which  formerly  projected 
into  the  water.  (U.  S.  Geol.  Survey.) 


376  ENGINEERING  GEOLOGY 


PLATE  LVIII.  —  This  represents  a  coast  line  showing  both  erosion  and  deposition. 
Near  the  southern  border  of  the  area  there  is  a  steep  though  low  cliff,  denoting 
wave  erosion.  Along  the  greater  part  of  the  coast,  however,  there  has  been 
recent  deposition  by  waves  and  shore  currents,  and  the  wind  has  piled  up  the 
beach  into  sand  dunes.  The  bar  which  shuts  in  Morro  Bay  appears  to  have 
been  deposited  by  currents  drifting  northward.  The  bar  has  been  built  in  a 
line  which  is  a  more  or  less  direct  continuation  of  the  shore  line  to  the  south. 
North  of  Morro  Bay  also,  deposition  is  going  on,  and  the  land  is  apparently 
advancing  on  the  sea.  At  the  head  of  Morro  Bay  a  delta  is  in  process  of  build- 
ing. 

Morro  rock  is  an  island  presumably  isolated  from  the  main  land  by  sub- 
sidence, by  wave  erosion,  or  by  both  subsidence  and  wave  erosion.  The  drain- 
age entering  the  bay  tends  to  prevent  the  completion  of  the  bar.  (U.  S.  Geol. 
Survey.) 


SCALE  OF  MILES 


(377) 


378  ENGINEERING  GEOLOGY 


PLATE  LIX.  —  This  shows  the  development  of  coastal  irregularities  through  proc- 
esses which  will  ultimately  result  in  coastal  simplification.  At  the  extreme 
south  the  sea  cliff  indicates  wave  erosion.  From  this  point  debris  has  been 
shifted  northward,  building  the  beach  which  terminates  at  the  north  in  Sandy 
Hook.  The  tendency  of  the  hook  to  turn  westward  is  the  result  of  its  position. 
The  waves  from  the  open  ocean  to  the  east  have  a  strong  westward  sweep 
when  winds  and  tides  favor. 

The  surface  of  the  deposits  made  by  the  waves  and  shore  currents  has  been 
notably  modified  by  the  wind,  which  has  developed  long  ridge-like  or  mound- 
shaped  dunes. 

At  the  northwest,  beach  deposition  is  also  in  progress,  straightening  the 
coast  line  northwest  of  Port  Monmouth.  Just  east  of  that  place  filling  by 
sedimentation  and  by  growth  of  vegetation  is  building  out  the  coast  line.  This 
region  is  now  protected  from  strong  waves  by  Sandy  Hook.  The  north  border 
of  the  Highlands  of  Navesink  is  marked  by  cliffs,  but  to  the  east  lowlands 
have  been  developed  by  deposition  at  the  base  of  the  cliffs.  These  cliffs  were 
doubtless  cut  by  wave  erosion  before  Sandy  Hook  had  its  present  position. 
The  Hook  now  protects  the  Highlands  against  the  waves. 

The  bays  marked  as  Navesink  River  and  Shrewsbury  River  form  one  of  the 
most  conspicuous  features  of  the  map.  These  bays  or  estuaries  are  probably 
the  result  of  recent  subsidence  of  the  area.  The  subsidence  has  drowned  the 
lower  ends  of  the  rivers,  converting  them  into  bays.  The  building  of  the  bar 
across  their  mouths  is,  therefore,  another  illustration  of  the  process  of  coast 
simplification.  (U.  S.  Geol.  Survey.) 


SANDY       HOOK      BAY 


(379) 


380 


ENGINEERING  GEOLOGY 


water.  Spits  are  also  at  times  built  out  from  projecting  spurs  of  the 
coast  line. 

A  strong  current,  even  of  temporary  character  flowing  past  the  end 
of  the  spit,  may  cause  it  to  curve  into  a  hook  (Figs.  184  and  185,  and 
Plate  LVI),  and  this  will  occasionally  change  its  position  because  of 
change  in  the  direction  of  the  wind. 

If  the  spit  completely  crosses  a  bay  and  becomes  tied  to  the  opposite 
shore  it  is  called  a  bar,  and  many  lakes  have  been  formed  by  the  up- 
building of  a  bar  across  the  mouth  of  a  bay.  Bars  sometimes  tie  islands 
to  the  mainland. 

Conditions  are  frequently  quite  different  where  an  active  stream 
enters  the  bay,  for  then  the  outflow  from  the  bay  may  be  strong  enough 
to  prevent  the  completion  of  the  bar. 

At  other  times  the  growth  of  the  raised  spit  across  a  bay  may  gradu- 
ally shift  the  stream  channel  towards  the  farther  side  of  the  bay,  con- 
sidering direction  of  shore  drift.  If  the  ridge  building  still  encroaches 


Sea  level 


FIG.  186.  —  Section  of  a  bar.     (After  Gilbert.) 

on  the  stream  channel  the  latter  may  break  through  the  spit  at  another 
point,  but  if  the  stream  is  completely  blocked  the  water  may  seep  out 
through  the  beach  gravels. 

Plate  LIV,  Fig.  2,  shows  an  interesting  occurrence  at  Inverness, 
Nova  Scotia.  Here  the  small  harbor  which  was  to  have  been  used  for 
shipping  coal  from  the  neighboring  mines  became  completely  closed 
by  a  bar  of  shore  drift  from  the  north.  The  stream  flow  was  too  weak 
to  keep  a  channel  way  open  across  the  bar,  and  dredging  was  equally 
ineffective.  Jetties  which  were  constructed  became  buried  in  the 
drifting  sand.  In  addition  the  wind  picked  up  the  sand  from  the  upper 
edge  of  the  beach  and  piled  it  into  dunes. 

Tidal  scour  is  another  factor  tending  to  maintain  a  channel  way 
(thorofare)  across  a  spit  or  barrier  beach.  Sediment  brought  in  by  the 
tidal  current  is  sometimes  deposited  inside  the  entrance  forming  a 
shoal,  which  is  obstructive  to  navigation. 

Although  shore  drift  may  move  in  opposite  directions  at  different 


WAVE  ACTION  AND  SHORE  CURRENTS,  ETC.  381 

times,  there  is  usually  a  positive  resultant  in  one  direction,  and  the 
determination  of  this  is  of  importance  in  bar  improvement. 

The  tidal  wave  can  produce  a  current  which  is  separate  from  the 
littoral  (shore)  current. 

Problems  of  Harbor  and  River  Mouth  Improvement 

Relation  of  wave  and  shore  current  work  to  harbors.  —  From 
what  has  just  been  said  the  engineer  will  readily  observe  that  harbors 
can  be  closed  or  silted  up,  or  bars  formed  which  shallow  the  channel, 
and  hence  in  many  cases  preventive  measures  are  necessary  to  combat 
the  work  of  wind  and  waves.  The  formation  of  these  features  of  shore 
and  sea-floor  topography  are  not  the  only  things  that  have  to  be  con- 
sidered. 

Equally  important  to  recognize  is  the  fact  that  many  of  them  are  of 
very  temporary  character.  Spits  and  bars  shift  at  times  with  remark- 
able rapidity  as  a  result  of  storms.  One  storm  may  close  up  a  thorofare 
at  one  point  and  open  up  a  new  one  some  distance  away. 

Haupt  says  that  the  "best  channel  across  an  outer  bar/'  in  a  state  of 
nature,  will  shift  to  leeward,  and  that  immediately  abreast  of  the  inlet 
or  "gorge, "  both  inside  and  out,  the  deposits  of  sand  will  be  the  greatest, 
while  a  "flood  channel"  will  be  formed  in  many  instances  seaward 
of  the  "windward  spit,"  and  "swash"  channels  may  be  formed  between 
it  and  the  main  or  ebb  channels.  In  tidal  inlets  the  full  prism  of  the 
flood  should  be  admitted  to  the  inner  bays,  so  that  the  ebb  may  have 
sufficient  volume  to  maintain  the  size  of  the  entrance. 

The  coast  of  New  Jersey  affords  some  excellent  examples  of  the  above, 
and  the  following  quoted  from  the  State  Geologist's  report  is  highly 
illustrative.1 

"  The  coast  of  New  Jersey  from  Sandy  Hook  to  Cape  May  is  of  great  importance 
in  many  respects.  It  forms  the  southern  approach  to  New  York  Harbor,  and  the 
large  tonnage  between  New  York  and  all  West  Indian,  Gulf,  South  and  Central 
American  ports  passes  close  at  hand.  Although  from  Sandy  Hook  to  Delaware 
Breakwater  is  only  134  miles,  records  of  the  United  States  Life-Saving  Service  show 
more  disasters  on  this  coast  than  on  any  other  of  equal  extent  on  the  United  States. 

"  This  danger  is  greatly  increased  by  the  absence  of  any  harbors  of  refuge  along 
the  coast,  and  the  shallow,  tortuous,  shifting  channels  at  the  various  inlets.  The 
presence  of  well-defined  and  fixed  channels  at  several  inlets  along  the  coast  would 
go  far  towards  eliminating  these  dangers. 

"  It  is  difficult  to  one  unfamiliar  with  the  action  of  waves  and  currents  to  ap- 
preciate what  great  changes  may  be  wrought  upon  the  beaches  during  even  a  single 
storm,  or  by  slow  accretion  at  one  locality  and  equally  slow  wasting  at  another. 

1  N.  J.  Geol.  Survey,  Ann.  Rept.,  1905. 


382 


ENGINEERING  GEOLOGY 


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WAVE  ACTION  AND  SHORE  CURRENTS,   ETC.  383 

The  channels  of  the  inlets  are  constantly  changing  in  depth  and  location  through 
certain  cycles  and  the  inlets  themselves  are  slowly  shifting  in  position. 

••  Much  money  has  been  spent,  with  comparatively  little  result,  due  in  part  to 
the  absence  of  any  concerted  action  embracing  considerable  areas  and  in  part  to 
improper  plans  followed." 

Case  of  Manasquan  Inlet  (Plates  LXI  and  LXII).1  —  "With  the  exception  of 
Shark  River,  Manasquan  Inlet  is  the  only  opening  remaining  in  the  fifty-three  mile 
stretch  of  coast  from  Sandy  Hook  to  Barnegat  Light.  It  has  ruling  depths  at  low 
water  of  about  three  feet  or  less,  with  constantly  shifting  channels,  which  have  at 
intervals  been  entirely  closed. 

"  It  has  a  drainage  area  of  80.5  square  miles,  while  the  immediate  tidal 
basin  covers  almost  exactly  2  square  miles.  With  this  small  ratio  between  land  and 
sea-water  area,  and  with  the  mouth  closed  by  a  bar,  it  would  be  only  a  short  time 
before  the  6-mile  reach  constituting  the  basin  would  be  converted  into  a  freshwater 
lake,  with  an  uninterrupted  outflow  to  the  great  detriment  of  the  country.  To 
preserve  its  maritime  features,  it  is  vital  that  as  much  sea  water  be  admitted  as 
possible,  and  to  prevent  gradual  shoaling,  the  tidal  currents  should  be  maintained 
to  their  fullest  extent,  without  causing  too  great  velocity  in  the  engorged  sections. 

"  These  results  will  be  accomplished  best  by  admitting  as  large  a  volume  of  the 
flood  tide  at  the  gorge  as  may  be  consistent  with  the  physical  conditions  and  riparian 
interests,  and  at  the  same  time  by  so  guiding  and  concentrating  the  movements  of 
the  ebb  tide  as  to  create  a  scour  and  consequent  deepening  across  the  outer  bar. 
Works  which  throttle  the  tides  and  violate  the  fundamental  condition  of  letting  hi 
the  largest  available  amount  of  water  in  order  to  maintain  the  currents  which  are 
the  main  factors  in  cutting  out  the  channels  at  ebb  tide  must  result  only  in  injury. 

"  The  physical  features  at  this  inlet  are  typical  and  suggestive  of  the  appropriate 
remedy.  Here  are  found  the  prevailing  northerly,  littoral  drift;  the  angular  wave 
movement,  also  working  to  the  north;  the  inwardly-curving  south  spit;  the  large 
inner,  middle  ground;  the  cresent-shaped  outer  bar,  lying  close  in  shore;  the  diverse 
channels  for  the  flood  and  ebb  movements,  and  the  deep  holes  or  pockets  caused  by 
the  reaction  of  the  currents  upon  obstructing  barriers  of  sand  or  wood. 

"  Reference  to  the  accompanying  map  (Plate  LXIII)  will  show  a  jetty  on  the  north 
side  about  1500  feet  in  length,  one  of  two  built  by  the  United  States  government 
in  1882.  That  portion  of  the  inlet  and  bay  adjacent  to  the  jetty  has  an  average 
width  of  nearly  1000  feet  and  contains,  approximately,  35  acres,  of  which  about 
13  acres  are  bare  at  low  water,  while  the  balance  is  too  shallow  for  anchorage  for 
coasters. 

"  Near  the  outer  end  of  the  jetty,  there  is  a  small  pocket  having  a  depth  of  10 
feet  for  a  width  of  about  100  feet. 

"  The  south  spit  of  sand  projecting  into  the  throat  of  the  inlet  and  forming  what 
is  known  as  the  gorge,  is  increasing  in  extent  and  curving  inward  by  the  deposits 
carried  up  by  the  flood  tide  so  that  at  low  water,  the  channel  is  reduced  to  only 
260  feet  in  width  and  its  cross-sectional  area  is  but  1260  square  feet. 

"  With  a  3-foot  tide,  this  sectional  area  would  be  doubled,  but  the  duration  of 
high  water  is  very  short  and  the  current  velocities  are  reduced  to  nothing.  By  the 
improvement  proposed  and  shown  on  the  plan,  this  cross-section  at  "  D-E,"  as  well 
as  the  one  on  the  crest  of  the  outer  bar,  will  be  more  than  doubled,  thus  admitting 

1  Quoted  from  report  by  L.  M.  Haupt,  N.  J.  Geol.  Survey,  Ann.  Rept.,  1907,  p.  72, 
1908. 


384 


ENGINEERING  GEOLOGY 


WAVE  ACTION  AND  SHORE  CURRENTS,   ETC.  385 

more  water  at  each  tide,  increasing  the  velocity  of  the  currents  and  creating  greater 
and  more  permanent  depths  of  channel. 

"  The  south  spit  and  inner-middle  ground  not  only  interfere  with  the  incoming 
tide,  with  its  load  of  sand,  but  by  checking  its  velocity  they  invite  deposits  in  the 
harbor." 

As  evidences  of  the  changes  at  this  inlet  reference  should  be  made  to  Plate  LXI, 
which  is  from  the  U.  S.  Government  chart  for  1878  and  shows  three  positions  for  the 
inlet,  from  which  it  appears  that  the  mouth  was  drifting  southward. 

Relation  of  bars  to  rivers.  —  Bars1  are  found  at  the  mouths  of 
many  rivers,  and  may  be  built  up  in  part  of  river  sediment  and  in  part 
of  sediment  brought  in  by  waves  and  tidal  currents. 

If  a  sediment-laden  river,  like  the  Mississippi,  Nile  or  Amazon,  enters 
directly  into  the  sea  or  lake,  checking  the  velocity  of  the  current  as  it 
meets  still  water,  will  cause  it  to  drop  its  load  of  sediment,  thus  forming 
a  bar. 

On  the  other  hand,  bars  at  the  outlets  of  lagoons  or  bays  which  empty 
into  tidal  seas,  and  which  receive  the  flow  of  a  river,  are  caused  chiefly 
by  the  action  of  the  winds  and  waves,  which  drive  material  into  and 
across  the  mouth.  The  tidal  currents,  however,  keep  the  mouth  from 
being  closed.  In  such  cases,  little  actual  river  silt  probably  reaches 
the  bar. 

In  the  case  of  rivers  which  discharge  through  a  tidal  estuary,  the  bar 
may  be  due  to  conflict  of  ebb  and  flood  currents  at  the  outfall,  which 
cause  eddies  and  still  water;  or  to  the  difference  in  duration  of  their 
scouring  action;  or  to  waves  and  sand  drift  along  the  shore. 

"  The  operation  of  the  laws2  governing  the  formation  and  the  improve- 
ment of  the  outlets  of  rivers  and  tidal  harbors  is  usually  complex  and 
difficult  of  any  close  analysis.  The  forces  at  work  are  generally  many 
and  varied,  and  while  the  effect  of  a  single  one  upon  a  plan  for  improve- 
ment might  be  foretold,  their  action  in  combination  can  only  be  approx- 
imated. 

"There  are,  for  example,  as  just  mentioned,  the  transportation  and 
deposit  of  sediment,  present  in  most  rivers;  the  effect  of  floods  and  tides; 
the  presence  or  absence  of  currents  along  the  coast;  and  the  gradual 
effects  of  storms  and  the  drift  of  shore  material,  which  with  small 
rivers  may  change  the  outlet  entirely,  as  with  the  Yare  River  on  the  east 
coast  of  England,  where  the  outlet  was  driven  south  4  miles  in  the 
course  of  years,  and  at  Aransas  Pass  in  America,  which  has  moved  to 
the  southwest  about  a  mile  in  the  past  50  years.  In  some  cases  such 

1  For  detailed  discussion  of  this  subject  see  Thomas  and  Watt,  Improvement  of 
Rivers,  2nd  ed.,  part  1,  p.  309,  1913.     (Wiley  &  Sons.) 

2  Thomas  and  Watt,  I.e.,  p.  310. 


II 


FIG.  1 


Oct.  8 


8.5 


Nov.  27, 


FIG.  2 


FIG.  3 


PLATE  LXII.  —  Maps  showing  changes  at  the  mouth  of  Manasquan  Inlet,  in  the 
year  1907.  Fig.  1,  Sept.  6;  Fig.  2,  Oct.  8;  Fig.  3,  Nov.  27.  (After  Haupt, 
N.  J.  Geol.  Survey,  Kept.,  1907.) 

(386) 


WAVE  ACTION  AND  SHORE  CURRENTS,   ETC. 


387 


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388  ENGINEERING  GEOLOGY 

causes  produce  daily  changes  in  the  channel,  as  with  the  Hoogly,  where 
ships  can  navigate  only  in  daytime  and  by  constantly  taking  soundings. " 

However,  close  study  of  the  charts  of  different  periods  may  indicate 
the  existence  of  certain  persistent  forces  at  work,  a  knowledge  and 
recognition  of  which  will  enable  the  engineer  to  attack  the  problem 
more  intelligently. 

Rivers  which  enter  tidal  estuaries  have  to  be  treated  differently 
from  those  which  have  non-tidal  outlets,  without  shore  currents,  or 
where  these  currents  are  slight. 

Improvement  of  tidal  rivers.  —  In  improving  tidal  rivers  the  following  principles 
have  to  be  borne  in  mind  :l 

(1)  "  The  tidal  flow  should  be  admitted  freely  up  the  river  as  far  as  possible  in 
order  to  reduce  period  of  slack  water  to  a  minimum.     In  this  way  the  area  of  in- 
evitable deposits  is  enlarged  and  there  is  hot  an  excessive  accumulation  at  one 
point  in  the  channel  when  the  fresh-water  discharge  is  small.     Moreover  the  volume 
of  tidal  water  flowing  from  the  outlet  is  increased. 

(2)  The  fresh-water  discharge  should  be  maintained  as  large  as  possible,  and 
not  abstracted  for  canal  and  other  purposes  so  that  it  can  have  the  fullest  possible 
effect  in  reinforcing  the  ebb  throughout  the  whole  of  the  tidal  course  of  the  river 
and  thus  keep  the  channel  scoured. 

(3)  The  form  of  the  estuary  should  be  regular  if  possible  so  as  to  enlarge  gradually 
as  it  approaches  the  sea,  and  thus  promote  regularity  of  flow  without  restricting  the 
tidal  capacity  above  the  outlet." 

This  is  sometimes  accomplished  by  low  training  banks  which,  while  directing 
and  concentrating  the  latter  half  of  the  ebb,  do  not  materially  impede  the  admission 
of  the  flood  tide  up  the  estuary.  Where  the  estuary  is  very  wide  and  irregular  and 
the  main  river  channel  through  it  is  very  tortuous  and  shifting,  high  embankments 
may  be  formed  on  each  side  widening  out  towards  the  sea  and  the  land  behind 
them  reclaimed. 

In  non-tidal  rivers  or  those  where  the  range  of  tide  is  very  small,  the  principles 
governing  them  are  somewhat  different  because  of  the  lack  of  tidal  influences  capable 
of  affecting  the  maintenance  of  their  outlets,  and  because  of  the  difference  in  form 
of  the  mouths  themselves. 

Here  the  stream  flow  is  always  in  the  same  direction  and  on  being  checked  at  its 
mouth  deposits  sediments,  thus  gradually  building  up  a  bar  which  forces  the  water 
in  various  directions  through  separate  outlets  across  the  foreshore  and  forms  what  is 
known  as  a  delta.2  The  development  of  several  arms  or  outlets  tends  to  reduce  the 
scouring  effect  of  the  currents  and  the  channels  become  too  shallow  for  navigation 
by  reason  of  the  deposit  of  material  brought  down  by  the  river. 

Thus  deltas  gradually  extend  into  the  sea  as  this  material  is  progressively  de- 
posited at  the  mouths  of  the  outlets.  Mr.  Vernon-Harcourt  lays  down  the  fol- 
lowing principles  for  improving  non-tidal  outlets  without  shore  currents  or  where 
the  currents  are  very  slight. 

"  (1)  The  only  method  of  deepening  the  outlet  of  sediment-bearing  rivers  flow- 

1  Mr.  Vernon-Harcourt  quoted  by  Thomas  and  Watt,  Improvement  of  Rivers, 
Vol.  1,  p.  310. 

2  See  Chapter  on  Rivers. 


WAVE  ACTION  AND  SHORE  CURRENTS,   ETC.  389 

ing  into  tideless  seas  is  to  prolong  one  of  their  delta  channels  by  parallel  jetties  out 
to  the  bar,  so  that  the  prolonged  current,  being  concentrated  across  the  bar,  may 
scour  a  deeper  channel,  and  carry  its  burden  of  sediment  into  deep  water  further  out. 

(2)  One  of  the  minor  outlets  should  be  selected  for  improvement,  if  its  delta 
channel  is  adequate,  or  can  easily  be  made  adequate  for  the  requirements  of  navi- 
gation;  and  the  discharge  of  the  other  outlets  should  not  be  interfered  with.     The 
advance  of  the  delta  at  one  of  the  minor  outlets  is  slower,  and  the  distance  out  to 
the  bar  is  less,  and  consequently  the  jetty  works  are  less  costly;  whilst  an  increased 
discharge,  produced  by  impeding  the  flow  through  the  other  outlets,  would  also 
increase  the  volume  of  sediment,  and  therefore  quicken  the  rate  of  advance  of  the 
delta,  and  hasten  the  necessity  of  prolonging  the  jetties. 

(3)  The  success  of  the  jetty  system  depends  on  a  rapid  deepening  of  the  sea  in 
front;   on  the  fineness  and  lightness  of  the  sediment  brought  down;   and  on  the  ex- 
istence of  a  littoral  current,  its  velocity,  and  the  depth  to  which  it  extends.     Any 
erosive  action  of  winds  and  waves  along  the  shores  of  the  delta  is  favorable  to  the 
system,  and  also  any  reduction  in  density  of  the  sea-water,  such  as  may  be  found 
in  an  inland  sea. 

(4)  If  the  sea-bottom  is  flat;   if  a  large  proportion  of  the  sediment  is  dense,  so 
that  it  is  carried  along  the  bed  of  the  river  or  close  to  it;    if  the  outlet  faces  the 
prevalent  winds;  and  if  no  littoral  current  exists,  it  is  possible  that  an  improvement 
of  the  outlet  may  not  be  practicable;  and  then  recourse  must  be  had  to  a  side  canal, 
starting  off  from  the  river  some  distance  up,  and  entering  the  sea  beyond  the  influ- 
ence of  the  alluvium  of  the  river. 

(5)  The  bars  in  front  of  the  outlets  of  tideless  rivers  being  formed  by  the  deposit 
from  the  river,  vary  in  form  according  to  the  nature  of  the  sediment  brought  down. 
When  the  material  is  composed  of  particles  of  very  variable  density,  it  is  gradually 
sifted  as  the  velocity  of  the  current  decreases  and  gives  a  flat  sea-slope  to  the  bar. 
When,  on  the  contrary,  most  of  the  material  is  heavy,  the  bar  has  a  flat  river  slope,  as 
in  the  first  case,  formed  by  the  gradual  arrest  of  the  sediment  rolled  along  the  bottom; 
but  as  little  of  the  material  is  carried  beyond  the  crest  of  the  bar,  the  sea-slope  is 
steep. 

(6)  The  jetty  system  does  not  constitute  a  permanent  improvement,  for  sooner 
or  later,  in  proportion  as  the  physical  conditions  are  unfavorable  or  the  reverse,  a 
bar  is  formed  further  out,  and  a  prolongation  of  the  jetties  becomes  necessary. 

The  last  rule  would  not  apply  if  there  were  a  prevailing  wind  which  caused  a 
shore  current  sufficient  to  carry  away  the  silt  as  fast  as  it  was  brought  out  by  the  river. 

The  conditions  at  an  outlet,  moreover,  are  often  complicated  by  the  shifting  of 
the  channel  due  to  the  drift  of  sand  along  the  coast  or  to  disturbances  produced  by 
storms  which  in  exposed  outlets  may  block  up  a  channel  and  cause  a  new  one  to 
open  in  a  very  short  time. 

Where  the  effect  of  the  shore  drift  is  small,  the  general  tendency  of  the  flow 
from  the  outlet  appears  to  be  along  the  shortest  path  to  deep  water,  this  being 
under  ordinary  conditions  the  line  of  least  resistance  and  frequently  nearly  at  right 
angles  to  the  adjoining  coast.  In  many  cases  there  exist  two  main  channels  to 
the  bar,  in  addition  to  the  small  side  or  swash  channels  which  deepen  and  shoal 
alternately  during  the  cycle  of  change  and  shift  their  location  within  a  sector  cover- 
ing an  angle  of  from  45  to  90  degrees. 

This  cycle  of  change  may  be  illustrated  by  taking  as  an  example  an  outlet 
whose  limits  of  change  lie  between  northeast  and  southeast  and  whose  main  channel 


390  ENGINEERING  GEOLOGY 

for  the  time  being  is  the  southerly  one.  After  this  channel  has  continued  in  exist- 
ence for  perhaps  some  years  it  will  begin  to  shoal,  possibly  from  a  single  gale  or  a 
period  of  gales,  possibly  from  more  obscure  causes.  At  the  same  time  the  northerly 
channel  will  begin  to  open,  and  the  closing  of  the  southerly  one  will  continue  until 
it  has  become  valueless  for  shipping.  Usually  more  or  less  change  of  intermediate 
location  occurs  during  this  period,  the  channels  sometimes  wandering  over  a  con- 
siderable portion  of  their  field  before  the  final  shoaling  or  opening  occurs.  The 
northerly  channel  will  then  pass  through  a  period  as  did  the  other  and  may  shift 
further  north  deteriorating  until  the  natural  forces  close  it,  and  the  water  breaks 
open  again  along  its  first  direction  towards  the  south." 

Shore  drift.  — "  Where  waves  break  in  a  considerable  depth  of 
water  or  where  outside  currents  flow  in  a  similar  depth,  the  bottom 
appears  to  be  slightly,  if  at  all,  disturbed;  but  where  the  depths  are 
shallow  the  waves  and  currents  will  stir  up  and  transport  the  material. 
Tests  made  in  1902  at  Cumberland  Sound  showed  that  coarse  sand  and 
shell,  when  stirred  up  by  breakers,  were  carried  to  a  considerable  dis- 
tance even  by  light  currents,  and  were  not  deposited  till  smooth  water 
was  reached.  The  same  materials  in  quiet  water  lay  undisturbed  by 
currents  flowing  as  swiftly  as  4  feet  per  second,  although  fine  sand  was 
found  to  be  moved  by  comparatively  slight  currents.  This  action  on 
exposed  coasts  leads  to  a  constant  movement  of  the  sand  or  shingle, 
and  if  the  storms  prevail  in  one  direction,  there  will  be  a  corresponding 
littoral  drift.  Where  jetties  or  breakwaters  are  built  under  such  cir- 
cumstances there  will  result  an  erosion  on  the  leeward  side  and  a  filling 
on  the  windward  side,  and  this  will  continue  until  the  latter  is  rounded 
out  and  the  sand  can  travel  past  the  ends  of  the  jetties  and  continue 
its  movement  along  the  coast.  The  construction  of  breakwaters  for 
the  harbor  of  Madras  led  to  an  erosion  of  the  neighboring  coast  for  a 
distance  of  several  miles  to  the  north,  in  which  whole  villages  were 
destroyed,  and  at  the  harbor  of  Ceara  in  Brazil,  a  similar  erosion  took 
place  and  continued  for  about  three  years,  until  the  littoral  drift  had 
silted  up  the  windward  side  and  the  entrance,  and  could  pass  along  as 
before.1  At  the  mouth  of  the  St.  John's  River  in  Florida,  the  beach 
to  the  south  was  similarly  eroded." 

The  four  general  methods  used  by  engineers  to  improve  navigable 
conditions  at  the  mouths  of  rivers  are:2 

1.  By  lateral  canals. 

2.  By  dredging. 

3.  By  jetties  and  dredging  combined. 

4.  By  jetties  only. 

1  Proceedings,  Inst.  C.  E.,  Vol.  CLVL 

2  For  excellent  discussion  of  this  see  Thomas  &  Watt,  Improvement  of  Rivers, 
Vol.  I,  p.  314. 


WAVE  ACTION   AND  SHORE  CURRENTS,   ETC.  391 

Case  of  the  Columbia  River,  Oregon.1  —  "  This  river  offers  an  interesting  ex- 
ample of  single  jetty  work.  It  flows  into  the  Pacific  through  a  wide  tidal  estuary, 
which  narrows  to  a  width  of  three  miles  at  the  mouth.  Its  bed,  the  shoals,  and  the 
bar  are  composed  of  a  fine  sand,  easily  shifted.  Little  sediment,  however,  is  brought 
down  by  the  river.  The  mean  tidal  variation  at  the  mouth  is  7.4  feet,  and  the 
maximum  9.5  feet,  the  effect  at  extreme  low  water  being  noticeable  for  150  miles 
from  the  coast.  The  tidal  outflow  is  estimated  as  from  1,350,000  as  the  average  to 
3,000,000  cubic  feet  per  second  as  the  maximum.  The  estimated  fresh  water  dis- 
charge is  from  90,000  to  a  maximum  of  1,500,000  cubic  feet  per  second.  The  main- 
channel  current  on  the  bar  during  the  ebb  runs  at  all  seasons  from  southwest  to 
west-southwest,  with  velocities  from  2£  to  5£  miles  per  hour;  the  flood  current' 
runs  from  north  to  north-northwest,  with  velocities  from  \\  to  3£  miles  per  hour. 
There  is  a  littoral  current  running  at  its  maximum  from  2  to  3  miles  per  hour  with  a 
marked  resultant  set,  due  to  prevailing  influences,  towards  the  north.  The  sand- 
drift  is  therefore  northward  also,  and  during  the  construction  of  the  jetty  the  sand 
accumulated  on  the  south  side  till  it  overtopped  the  work.  There  has  been  mani- 
fest at  all  times  a  noticeable  tendency  for  the  channel,  or  channels  where  two  ex- 
isted, to  cross  the  bar  on  a  southwest  course.  The  depths  on  the  bar  varied  from 
19  to  27  feet. 

In  the  earliest  existing  chart  of  the  entrance,  made  in  1792  by  Admiral  Van- 
couver, only  one  channel  appears,  running  almost  due  west  and  carrying  27  feet 
over  the  bar.  The  next  survey,  made  in  1839,  shows  two  channels;  a  southerly 
one  with  a  bar  depth  of  27  feet,  and  a  northerly  one  with  a  corresponding  depth  cf 
19  feet.  This  condition  remained  a  typical  one  for  more  than  forty  years,  the 
principal  changes  being  the  gradual  lengthening  of  Clatsop  Spit,  and  the  disappear- 
ance of  the  Middle  Sands  when  the  currents  tended  to  reunite  into  a  single  channel. 
During  this  period  the  bar  depths  of  the  two  channels  varied  between  19  and  27 
feet.  By  1885,  however,  the  north  channel  had  practically  disappeared,  since 
which  time  the  south  channel  alone  has  been  in  existence,  although  about  1881  a 
minor  channel  opened  still  more  to  the  south,  which  promised,  until  checked  by  the 
jetty,  to  create  a  second  main  channel. 

During  the  construction  of  the  jetty  between  1885  and  1896,  the  channel 
swung  northward,  and  in  1895  had  a  depth  of  30  feet  over  a  width  of  seven-eighths 
of  a  mile,  and  ran  almost  due  west  to  the  bar.  This  was  the  best  condition  attained. 
The  northward  trend  continued,  however,  and  by  1902  the  depth  had  decreased  to 
22  feet,  the  remains  of  the  old  channel  then  pointing  to  the  north,  and  two  new 
channels  of  almost  equal  depth  had  become  apparent.  It  is  worthy  of  notice  that 
during  all  the  changes  between  1885  and  1897  the  channel  across  the  bar  pointed 
persistently  to  the  southwest,  and  that  when  it  swung  to  the  northwest  during  1897 
and  1898  its  deterioration  commenced.  This  change  of  direction  was  due  to  the 
sand-drift  from  the  south  flowing  round  the  end  of  the  jetty,  which  ended  in  com- 
paratively shallow  water.  The  evidence  shows  that  this  drift  is  principally  due  to 
local  movements  of  the  sand,  and  that  there  has  been  no  extension  of  the  southwest 
face  of  the  bar  since  1839. 

The  south  jetty,  constructed  between  1885  and  1896,  was  intended  to  secure 
30  feet  of  water  in  the  channel,  hi  which  object  it  was  for  some  time  successful.  It 

1  This  and  the  next  case  are  quoted  from  Thomas  &  Watt,  Improvement  of 
Rivers. 


392  ENGINEERING   GEOLOGY 

was  later  proposed  (1905)  to  obtain  a  depth  of  40  feet  with  a  width  of  not  less  than 
one-half  mile.  For  this  purpose  the  south  jetty  was  to  be  extended  1\  miles  to 
deep  water,  and  to  be  raised  to  mean  tide  level,  and  should  this  fail  to  secure  and 
maintain  the  desired  channel,  a  north  jetty  1\  miles  long  or  less  was  to  be  built. 
This  would  locate  the  outlet  on  a  portion  of  the  bar  that  had  remained  practically 
unchanged  since  1839.  To  expedite  the  action  of  the  water,  suction  dredging  was 
commenced  and  has  continued  steadily. 

The  north  jetty  was  not  commenced,  however,  when  proposed,  and  in  1910 
further  recommendation  was  made  for  its  construction,  with  a  view  to  obtaining  a 
deeper  and  more  permanent  crossing  than  the  single  jetty  appeared  able  to  secure. 
At  that  time  there  was  a  channel  over  the  bar  with  a  width  of  8000  feet  and  a  least 
depth  of  24  feet,  its  center  portion  having  a  least  depth  of  26  \  feet  with  a  width  of 
1000  feet. 

Conditions  at  this  outlet  are  unusually  difficult,  as  the  coast  is  exposed  to  very 
heavy  seas,  and  there  is  a  considerable  sand-drift  working  north." 

Case  of  Mississippi  River,  South  Pass.  —  "  The  mouth  of  the  Mississippi  River, 
which  drains  nearly  a  million  and  a  quarter  square  miles,  is  divided  into  three  main 
outlets  —  Southwest  Pass,  South  Pass,  and  Pass-a-1' Outre.  As  early  as  1726  an 
improvement  of  the  outlet  was  attempted  by  harrowing  the  bottom,  and  that  and 
other  means,  such  as  dredging  and  partial  jetties,  were  tried  for  many  years  with 
small  success.  Finally,  the  construction  of  a  canal  at  an  estimated  cost  of  ten  mil- 
lion dollars  was  recommended,  but  the  project  was  suspended  by  the  porposal  of 
James  B.  Eads  in  1874  to  construct  at  his  own  risk  the  present  jetties.  After  numer- 
ous delays  this  proposal  was  accepted,  and  the  jetties  were  built  between  1875  and 
1879.1 

South  Pass  is  about  12.9  miles  long,  with  an  average  width  of  750  feet,  and  a 
least  original  depth  in  the  channel  inside  of  29  feet.  The  original  depth  on  the  bar 
was  8  feet,  and  the  depth  on  the  shoal  at  the  head  of  the  passes,  17  feet.  The  dis- 
charge per  second  at  New  Orleans  in  extreme  high  water  has  been  given  as  1,740,000 
cubic  feet,  and  the  amount  of  solid  matter  carried  in  suspension  at  such  periods  as 
2000  cubic  feet  per  second.  The  range  between  high  and  low  water  there  is  about 
21 1  feet;  at  the  head  of  South  Pass  it  is  about  2|  feet.  The  velocity  at  the  latter 
point  is  5  feet  per  second,  and  the  fall  per  mile  in  the  pass,  2|  inches.  This  outlet 
before  improvement  was  estimated  to  have  carried  about  ten  per  cent  of  the  dis- 
charge of  the  three  passes;  the  remainder  was  divided  almost  equailv  between  the 
other  two  main  passes.  In  1910  it  carried  11.2  per  cent.  Its  low-water  discharge 
was  about  25,000  cubic  feet  per  second,  and  it  carried  to  the  sea  about  22,000,000 
cubic  yards  of  sediment  per  annum. 

The  jetties  were  built  by  James  B.  Eads,  who  contracted  with  the  United 
States  Government  to  provide  a  channel  26  feet  deep  and  not  less  than  200  feet  in 
width,  and  with  a  center  depth  of  30  feet,  and  to  maintain  the  same  for  twenty 
years  for  a  total  cost  of  eight  million  dollars.  It  is  stated  that  this  channel  was 
maintained  for  the  twenty  years  (1879-1899)  with  the  exception  of  about  500  days. 
The  jetties  at  the  mouth  were  placed  1000  feet  apart,  considerably  more  than  the 
width  of  the  river  above,  but  they  were  contracted  later  by  inner  jetties  to  a  width 
of  650  feet  and  by  spur  dikes  to  a  width  of  600  feet.  The  head  of  the  pass  had  also 

1  For  a  history  of  the  improvements,  see  Annual  Report,  Chief  of  Engineers, 
U.  S.  Army,  1899,  p.  1914. 


WAVE  ACTION    AND  SHORE  CURRENTS,   ETC.  393 

to  be  improved  by  jetties  in  order  to  secure  a  deeper  channel  and  mattress  sills 
were  placed  across  the  entrances  to  Southwest  Pass  and  Pass-a-1' Outre  in  order  to 
prevent  their  enlargement  and  a  consequent  diversion  of  part  of  the  flow  from 
South  Pass.  The  conditions  here  are  but  slightly  affected  by  the  action  of  storms 
or  sand-drift. 

By  1910  there  had  been  secured  throughout  the  channel  over  the  bar  a  least 
available  depth  of  31  feet;  this  deepening  had  been  obtained  almost  entirely  by 
scour,  although  dredging  was  used  to  some  extent. 

The  disposition  of  the  sediment  by  the  river  is  worthy  of  notice.  A  com- 
parison of  the  conditions  from  1875  to  1903  indicates  that  the  bar  has  advanced 
very  little;  that  the  river  has  maintained  a  deep  channel  to  the  open  sea;  and  that 
the  greater  part  of  the  vast  amount  of  sediment  brought  down  in  28  years  has  been 
deposited  to  the  east  and  west  of  the  channel  and  behind  the  jetties.  The  Survey 
of  1910  shows  the  same  general  location  of  channel  across  the  bar  as  the  survey  of 
1903.  The  bank  on  the  south  side  immediately  opposite  the  ends  of  the  jetties 
and  the  somewhat  abrupt  turn  necessitated  thereby  are  a  source  of  some  incon- 
venience to  ships  descending  in  any  current,  and  not  infrequently  they  go  aground 
broadside  before  they  can  make  change  of  course." 

Conditions  along  coast  of  United  States.  —  The  engineer  engaged 
in  harbor  improvement  along  the  United  States  coast  line  has  to  consider 
a  variety  of  conditions.  Along  the  coast  from  Maine  to  Cape  Cod 
and  to  New  York  along  the  shore  of  the  mainland,  the  coast  is  mostly 
rock  bound,  and  the  bays  often  represent  valleys  that  were  modified  by 
glacial  erosion  when  the  land  stood  higher,  but  have  now  become  partly 
submerged  by  subsequent  sinking  of  the  coast  line.  At  the  mouth  of 
some  of  these  there  are  obstructions  which  consist  of  rock  ledges  or 
glacial  detritus.  The  tidal  rise  is  moderate  at  Cape  Cod,  but  increases 
to  the  northward.  The  rock  is  resistant,  and  hence  changes  by  wave 
action  are  not  very  noticeable. 

Improvement  in  these  harbors  consists  mainly  of  dredging  and  rock 
removal. 

From  Cape  Cod  to  New  York  there  are  a  number  of  island  harbors 
like  those  of  Nantucket,  Vineyard  Haven,  Block  Island,  and  Long 
Island,  all  of  which  are  peculiar,  and  seem  to  be  due  to  irregularities  in 
the  moraine.  The  tidal  rise  is  only  a  few  feet,  but  on  account  of  the 
great  interior  sounds  to  be  filled,  the  tidal  currents  in  some  places  are 
quite  strong.  The  material  of  the  coast  is  all  unconsolidated.  Storms 
are  severe  along  this  part  of  the  coast,  and  wave  effect  on  the  finer 
materials  is  often  considerable. 

The  improvement  of  the  harbors  is  by  dredging  and  by  the  con- 
struction of  works  for  the  contraction  and  protection  of  the  tidal  channels. 

The  shore  of  the  south  Atlantic  coast  or  that  portion  extending  from 
New  Jersey  to  Florida,  is  composed  mostly  of  fine  materials,  which 


394  ENGINEERING  GEOLOGY 

are  easily  eroded  and  afford  good  conditions  for  the  waves  and  currents. 
The  sea  floor  extends  seaward  from  50  to  100  miles,  with  a  uniform 
slope  of  10  feet  to  the  mile.  Tidal  rise  varies  from  2J  to  7  feet  at 
different  points. 

Wave  action  on  the  whole  is  moderate,  especially  on  the  southern 
part  of  the  coast,  but  nevertheless,  the  wind  waves  do  considerable 
work.  The  harbors  are  improved  by  contraction,  and  protection  work 
and  dredging. 

Along  the  Gulf  of  Mexico,  the  coast  can  be  divided  into  two  sections. 
The  eastern  part  is  not  much  exposed  to  storms,  but  the  on-shore  winds 
of  the  western  portion  are  strong  and  continuous.  The  materials 
along  both  sections  are  easily  eroded,  and  the  tidal  rise  is  about  one  foot. 
Methods  of  improvement  are  similar  to  those  used  along  the  southern 
Atlantic  coast. 

Turning  to  the  Pacific  coast  we  find  that  the  materials  of  the  southern 
part  are  easily  eroded,  that  the  tidal  range  is  large,  but  that  the  wind 
action  is  small. 

On  the  northern  part  of  the  Pacific  coast  line,  material  which  can  be 
moved  by  the  waves  is  abundant,  but  many  rocky  headlands  make 
the  problem  somewhat  complex.  The  wave  action  is  tremendous  and 
there  are  great  ocean  currents  that  may  have  some  effect.  Tidal  action 
is  also  strong. 


References  on  Waves  and  Shore  Currents 

1.  Allanson-Winn,  Trans.  Amer.  Soc.  Civ.  Engrs.,  L,  p.  66,  1903. 
(Wave  work.)  2.  Black,  Trans.  Amer.  Soc.  Civ.  Engrs.,  XXIX,  p. 
223,  1893.  (Improvement  of  South  Atlantic  coast  harbors.)  3. 
Chamberlin  and  Salisbury,  Geology,  I,  p.  330,  2nd  ed.,  1905.  (Holt 
&  Co.)  4.  Cooper,  Trans.  Amer.  Soc.  Civ.  Engrs.,  XXXVI,  p  139, 
1896.  (Ocean  and  Wave  Force.)  5.  Fenneman,  Wis.  Geol.  Surv., 
Bull.  VIII,  1902.  (Theory  of  wave  action.)  6.  Geikie,  Textbook 
of  Geology,  3rd  ed.,  p.  433,  1893,  London.  7.  Gilbert,  U.  S.  Geol. 
Surv.,  Fifth  Ann.  Kept.,  p.  80.  (Shore  line  features.)  8.  Gulliver, 
Proc.  Amer.  Acad.  Arts  and  Sci.,  XXXIV,  p.  151,  1899.  (Shore  line 
topography.)  9.  Harts,  U.  S.  A.  Corps  of  Engrs.,  Prof.  Memoirs,  Oct. 
to  Dec.,  1911.  (Harbor  improvement  on  Pacific  Coast.)  10.  Haupt, 
Trans.  Amer.  Soc.  Civ.  Engrs.,  XXIII,  p.  123,  1890.  (Littoral  Move- 
ments of  the  New  Jersey  Coast.)  11.  Thomas  and  Watt,  Improve- 
ment of  Rivers,  New  York,  1913.  (Wiley  and  Sons.)  12.  Sanborn, 


WAVE  ACTION  AND  SHORE  CURRENTS,   ETC.  395 

Trans.  Amer.  Soc.  Civ.  Engrs.,  LXXI,  p.  284,  1911.  (Theory  of  the 
water  wave.)  13.  Stevenson,  Design  and  Construction  of  Harbors, 
2nd  ed.,  1874,  Edinburgh.  14.  Vedel,  Trans.  Amer.  Soc.  Civ.  Engrs., 
LIV,  Pt.  A.,  p.  139, 1905.  (Island  harbors  and  accumulation  of  material 
caused  by  detached  works.)  15.  Wheeler,  Tidal  Rivers,  1893.  (Long- 
mans, Green  &  Co.,  New  York.) 


CHAPTER  IX 

LAKES:  THEIR  ORIGIN  AND   RELATION  TO 
ENGINEERING  WORK 

Definition.  —  A  lake  may  be  defined  as  a  body  of  water  occupying 
a  more  or  less  basin-shaped  depression  in  the  earth's  surface.  A  small 
lake  is  called  a  pond,  and  a  very  large  lake  is  sometimes  referred  to  as 
an  inland  sea.  These  terms  are,  however,  loosely  used. 

Relation  to  engineering  work.  —  Engineers  in  the  different  branches 
of  their  work  often  have  to  deal  with  lakes  for  the  following  reasons: 
(1)  Lakes  frequently  serve  as  sources  of  water  supply  for  municipal  or 
steaming  purposes,  hence  their  volume,  and  the  chemical  composition 
of  the  water  have  to  be  considered;  (2)  many  navigable  lakes  of  large 
size  show  changes  of  shore  lines  due  to  wave  action  and  shore  currents, 
and  problems  of  coast  protection  and  harbor  maintenance  have  to  be 
dealt  with  as  along  the  sea  coast;  (3)  by  a  natural  process  lakes  are 
often  converted  into  swamps,  across  which  railroad  lines  have  to  be 
laid,  these  tracts  often  giving  considerable  trouble  in  road-building 
and  maintenance. 

TYPES  OF  LAKES 

The  formation  of  lakes  is  sometimes  complex,  and  their  origin  may 
be  due  to  a  number  of  causes;  moreover,  even  after  the  lake  has  been 
formed  it  is  frequently  modified  in  different  ways,  especially  in  depth. 
In  North  America  there  are  many  lakes  of  varying  size  and  depth,  and 
the  table  on  the  following  page  contains  data  regarding  some  of  the  more 
important  ones. 

Lakes  may  be  classified  according  to  origin,  and  the  following  group- 
ing has  been  suggested  by  Davis:  (1)  Original  consequent  lakes;  (2) 
lakes  of  normal  development;  and  (3)  lakes  due  to  accident. 

Original  Consequent  Lakes 

This  class  includes  those  lakes  which  occupy  original  depressions  in 
a  land  surface.  They  may  be  irregularities  of  the  ocean  bottom  which 
were  preserved  when  it  was  lifted  above  sea-level.  The  Everglades  of 
Florida  occupy  such  a  depression.  Other  examples  of  this  type  are 
lakes  occupying  depressions  on  the  surface  of  lava  flows ;  depressions  in 

396 


LAKES:    THEIR  ORIGIN,   ETC. 


397 


Name. 

Average 
depth,  feet. 

Maximum 
sounded 
depth,  feet. 

Area, 
square  miles. 

Area  of 
watershed, 
square  miles. 

Athabasca    \lta-Sask.                 

2,842 

Cayuga   N   Y 

435 

66.3 

1,571.6 

400 

436  7 

7,750 

Crater,  Ore.                                               

1975 

Erie 

70 

204 

10,000 

22,700 

Geneva  Wis 

142 

8  6 

George,  N.  Y. 

60? 

170 

43.6 

227 

Great  Bear  Lake  N  W  Ty. 

11,820 

Great  Salt  Lake  
Great  Slave  Lake,  N.  W.  Ty.. 

15  to  18 

50 

2,000  (variable) 
10,719 

2,443* 

25.4 

Huron  
Mendota,  Wis. 

210 

702 
84 

23,200 
15.2 

31,700 

335 

870 

20,200 

37,700 

Mono,  Cal  
Oconomowoc,  Wis. 

61 

152 
49.2 

87 
819* 

7,000 

Okechobee,  Fla. 

730t 

5,366 

Oneida,  N.  Y  

78 

1,352.5 

Ontario                                                    .   . 

300 

738 

7,260 

21,600 

Owens,  Cal 

75 

Seneca,  N.  Y  

612 

67.2 

708.1 

Superior 

475 

1008 

31,800 

51,600 

Tahoe,  Cal. 

1645 

195 

324 

2,086 

Winnipeg  

70 

9,457 

*  Acres. 

t  When  surface  stands  20  feet  above  the  Gulf. 

sand  dunes,  as  on  Long  Island,  N.  Y.,  and  depressions  in  the  glacial 
till  (p.  421)  or  modified  glacial  drift  (p.  422).  Lakes  of  the  last  two 
types  are  not  uncommon  for  example  in  Wisconsin.1  Lakes  in  the  drift 
may  be  fed  by  streams,  or  by  springs  issuing  along  the  sides  of  the  de- 
pression which  the  lake  occupies.  Their  level  may  coincide  in  a  general 
way  with  that  of  the  water  table  (p.  298)  of  the  surrounding  region,  a 
good  example  of  which  is  Lake  Ronkonkoma  on  Long  Island,  N.  Y. 

Lakes  of  Normal  Development 

This  type  includes  all  those  lakes  which  have  been  formed  in  con- 
nection with  the  development  of  river  valleys.  Several  subtypes 
deserve  notice. 

Oxbow  lakes.  —  The  formation  of  these  (Plate  XLII)  has  been 
described  under  Rivers  (Chapter  V).  They  are  usually  shallow,  and 
of  no  particular  economic  importance. 

Beaches  across  inlets.  —  The  inlet  into  which  a  river  discharges  is 
to  be  regarded  as  the  lower  extremity  of  its  valley.  As  explained  under 
Waves  and  Shore  Currents,  Chapter  VIII,  a  bar  may  sometimes  form 
across  a  harbor  (Plate  LVII)  or  inlet  mouth,  and  be  gradually  built  up 
to  a  beach,  thus,  more  or  less  completely  shutting  off  any  open  con- 
nection between  inlet  and  sea,  so  that  a  lake  is  formed  behind  the 
beach.  Even  if  there  remains  no  open  channel  between  the  lake  and 
1  Fenneman,  Wis.  Geol.  Survey,  Bull.  VIII,  1902. 


PLATE  LXIV,  FIG.  1.  —  Gravelly  beach  formed  by  wave  action,  Kootenay  Lake, 
British  Columbia.     (H.  Ries,  photo.) 


FIG.  2.  —  Lake  formed  by  barrier  of  lava,  Central  France.     (H.  Ries,  photo.) 
(398) 


LAKES:    THEIR  ORIGIN,   ETC.  399 

the  outer  water,  the  water  of  the  lake  may  still  escape  by  seepage 
through  the  sand  of  the  beach  ridge.  Lakes  of  this  sort  are  found,  for 
example,  along  lakes  Erie  and  Ontario,  on  Long  Island,  and  along  the 
Massachusetts  coast  (Plate  LVII). 

Sink-hole  lakes.  —  The  formation  of  sink  holes  in  limestone  for- 
mations is  explained  in  Chapter  VI.  In  some  cases  these  become 
clogged  with  debris  so  that  the  surface  water  accumulates  in  the  depres- 
sion, but  the  pond  is  never  of  large  size,  although  in  some  instances 
the  extended  breaking  down  of  the  limestone  by  subterranean  solution 
may  afford  a  depression  of  some  size.  In  other  cases  there  may  be  an 
outflow  through  one  or  more  sink  holes  in  the  bottom  of  the  lake,  but 
the  level  is  not  lowered  unless  the  escape  exceeds  the  supply. 

Thus  in  the  case  of  Lake  Miccosukee,  Florida,  which  has  an  area  of 
about  5000  acres  (Ref.  6)  it  was  found  that  when  a  channel  entering 
from  the  southwest  was  discharging  about  200  gallons  per  minute,  the 
lake  level  was  being  gradually  lowered,  but  when  the  same  stream  was 
bringing  in  approximately  7000  gallons  per  minute  the  lake  was  rapidly 
filling. 

Crustal  movement  lakes.  —  Owing  to  movements  of  the  earth's 
crust,  depressions  capable  of  holding  water  are  sometimes  formed.  The 
simplest  case  would  be  a  depression  formed  by  warping  of  the  rocks 
of  the  earth's  crust,  either  to  form  a  new  basin,  or  else  lift  up  the  ends 
of  a  pre-existing  trough. 

Lake  Temiskaming  hi  Ontario  for  example  is  regarded  as  a  case  of 
the  latter.1  This  lake  is  nearly  70  miles  long,  its  outlet  being  marked 
by  the  Long  Sault  Rapids.  The  lake  is  bounded  by  rocky  shores 
through  much  of  its  length,  and  is  supposed  to  represent  a  pre-Glacial 
canyon  which,  by  the  down-warping  in  its  middle  part,  has  become 
flooded.  The  total  amount  of  down-warp  is  estimated  at  as  much  as 
500  feet  in  the  center  of  a  distance  of  50  miles. 

Lakes  appear  to  be  formed  sometimes  as  the  result  of  faulting,  as  in  the  case  of 
the  Warner  Lakes  in  Oregon.  Here  large  rectangular  blocks  of  the  earth's  crust 
have  been  tilted  by  faulting,  so  that  corresponding  corners  of  neighboring  blocks 
have  been  tilted  downward  to  the  same  degree.  Such  lakes  are  roughly  triangular 
in  outline,  and  bounded  on  two  sides  by  cliffs,  along  which  the  water  may  be  deepest, 
and  shoals  off  towards  the  third  side. 

Lakes  Due  to  Accident 

This  would  include  those  lakes  located  along  lines  of  drainage  which 
have  become  dammed  by  one  cause  or  another.  They  are  of  variable 
size  and  differ  hi  their  degree  of  permanency,  some  being  but  short-lived. 

1  Pirsson,  Amer.  Jour.  Sci.,  4th  series,  XXX,  p.  25,  1910. 


400  ENGINEERING  GEOLOGY 

Drift-dam  lakes.  —  In  many  cases  where  a  river  valley  was  formed 
prior  to  the  Glacial  period,  a  dam  of  glacial  drift  was  deposited 
across  the  stream's  course  at  some  point,  which  served  to  impound  the 
river  waters.  Lake  George  in  New  York  State  is  a  lake  of  this  type. 
The  valley  above  the  dam  may  be  in  part  filled  with  drift.  The  tight- 
ness of  the  drift  dam  will  depend  on  whether  it  is  dense  till  or  gravelly 
and  sandy  modified  drift. 

This  is  probably  the  most  extensive  type  of  glacial  lake.  Plate 
LXVI,  Fig.  2,  shows  a  lake  that  is  being  held  in  a  valley  by  a  terminal 
moraine  (p.  421). 

The  bottom  of  a  lake  originating  in  the  manner  described  above,  may  be  the 
original  rock  floor  of  the  valley,  but  is  more  likely  to  be  formed  of  the  glacial  drift 
which  partly  fills  the  pre-Glacial  valley. 

In  some  cases  a  lake  may  form  behind  the  terminal  moraine  of  an  existing  glacier, 
being  held  in  on  one  side  possibly  by  the  ice  itself  (Plate  LXV,  Fig.  1).  Small  lakes 
of  this  sort  are  not  uncommon  in  regions  of  existing  glaciers. 

Lake  Como,  in  the  Bitter  Root  Valley,  Montana,  is  described  as  a  deep  natural 
lake  basin  formed  by  a  terminal  moraine  of  fine  and  coarse  gravel,  sand,  and  rock 
flour.  The  Twin  Lakes,  near  Leadville,  Colo.,  are  said  to  be  located  between  two 
great  lateral  moraines,  and  held  in  by  a  terminal  moraine,  which  consists  chiefly  of 
rock  flour  and  is  practically  impervious  to  water.1 

Landslide  lakes.  —  The  name  of  this  type  explains  the  manner  of 
origin,  for  wherever  a  landslide  of  more  or  less  water-tight  material 
crosses  a  valley  occupied  by  a  stream,  a  lake  is  likely  to  be  formed. 
Lakes  of  this  type  are  rarely  of  great  extent.  The  dam  that  holds  them 
in  may  occasionally  be  of  considerable  width,  and  contain  much  stony 
material,  so  that  it  involves  time  and  trouble  to  cut  a  drainage  channel 
across  it.  The  landslides  causing  an  obstruction  of  the  stream  may 
either  be  material  dislodged  from  the  valley  slopes,  or  soft  unconsoli- 
dated  material  that  has  been  undermined  by  the  stream.  The  last 
type  is  not  effective  except  in  the  case  of  small  streams  and  even  then 
the  slide  may  only  obstruct  the  river  temporarily.2  f 

Schuyler3  describes  "  a  natural  dam  on  a  branch  of  the  Umpqua  river  in  Oregon, 
over  300  feet  high,  formed  by  a  landslide  from  the  adjacent  sandstone  cliff.  The 
base  of  this  dam  is  not  over  3000  to  4000  feet.  Floods  of  several  thousand 
second-feet  pass  over  the  top  of  it  every  year,  and  it  is  practically  water-tight,  as 
it  holds  back  a  good-sized  lake.  This  is  a  natural  rock-fill  dam  composed  of  enor- 
mous blocks  of  stone,  whose  voids  are  filled  with  smaller  stone  and  rock  dust  ground 
up  in  the  process  of  falling."  Crystal  Lake  in  Colorado  is  a  lake  of  the  landslide 
type. 

Lava  dams.  —  In  some  regions  of  volcanic  activity,  a  lava  flow  occasionally 
obstructs  a  valley,  so  that  the  water  becomes  ponded  behind  it.  No  large  water 

1  Schuyler,  Reservoirs,  pp.  483  and  487,  1908. 

2  See  G.  M.  Dawson,  Geol.  Soc.  Amer.,  Bull.  X,  p.  484,  1899. 

3  Schuyler,  Reservoirs,  p.  483,  1908. 


LAKES:    THEIR  ORIGIN,   ETC.  401 

bodies  of  this  type  are  known.     Plate  LXIV,  Fig.  2,  shows  such  a  lake  in  south 
central  France.     Snag  Lake  in  California  is  also  of  this  type. 

Crater  lakes.  —  The  craters  of  many  extinct  volcanoes  are  often  more  or  less 
filled  with  water,  but  so  far  as  known  they  have  never  been  used  for  economic  pur- 
poses. Indeed  they  are  not  very  abundant.  Plate  LXV,  Fig.  2,  shows  a  crater 
lake  in  the  volcano  of  Toluca,  Mex.,  14,000  feet  altitude.  Crater  Lake,  Oregon, 
which  has  a  diameter  of  about  six  miles  is  one  of  the  largest  known.  In  some  regions 
of  present  volcanic  activity,  there  may  be  bubbling  in  the  lake  due  to  escape  of  steam 
or  other  gas.1  Crater  lakes  must  perforce  have  a  small  drainage  basin  and  can  hardly 
be  drawn  upon  as  a  source  of  water  supply  for  any  purpose. 

Glacial  dams.  —  The  advance  of  a  glacier  across  a  river  valley  may 
dam  the  flow  sufficiently  to  form  a  lake.  In  regions  of  alpine  glaciers 
they  are  seldom  of  large  size,  and  are  not  to  be  considered  except  for 
threatened  danger  from  floods  in  the  event  of  their  sudden  release. 

In  Alaska,  however,  a  region  which  will  attract  the  engineer's  atten- 
tion to  an  increasing  degree  in  the  future,  the  effects  of  living  glaciers 
on  drainage  obstruction  may  have  to  be  occasionally  reckoned  with. 
Thus,  the  constriction  of  the  Copper  River  by  Child's  glacier  gave  rise 
to  the  lake  in  which  Miles  glacier  terminates.  The  lake  was  crossed  by 
a  car  ferry  until  the  bridges  on  the  Copper  River  railroad  had  been 
completed.2 

LAKE  WATERS 
Waves  and  Currents 

Wave  and  ice  action.  —  Wherever  a  lake  is  of  sufficient  size  to 
permit  waves  and  shore  currents  of  any  importance  to  develop,  and 
the  coast  line  is  composed  of  soft  materials,  we  find  the  same  erosion 
and  deposition  going  on  as  along  the  ocean  coast  line.  These  phe- 
nomena are  described  in  Chapter  VIII,  and  need  not  therefore  be  re- 
peated here. 

A  phenomenon  seen  in  some  lakes,  not  observed  in  the  ocean,  is  the 
development  of  ice  ramparts.  In  many  lakes  the  water  becomes  en- 
tirely covered  by  ice  during  cold  weather.  If  the  ice  covering  has  a 
temperature  of  say  20°  F.,  and  the  temperature  is  lowered  to  say 
- 10°  F.,  the  ice  contracts,  which  results  in  its  either  pulling  away  from 
the  shore,  or  cracking.  If  the  former  the  water  uncovered  at  once 
freezes;  if  the  latter  the  water  filling  the  cracks  does  the  same. 

When  the  temperature  rises  again,  the  ice  expands,  and  either  crowds 
up  against  the  shore  or  arches  up  at  some  other  point.  Where  the 
shore  is  gravelly  or  composed  of  other  soft  material,  it  is  sometimes 

1  Hovey,  Nat.  Geog.  Mag.,  1902. 

2  Martin,  Bull.  Ainer.  Geog.  Soc.,  XLV,  p.  801,  1913. 


PLATE  LXV,  FIG.  1.  — Lakelet  held  in  by  terminal  moraine  and  glacier.     (R.  D. 

George,  photo.) 


FIG.  2.  —  Crater  lake,  volcano  of  Toluca,  Mexico.     (H.  Ries,  photo  ) 
(402) 


LAKES:    THEIR  ORIGIN,   ETC.  403 

pushed  up  into  ridges.  These  often  differ  from  beaches  or  bars  in  that 
the  material  may  be  entirely  unassorted. 

Such  ice  terraces  were  noted  by  Buckley,  and  have  since  been  de- 
scribed by  Fenneman  for  many  of  the  Wisconsin  lakes.1  Where  struc- 
tures occur  along  the  shores  of  the  lake  considerable  damage  may  be 
caused  by  the  ice  thrust. 

Lake  currents.  —  Currents  of  either  temporary  or  permanent 
nature  may  be  present  in  many  lakes,  but  in  most  cases  they  are  so 
weak  as  to  attract  little  attention.  These  currents  may  be:  (1)  The 
general  movement  of  the  water  from  inlet  to  outlet  of  lake,  the  body 
current,  whose  speed  is  slow;  (2)  a  surface  current  due  to  prevailing 
winds;  (3)  return  currents;  and  (4)  surf  motion,  which  produces  a 
general  drift  towards  the  shore,  and  in  some  cases  a  shore  current  if 
the  waves  approach  the  shore  line  obliquely. 

The  first  of  these  may  be  noticeable  only  at  the  head  and  foot  of 
the  lake,  and  is  not  necessarily  a  direct  flow  from  head  to  foot.  The 
second  will  be  in  the  direction  of  the  prevailing  wind.  The  third  will 
depend  to  a  large  degree  upon  the  capacity  of  the  outlet,  whether  it 
can  take  care  of  all  the  water  that  is  driven  towards  it. 

Some  years  ago  the  U.  S.  Weather  Bureau2  attempted  to  ascertain 
the  direction  of  currents  in  the  Great  Lakes.  It  was  found  that  in 
Lake  Superior  the  return  current  was  along  the  southern  shore;  in  Lake 
Michigan  along  the  eastern  shore;  in  Lake  Huron  along  the  western 
shore;  but  in  Lakes  Erie  and  Ontario  it  was  not  so  clear. 

Variations  in  lake  level.  —  The  surface  level  of  all  lakes  is  liable  to 
fluctuations,  which  may  be  gradual  or  sudden. 

Gradual  variations.  —  These  can  usually  be  correlated  with  rainfall. 
During  a  rainy  season,  a  lake  with  outlet  may  be  supplied  with  water 
by  surface  streams  and  springs  faster  than  the  outlet  can  carry  it  off, 
and  the  level  of  the  lake  rises,  it  may  be  only  a  few  inches,  or  it  may  be 
several  feet.  Such  variations  are  not  confined  to  small  lakes,  but  are 
sometimes  quite  noticeable  in  large  ones. 

It  is  said,  for  example,  that  "since  the  settlement  of  the  Great  Lakes 
region  the  level  of  lakes  Michigan  and  Huron  has  fluctuated  noticeably. 
Not  only  is  there  a  regular  seasonal  fluctuation  of  about  one  and  one- 
half  feet  (high  water  coming  in  June  or  July,  and  low  water  in  mid- 
winter), but  there  are  greater  changes  through  periods  of  several  years. 
In  1886  Lake  Michigan  was  about  two  feet  higher  and  in  1896  nearly 
three  feet  lower  than  in  1906.  At  high  water  in  1838,  the  same  lake 

1  Wis.  Geol.  Survey,  Bull.  VIII. 

2  Bull.  B,  1894. 


404  ENGINEERING  GEOLOGY 

stood  nearly  six  feet  higher  than  at  low  water  in  1896.  When  these 
secular  changes  of  level  are  plotted  next  to  a  rainfall  curve1  the  con- 
nection between  periods  of  unusual  rainfall  or  drought  and  periods  of 
high  or  low  water  is  evident."2 

Sudden  variations.  —  Lake  waters  are  sensitive  to  changes  of  atmos- 
pheric pressure.  It  is  sometimes  noticed  that  in  calm  weather  the  lake 
level  may  show  a  variation  of  several  feet  in  less  than  an  hour.  Such 
oscillations  are  known  as  seiches.3  Of  course  on  small  lakes  the  seiche 
is  smaller  than  on  large  ones  and  in  many  is  hardly  appreciable.  In 
addition  to  these,  rhythmical  pulsations  producing  a  difference  in  level 
of  as  much  as  four  or  five  inches  during  calms,  unaccompanied  by 
variations  in  atmospheric  pressure,  have  been  observed,  but  these  are 
little  understood  (Russell). 

Effect  of  strong  wind.  —  If  a  strong  wind  blows  over  a  lake  surface 
for  some  time  in  one  direction,  the  water  is  forced  towards  one  end,  re- 
sulting in  a  marked  difference  in  level  at  the  two  extremities  of  the 
lake.  In  the  case  of  Lake  Erie,  this  difference  may  sometimes  amount 
to  as  much  as  15  feet. 

Temperature  of  lakes.  —  Lake  waters  may  be  warmed,  either  by 
the  sun's  heat,  or  by  contact  with  the  air,  but  since  water  is  a  poor 
radiator  as  well  as  a  poor  conductor  of  heat,  it  will  not  respond  to 
atmospheric  temperature  changes  as  readily  as  solid  mineral  masses 
like  rocks.  A  shallow  lake  may  be  warmed  to  the  bottom  by  the 
summer's  heat,  and  equally  chilled  by  the  winter's  cold,  although  its 
temperature  will  be  more  uniform  than  that  of  the  air. 

The  subject  of  the  temperature  of  ponds  and  lakes  is  of  considerable 
practical  importance,  where  these  are  to  be  used  for  water  supply, 
since  it  is  desirable  to  obtain  water  not  only  of  good  quality,  but  some- 
times at  a  uniform  temperature. 

Engineers  connected  with  waterworks  should  be  familiar  with  the 
seasonable  changes  of  temperature  in  lakes  and  reservoirs  used  for 
water  supply.  This  is  especially  true  of  deep  ponds  (say  those  deeper 
than  50  feet),  because  in  these  the  temperature  changes  may  produce 
or  prevent  vertical  currents  at  different  seasons,  which  often  exert  an 
important  influence  on  the  quality  of  the  water  at  different  depths. 

If  in  a  given  lake  a  series  of  temperature  determinations  be  made  at 
different  depths  throughout  the  year,  it  will  be  found  that  the  shallower 
layers  of  water  show  the  greatest  variation,  warming  in  summer  and 

1  Lane,  A.  C.,  Mich.  Geol.  Survey,  VII,  Plate  V. 

2  Atwood  and  Goldthwait,  111.  Geol.  Survey,  Bull.  7,  p.  68,  1908. 

3  Perkins,  American  Meteorological  Journal,  Oct.,  1893. 


LAKES:    THEIR  ORIGIN,   ETC. 


405 


cooling  in  winter,  while  at  greater  depths,  beginning  even  as  low  as  50 
feet,  the  change  from  season  to  season  is  comparatively  slight. 

Even  during  warm  summer  weather,  the  deeper  layers  of  a  fresh- 
water lake  may  be  quite  cool.  This  is  due  to  the  fact  that  water  is 
densest  at  39.2°  F.,  and  the  water  which  becomes  cooled  in  winter  sinks 
to  the  bottom.  Moreover,  water  is  a  poor  conductor  of  heat,  hence  the 
cold  lower  layers  are  not  warmed  in  summer.  This  difference  in  weight 
of  water  at  several  temperatures  above  and  below  its  point  of  maximum 
density  is  shown  by  the  following  figures. 


Temperature 
of  water. 

Density. 

Degrees  F. 

32 

0.99987 

39.2 

1.00000 

50 

0.99974 

70 

0.99800 

86 

0.99577 

The  changes  taking  place  in  Lake  Cochituate  in  Massachusetts  have  been  well 
described  as  an  example  of  those  occurring  in  any  lake  of  moderate  or  good  depth 
which  freezes  over  in  winter.1  The  temperature  curves  given  in  Fig.  187  are  con- 
sidered to  represent  approximately  those  of  bodies  of  water  with  depths  varying 
from  20  to  80  feet,  and  exposed  to  the  same  climatic  conditions  as  those  prevailing 
in  the  vicinity  of  Boston.  It  is  seen  there  (consult  Fig.  187),  that  from  the  time 
of  the  breaking  up  of  the  ice  in  March,  the  surface  warms  considerably  more 
than  the  mid-depths  and  bottom,  and  that  after  September  the  surface  temperature 
drops  rapidly. 

With  regard  to  the  bottom  temperatures,  Fitzgerald  states  that  if  a  pond  is 
less  than  about  25  feet  deep,  the  bottom  temperature  does  not  differ  much  from  the 
surface,  although  in  winter  it  may  be  5°  or  6°  warmer,  and  in  summer  as  many 
degrees  cooler.  Such  shallow  ponds  are  stirred  to  their  depths  by  winds,  which 
help  to  keep  the  temperature  equalized. 

In  deeper  lakes,  however,  like  Lake  Cochituate,  when  the  surface  freezes  over 
about  January  first,  the  bottom  temperature  is  near  39.2°;  or  it  may  be  much  below 
this  if  the  weather  has  been  severe  and  the  winds  high  prior  to  freezing  over.  The 
several  layers  in  the  lake  will  of  course  lie  in  the  order  of  their  density,  the  tem- 
perature increasing  gradually  upwards,  until  within  a  few  feet  of  the  surface,  when 
it  suddenly  falls  to  the  freezing  point.  The  water  will  remain  so  until  the  ice  breaks 
up.  The  warming  up  of  the  surface  about  April  first  to  about  the  same  temper- 
ature as  the  bottom  causes  a  state  of  unstable  equilibrium,  and  circulation  begins 
from  top  to  bottom.  This  is  spoken  of  as  the  working  or  overturning  of  the  lake 
waters. 

When  by  May  first,  as  in  Lake  Cochituate,  the  surface  temperature  exceeds  the 
bottom  by  about  5°,  the  difference  in  density  seems  to  be  sufficient  to  prevent  further 
warming  up  of  the  bottom  layers.  There  follows  then  a  period  of  stagnation  which 
lasts  until  about  the  middle  of  November,  when  a  second  and  stronger  period  of 
circulation  begins  which  lasts  until  the  lake  freezes. 

During  the  summer  stagnation,  the  winds  may  not  stir  the  lake  much  deeper 
than  15  feet,  although  this  depends  on  the  difference  of  density  of  the  several  layers. 
1  Fitzgerald,  Trans.  Amer.  Soc.  Civ.  Eng.,  XXXIV,  p.  67,  1895. 


406 


ENGINEERING  GEOLOGY 


7 


s 

\ 

i 


/  i 

\  \M 


LAKES:    THEIR  ORIGIN,   ETC. 


407 


The  effects  of  stagnation  are  of  importance  in  relation  to  municipal 
water  supplies.  During  stagnation,  if  there  is  much  organic  matter  in 
the  lake,  it  collects  in  the  lower  quiet  layers,  and  decay  continues  until 
all  the  oxygen  is  used  up.  The  water  gets  darker,  and  has  a  bad  odor. 
Free  ammonia  and  other  decomposition  products  accumulate.  With 
the  overturning  of  the  lake  in  autumn  this  decayed  matter  is  brought 
to  the  surface. 

The  phenomena  just  referred  to  may  be  lacking  in:  (1)  A  lake  that  is 
free  from  organic  matter;  (2)  one  so  large  that  the  organic  matter 
brought  in  by  feeding  streams  is  completely  oxidized;  (3)  hi  a  large 
artificial  reservoir  constructed  on  sanitary  principles.  It  is  however 
rare  to  find  a  lake  in  which  the  water  at  the  bottom  is  as  pure  as  that 
at  the  surface,  at  the  end  of  summer. 

In  deep  lakes  covered  with  ice  in  winter,  there  are  two  lines  or  curves 
of  profile  which  confine  the  variations  of  temperature  within  certain 
limits  —  the  winter  curve  and  the  summer  curve.  These  curves  very 
nearly  meet  at  the  bottom  if  it  is  a  deep  lake,  and  are  separated  by  a 
considerable  interval  as  the  lake  becomes  more  shallow.  Recognition 
of  the  phenomena  described  above  will  enable  the  water-works  engineer 
to  locate  the  off-take  pipes  so  as  to  obtain  water  writh  regard  to  uni- 
formity of  temperature  and  purity.  In  artificial  reservoirs  of  depth  a 
low  off-take  may  be  provided  for  drawing  off  the  impure  water. 

Lake  Thun,  Switzerland.  —  Recent  data  on  the  temperature  of  deep 
lakes  seem  to  be  lacking,  but  the  following  table1  gives  a  valuable  series 
of  determinations  made  by  Fischer-Ooster  and  Brunner  in  Lake  Thun 
in  1848—1849.  This  table  shows  the  diminution  hi  temperature  through- 
out the  mass  in  winter,  and  the  heating  of  the  lower  strata  long  after 
the  surface  had  begun  to  cool  off.  It  is  seen  that  the  maximum  tem- 
perature at  depths  of  10  to  80  feet  occurs  in  September;  at  80  to  120 
feet  in  October;  and  at  120  to  350  feet  in  November. 

TEMPERATURES  AT  DIFFERENT  DEPTHS  IN  LAKE  THUN 


English 
feet. 

Mar.  28, 

1848. 

May  13. 

July  3. 

Aug.  5. 

Sept.  6. 

Oct.  28. 

Nov.  26. 

Feb.  3, 
1849. 

Surface 

42.3 

59.1 

59.6 

62.8 

65.6 

53.4 

46.3 

40.8 

10.7 

41.4 

51.4 

59.0 

60.4 

61.8 

53.2 

46.3 

41.0 

21.3 

41.3 

49.2 

57.2 

57.3 

59.1 

53.0 

46.4 

41.2 

32.0 

41.2 

48.0 

53.2 

55.5 

57.7 

53.0 

46.2 

41.0 

42.6 

41.1 

46.6 

52.1 

54.4 

56.2 

53.2 

46.1 

40.7 

63.9 

40.9 

44.8 

49.6 

52.6 

53.8 

53.0 

46.1 

40.7 

85.3 

40.8 

44.2 

46.3 

50.7 

50.9 

52.1 

46.1 

40.7 

127.9 

40.4 

41.8 

42.3 

43.7 

43.4 

43.6 

44.0 

40.7 

170.5 

40.4 

41.5 

41.4 

41.8 

41.7 

42.1 

42.1 

41.0 

266.5 

40.4 

40.8 

41.0 

41.1 

41.4 

41.0 

41.3 

40.7 

373.0 

40.6 

40.9 

40.9 

41.1 

41.1 

40.8 

40.7 

40.6 

479.6 

40.7 

40.7 

40.8 

40.9 

40.8 

40.9 

40.6 

40.7 

586.3 

40.7 

40.7 

40.8 

40.7 

40.8 

- 

Abstracted  in  Fitzgerald's  paper  referred  to  above. 


408  ENGINEERING  GEOLOGY 

Composition  of  Lake  Waters 

The  waters  of  lakes  may  show  a  wide  range  in  composition.  Those 
of  fresh-water  lakes,  that  is,  those  having  an  outlet,  do  not  differ  so 
much  from  river  waters,  although  of  course  a  lake  receiving  tributaries 
that  have  flowed  over  different  formations  might  show  a  composition 
expressing  more  or  less  the  average  of  these.  It  is  in  the  inland  lakes, 
without  outlet,  and  which  often  show  high  salinity  (p.  288),  that  the 
most  marked  variation  in  composition  is  found.  The  waters  of  the 
fresh-water  lakes  often  run  high  in  carbonates. 

In  the  table  given  on  page  409  will  be  found  the  analyses  of  a  number 
of  lake  waters  of  which  1  to  8,  and  10  are  from  fresh-water  lakes;  the 
others  are  from  more  or  less  strongly  saline  ones.  % 

These  saline  ones  are  representative  of  the  following  types:  (1)  Chlo- 
ride type,  solid  matter  mostly  sodium  chloride,  as  in  Great  Salt  Lake, 
No.  9;  (2)  carbonate  or  alkaline  type,  in  which  sodium  carbonate  is 
largely  in  excess,  as  in  Goodenough  Lake,  No.  16;  (3)  carbonates  and 
chlorides  predominating,  with  sulphates  subordinate,  as  in  Pyramid 
Lake,  No.  11;  (4)  " triple"  type,  with  chlorides,  sulphates  and  car- 
bonates all  present  in  notable  amounts,  as  in  Owens  Lake,  No.  14, 
Mono  Lake,  No.  13,  and  Tulare  Lake,  No.  17;  (5)  sulphate-chloride 
type,  as  Devil's  Lake,  No.  15. 

Type  No.  2  exhibits  the  nearest  relationship  to  river  waters. 

OBLITERATION  OF  LAKES 

Lakes  may  be  naturally  obliterated  in  several  ways:  (1)  By  evapo- 
ration; (2)  by  cutting  down  of  outlet;  (3)  by  filling  of  the  basin  with 
sediment,  plant  growth,  or  both;  (4)  by  lowering  of  surrounding  ground- 
water  level;  and  (5)  by  a  combination  of  several  of  these. 

Obliteration  by  evaporation.  —  In  many  arid  regions,  there  are  a 
few  lakes  which  have  no  outlet,  and  whose  waters  escape  chiefly  by 
evaporation.  These  lakes  are  all  of  saline  character.  In  some  in- 
stances lakes  of  large  area  and  great  depth  have  almost  completely 
disappeared  in  this  manner,  only  small  remnants  being  now  left.  One  of 
these,  Lake  Lahontan,1  which  covered  parts  of  Nevada,  had  an  area  of 
8400  square  miles;  another  one,  Lake  Bonneville,2  of  which  Great  Salt 
Lake  is  a  remnant,  had  an  area  of  17,000  square  miles,  and  a  depth  of 
about  1000  feet. 

Some  lakes  without  outlet,  found  in  arid  regions,  belong  to  the 

1  Russell,  U.  S.  Geol.  Survey,  Mon.  XI. 

2  Gilbert,  U.  S.  Geol.  Survey,  Mon.  I. 


LAKES:    THEIR  ORIGIN,   ETC. 


409 


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410  ENGINEERING  GEOLOGY 

periodic  type,  that  is  to  say,  they  evaporate  to  dryness  during  a  portion 
of  the  year,  or  sometimes  for  a  longer  period.  Such  lakes  are  also 
known  as  play  as. 

As  a  rule  lakes  with  no  outlet  occur  in  sparsely  inhabited  regions 
where  they  cause  little  trouble,  but  when  it  is  necessary  to  drain  them, 
it  sometimes  involves  considerable  work. 

An  interesting  case  of  this  was  the  draining  of  Texcoco  and  other 
lakes  in  the  valley  of  Mexico  City.  Here  there  is  a  group  of  lakes  which 
have  no  outlet,  but  receive  the  drainage  from  the  surrounding  hills. 
During  the  rainy  season  the  lake  level  rose  to  such  an  extent  as  to 
flood  much  of  the  surrounding  country,  and  even  affected  Mexico 
City.  The  drainage  system,  which  was  built  to  carry  off  not  only  the 
surplus  waters  of  the  lakes  but  also  the  sewerage  of  Mexico  City,  in- 
volved the  construction  of  a  canal  about  47J  kilometers  long,  and  a 
tunnel  about  10  kilometers  in  length,  from  which  the  water  drained  into 
the  Gulf  of  Mexico. 

Cutting  down  of  outlet.  —  Where  a  lake  is  held  in  by  a  rock  barrier, 
the  escaping  water  flowing  over  this  will  perform  little  erosion,  for  the 
reason  that  the  lake  acts  as  a  settling  basin,  and  so  the  water  becomes 
drained  of  much  of  its  suspended  load  in  passing  through  it.  The  clear 
water  flowing  off  at  the  lower  end  cuts  but  slowly. 

If  the  barrier  which  impounds  the  waters  of  the  lake  is  of  unconsoli- 
dated  material,  erosion  will  proceed  more  rapidly,  but  not  with  startling 
rapidity.  In  the  latter  case,  however,  an  artificial  outlet  could  be  cut 
more  readily  than  in  the  former. 

Obliteration  by  filling.  —  This  is  a  more  frequent  cause  of  oblitera- 
tion, which  can  be  noticed  in  progress  in  many  localities,  but  which 
does  not  become  effective  in  a  comparatively  short  period  of  time 
unless  the  lake  is  small  and  shallow.  It  may  be  due  to  two  causes: 
(1)  Sedimentation  and  (2)  plant  growth. 

Many  streams  flowing  into  lakes  carry  considerable  sediment.  This, 
of  course,  is  dropped  at  the  mouth  of  the  stream,  forming  a  delta  which 
gradually  extends  out  into  the  lake  (Plate  LXVI,  Fig.  2).  At  the  same 
time  the  finer  sediment  is  spread  out  over  the  lake  bottom.  A  small 
lake  or  artificial  reservoir  may  thus  sometimes  become  silted  up  to  a 
noticeable  degree  in  a  comparatively  short  time;  indeed  the  process  is 
to  be  seen  under  way  in  dozens  of  lakes. 

At  the  head  of  some  lakes  these  delta  deposits  have  encroached  some 
distance  and  are  of  considerable  thickness.  Thus  at  the  head  of 
Cayuga  Lake  they  are  over  400  feet  thick.1  At  the  head  of  Seneca  Lake 
1  Part  of  this  is  glacial  drift. 


LAKES:    THEIR  ORIGIN,   ETC.  411 

the  end  had  been  advanced  northward  some  two  miles  by  deposition. 
Kootenay  Lake  in  British  Columbia  has  been  filled  in  for  a  distance  of 
several  miles  at  its  head  or  southern  end  with  the  sediments  deposited 
by  the  Kootenay  River.  The  delta  built  by  the  Rhone  into  Lake 
Geneva  is  several  miles  in  length,  and  has  been  lengthened  nearly  two 
miles  since  the  time  of  the  Roman  occupation  (Chamberlin  and  Salis- 
bury). 

Another  less  important  process  of  lake  filling  is  by  the  accumulation  of 
bog  lime  on  the  lake  bottom,  but  this  is  slow,  and  to  be  looked  for  only 
in  regions  of  calcareous  waters,  such  as  occur  in  some  of  the  northern 
central  states.  The  deposits  thus  accumulated  sometimes  underlie 
several  hundred  acres  to  a  depth  of  10  or  20  feet,  and  are  often  of 
sufficient  purity  to  be  of  commercial  value. 

Filling  by  plant  growth  is  a  widespread  and  sometimes  important 
process.  Around  the  edge  of  many  ponds  there  is  a  growth  of  water- 
loving  plants,  which  gradually  extend  out  towards  the  middle  of  the 
lake  as  the  water  becomes  shoaled  by  the  deposition  of  sediment. 

By  a  combination  of  these  two  processes  the  pond  may  be  gradually 
converted  into  a  swamp.  Many  swamps  and  bogs  are  the  last  stage  in 
lake  obliteration.  Consequently  in  section  they  often  show  an  upper 
series  of  layers  of  muck  or  peaty  material,  and  a  lower  series  of  sand 
and  mud,  or  sometimes  bog  lime,  the  whole  more  or  less  softened  by 
water. 

In  not  a  few  cases  railroad  tracks  have  been  laid  across  these  swampy 
tracts,  and  many  an  engineeer  of  practical  experience  can  recount 
numerous  troubles  which  he  has  had  with  such  ground.  In  some  in- 
stances the  road  bed  has  sunk  out  of  sight  over  night.  In  other  cases 
load  after  load  has  been  added  to  the  road  bed,  without  appreciably 
raising  its  level,  until  it  was  discovered  that  the  underlying  material 
was  quietly  flowing  out  laterally,  in  one  instance,  to  a  lake  a  quarter  of 
a  mile  away. 

A  case  which  is  typical l  of  many  occurred  along  the  line  of  the  Panama 
Railroad  where  it  was  necessary  to  make  a  90-foot  fill.  A  30-foot 
trestle  was  built  and  filled  without  any  trouble.  A  second  30  feet 
was  added  and  stood.  It  was  then  raised  to  85  feet,  and  the  next 
morning  was  out  of  sight,  leaving  a  90-foot  lake,  400  or  500  feet  long. 
Two  more  trestles  were  lost  and  then  the  engineers  began  loading  up 
the  outer  edge  of  the  soft  area  to  counterbalance  the  fill. 

One  more  phase  of  the  matter  should  be  mentioned.  In  some  cases 
of  lake  filling,  encroaching  vegetable  growth  forms  a  floating  mat  (Fig. 
1  Slifer,  W.  Soc.  Eng.,  XVIII,  p.  609,  1913. 


412  ENGINEERING  GEOLOGY 

200)  which  eventually  completely  covers  the  pond  and  becomes  so 
thick  and  solid  as  to  support  tree  growth,  even  though  there  be  clear 
water  underneath.  These  mats  are  sometimes  mistaken  for  solid 
ground. 

Such  bogs  have  given  much  trouble  along  the  lines  of  the  Pere  Mar- 
quette,  Ann  Arbor,  Michigan  Central,  Grand  Trunk  and  other  railroads, 
in  Michigan.  Many  are  known  to  occur  in  Minnesota  and  are  crossed 
by  railroads. 

A  most  interesting  case  and  one  that  serves  well  for  illustrative  purposes,  oc- 
curred along  the  line  of  the  Grand  Trunk  Railway  at  Haslett  Park.  According  to 
Davis,1  "  The  road  was  originally  built  with  a  single  track,  and  a  large  amount  of 
timber  was  used  to  form  a  foundation  for  the  road  bed,  which  was  built  above  it. 
For  a  considerable  time  this  single  track  was  sufficient  for  the  needs  of  the  road,  and 
little  difficulty  was  experienced  from  instability  of  the  stratum  until  1902, when  the 
track  was  doubled.  In  the  process  of  this  work  the  dirt,  which  was  dumped  by  the 
side  of  the  existing  embankment,  gradually  sank  out  of  sight,  leaving  a  pond  of 
water,  at  the  same  time  forcing  the  track  and  right-of-way  fences  out  of  line.  The 
displacement  of  the  fence  was  38  feet  from  its  original  position,  and  of  the  track 
more  than  19  £  feet.  The  weight  of  the  material  for  widening  the  old  embankment 
broke  the  mat,  and  carried  down  with  it  a  portion  of  the  old  filling  as  well  as  the  peat 
below  it,  so  that  the  track  sank  whenever  trains  passed,  sometimes  a  half  foot,  and 
this  would  have  to  be  raised  by  filling  before  the  track  could  be  used  again.  In 
filling  the  opening  permanently  about  30,000  cubic  yards  of  material  were  used  be- 
fore the  track  stopped  sinking.  The  greatest  depth  of  the  hole  under  the  mat  was 
28  feet.  In  another  larger  bog,  on  the  line  of  the  same  railway,  and  less  than  6 
miles  from  the  one  described,  there  were  used  more  than  60,000  cubic  yards  of  filling 
in  making  the  changes  from  single  to  double  track.  This  depression  was  55  feet 
deep." 

Obliteration  by  lowering  of  groundwater  level.  —  As  noted  else- 
where, the  lake  surface  may  coincide  with  the  groundwater  level.  Any 
cause  which  tends  to  permanently  depress  the  level  will  operate  to  de- 
stroy the  lake.  In  some  cases  the  opening  of  land  for  agriculture, 
with  the  clearing  off  of  forests  and  consequent  increased  run-off,  may 
be  an  active  cause.  This  lowering  of  the  water  level  will  be  most 
noticeable  in  porous,  gravelly,  or  sandy  formations. 

A  case  in  point  is  seen  in  southeastern  Portage  County,  Wis.3  There  "in  the 
broad  level  areas  of  alluvial  plains  bordering  the  Green  Bay  Moraine  in  this  part 
of  the  area,  the  level  of  the  groundwater  has  been  lowered  to  depths  varying  from 
a  'few  feet  up  to  40  feet  since  the  region  was  opened  to  agriculture.  It  is  a  note- 
worthy fact  also  that  in  this  area,  where  the  groundwater  has  been  appreciably 
lowered,  the  lakes  have  become  greatly  contracted  and  many  of  them  are  entirely 
extinct.  Most,  if  not  all,  of  these  contracted  lakes,  long  ago  lost  their  outlets  and 
their  bottoms  do  not  contain  an  appreciable  amount  of  filling  due  to  wash  or  to 

1  Mich.  Geol.  Surv.,  Rept.  for  1906,  p.  155,  1907,  also  Waterbury,  The  Michigan 
Engineer,  1903,  p.  38. 

2  Weidman,  Wis.  Geol.  and  Nat.  Hist.  Surv.,  Bull.  XVI,  p.  613,  1907. 


LAKES:    THEIR  ORIGIN,   ETC.  413 

organic  agencies.  The  natural  inference  is,  therefore,  that  these  lakes  are  being  de- 
stroyed by  the  same  causes  which  have  operated  to  lower  the  level  of  the  ground- 
water  of  the  area.  In  those  parts  of  the  area  where  the  underlying  formation 
consists  of  an  abundance  of  clay  or  other  impervious  rock,  where  little  change  in 
the  level  of  the  groundwater  has  been  wrought  by  cultivation,  this  process  of  lake 
extinction  is  relatively  unimportant." 

Extinct  lakes.  —  We  find  records  in  many  parts  of  the  country  of 
pre-existing  lakes,  some  of  them  of  vast  size.  In  some  cases  they 
occupied  natural  basins  of  the  earth's  crust,  but  in  other  instances 
were  evidently  due  to  obstruction  of  the  surface  drainage  by  the  ice 
sheet  which  once  covered  the  northern  states,  and  remained  as  long  as 
the  cause  did. 

The  former  existence  of  these  lakes  is  recognized  in  various  ways. 
Sometimes  we  find  a  natural  basin  partly  filled  with  lake  sediments, 
forming  an  extensive  flat,  with  characteristic  fossils  present  in  the  beds. 

In  other  cases  the  former  existence  of  the  lake  is  recognized  by  old 
shore  lines  formed  by  wind,  stream,  and  wave  action.  Not  only  these 
forms  of  shore  lines  are  shown  but  there  may  also  be  preserved  spits, 
hooks,  bars,  deltas,  arid  beaches  as  in  the  ancient  Lake  Bonneville,  the 
ancestor  of  the  present  Great  Salt  Lake,  in  Utah. 

The  waters  of  the  Great  Lakes  formerly  covered  a  much  larger  area 
than  they  do  now  as  their  outlets  were  closed  up  by  the  continental  ice 
sheet.  Their  old  shore  lines  sometimes  serve  as  natural  grades  for 
roads,  the  well-known  Ridge  road  along  Lake  Ontario  being  one. 


References  on  Lakes 

1.  Davis,  C.  A.,  Mich.  Geol.  Survey,  Ann.  Kept.,  1906,  p.  105, 
1907.  (Peat  bogs.)  2.  Fairchild,  H.  L.,  Geol.  Soc.  Amer.,  Bull., 
Vol.  XXIV,  p.  133,  1913.  (Extinct  lakes  in  N.  Y.)  3.  Fenneman, 
Wis.  Geol.  Survey,  Bull.,  VIII,  1902.  (s.  e.  Wis.)  4.  Nichols,  W.  R., 
Bos.  Soc.  Nat.  Hist.,  1880.  (Temperature  of  freshwater  lakes.) 
5.  Russell,  I.  C.,  Lakes  of  North  America,  Ginn  &  Co.  (New  York, 
1895.)  6.  Sellards,  Fla.  Geol.  Survey,  3rd  Ann,  Rept.,  1910.  (Florida.) 
7.  Smith,  Jr.,  H.,  Trans.  Amer.  Soc.  Civ.  Eng.,  XIII,  p.  73,  1884". 
(Temperature  lakes  and  ocean.)  8.  Smyth,  Jr.,  C.  H.,  Amer.  Geolo- 
gist, XI,  p.  85,  1893.  (Lake  filling.)  9.  Tarr,  R.  S.,  Physical  Geog- 
raphy of  New  York  State,  Chapter  VI,  The  Macmillan  Co.  (New 
York,  1902.)  10.  Wilson,  A.  W.  G.,  Bull.  Geol.  Soc.  Amer.,  XIX, 
p.  471,  1907.  (Shorelines  on  Lakes  Ontario  and  Erie.) 


CHAPTER  X 

GLACIAL  DEPOSITS:    THEIR   ORIGIN, 
STRUCTURE,  AND  ECONOMIC   BEARING 

Origin  and  Nature  of  Glaciers 

GLACIERS  are  not  of  great  importance  to  the  engineer,  even  though 
they  may  be  of  considerable  scientific  interest,  but  the  work  which  they 
have  performed  in  the  past,  and  the  deposits  which  they  have  built  up 
are  matters  of  considerable  value  to  him,  and  often  present  interesting 
problems  in  connection  with  various  subsurface  operations,  such  as 
tunneling,  dam  foundations,  aqueduct  construction,  underground  water 
supply,  etc.  Glacial  deposits  sometimes  serve  also  as  a  source  of 
materials  of  economic  importance. 

While  we  shall  concern  ourselves  especially  with  the  latter  phase  of 
the  subject,  it  must,  for  the  sake  of  intelligent  understanding,  if  for  no 
other  purpose,  be  prefaced  at  least  by  a  few  remarks  on  the  way  in 
which  glaciers  originate,  and  the  work  they  perform. 

Formation  of  snow  fields.  —  In  cold  regions,  such  as  high  mountain 
tops,  and  in  polar  lands,  the  snowfall  if  heavy  may  remain  throughout 
the  year,  forming  a  perennial  snowfield. 

At  any  point  on  the  earth's  surface,  therefore,  we  find  a  level,  above 
which  the  snow  accumulates,  this  level  being  known  as  the  snowline. 
In  the  tropics  the  snow-line  is  from  15,000  to  16,000  feet  above  sea- 
level,  in  the  Rocky  Mountains  of  the  United  States  about  10,000  feet, 
in  the  Selkirks  about  8,000  feet,  while  at  the  poles  it  is  nearly  at  sea- 
level. 

The  snow  which  thus  collects  above  the  snow-line  is  disposed  of: 
(1)  By  evaporation,  especially  in  dry  regions;  (2)  by  avalanches, 
when  the  snow  collects  on  steep  slopes;  (3)  by  melting  during  warm 
days,  and  (4)  by  glaciers,  in  those  regions  where  it  cannot  be  entirely 
disposed  of  in  some  of  the  other  ways. 

Change  of  snow  to  ice.  —  If  snow  accumulates  on  the  surface  in 
quantity,  the  supply  exceeding  the  waste,  the  mass  becomes  gradually 
compacted  by  its  own  weight,  and  also  by  alternate  freezing  and  thaw- 
ing, so  that  during  the  day  when  the  surface  layer  of  snow  melts,  the 
water  trickles  down  through  the  cracks  or  pores  and  freezes  again. 

414 


GLACIAL  DEPOSITS:    THEIR  ORIGIN,  ETC.  415 

We  thus  get  a  granular  mass  which  is  between  snow  and  ice  in  its 
character  and  is  known  as  the  neve.  At  a  greater  depth  the  neve  be- 
comes still  more  compact  and  grades  into  ice. 

Ice  motion.  —  If  the  snow  and  ice  of  a  perennial  snowfield  accu- 
mulate in  sufficient  thickness,  the  ice  begins  to  move,  and  while  the 
exact  nature  of  this  motion  is  not  clearly  understood,  the  material 
seems  to  behave  much  like  a  viscous  body.  Such  a  mass  of  moving 
ice  is  called  a  glacier. 

If  the  ice  sheet  accumulates  on  a  comparatively  flat  surface,  the 
flow  may  take  place  in  all  directions  from  a  central  point,  but  if  the 
accumulation  is  on  a  slope,  the  latter  will  guide  the  direction  of  flow. 
Moreover,  if  such  a  slope  is  composed  of  valleys  and  ridges,  the  ice  will 
be  deeper  in  the  former,  or  be  confined  to  them  entirely  in  its  downward 
course. 

Conditions  essential  to  the  formation  of  glaciers.  —  These  are:  (1) 
Sufficient  atmospheric  moisture;  (2)  temperature  low  enough  during  a 
part  of  the  year  to  precipitate  the  moisture  as  snow;  and  (3)  the 
snowfall  during  at  least  a  part  of  the  year  must  be  in  excess  of  the 
summer's  melting,  so  that  the  accumulation  of  one  year  is  added  to 
the  fall  of  the  next,  etc.,  for  a  period  of  time. 

Types  of  glaciers.  —  Depending  then  on  the  conditions  of  accumu- 
lation we  can  recognize  three  types  of  glaciers:  (1)  Continental  glaciers, 
or  those  forming  an  ice  cap  covering  a  large  part  of  a  continent.  (2)  Val- 
ley glaciers,  or  those  extending  either  from  the  edge  of  an  ice  cap  as 
polar  glaciers  (ice  tongues)  or  from  a  neve  hi  the  mountains,  down  into  the 
valley  forming  alpine  glaciers  (Plate  LXVI,  Fig.  1).  (3)  Piedmont 
glaciers,  or  those  formed  by  the  merging  of  valley  glaciers  which  have 
descended  to  the  plain. 

General  features  of  glaciers.  —  The  motion  of  a  glacier,  especially 
the  valley  type,  bears  some  resemblance  to  that  of  rivers,  the  middle 
and  top  flowing  faster  than  the  bottom  and  sides,  because  these  are 
retarded  by  the  friction  of  the  ice  against  the  ground.  While  the  ice 
flows,  it  is  not  exceedingly  elastic,  and  comparatively  slight  irregularities 
of  its  bed,  cause  it  to  crack.  It  is  therefore  sometimes  much  broken 
by  crevasses. 

The  rate  of  flow  of  the  glacier  ice  depends  mainly  on  the  supply  of 
snow,  the  grade,  and  the  seasonal  temperatures.  The  glaciers  of  the 
Alps  advance  at  a  rate  of  from  two  to  fifty  inches  per  day  in  summer 
and  about  half  that  rate  in  winter,  while  the  vastly  larger  glacier  which 
enters  Glacier  Bay  in  Alaska  has  a  summer  velocity  of  70  feet  per  day 
in  the  middle  (Scott). 


PLATE  LXVI,  FIG.  1.  —  General  view  of  an  alpine  glacier,  the  Asulkan,  near  Glacier, 
B.  C.  Shows  the  reservoir  or  neve,  with  glacier  descending  from  it;  two  lateral 
moraines  on  either  side,  which  have  been  left  as  the  glacier  shrank  in  width; 
the  crevassed  ice  fall,  represented  by  roughened  dark  surface,  just  above  curve 
in  glacier.  (H.  Hies,  photo.) 


FIG.  2.  —  General  view  of  Lake  Louise,  Alberta,  from  the  Victoria  glacier.  The  lake 
occupies  a  hanging  valley,  its  waters  being  held  in  by  a  moraine  at  the  lower 
end.  In  the  foreground  the  debris-covered  surface  of  the  Victoria  glacier,  with 
two  moraines  at  either  side  beyond.  Sediment  carried  down  by  glacial  stream 
is  building  out  a  delta  at  head  of  lake.  (H.  Ries,  photo.) 
(416) 


GLACIAL  DEPOSITS:    THEIR  ORIGIN,  ETC.  417 

As  the  ice  stream  descends  from  the  snowfield  or  reservoir  to  lower 
levels,  it  melts  slowly  and  diminishes  in  thickness,  but  the  effect  of 
melting  is  most  noticeable  at  the  lower  end. 

If  now  the  rate  of  melting  back  at  the  lower  extremity,  and  the  rate 
of  advance  of  the  ice  are  balanced,  the  glacier  appears  to  be  stationary; 
if  rate  of  advance  exceeds  rate  of  melting,  the  ice  front  advances,  while 
under  reversed  conditions  it  appears  to  retreat  (Plate  LXVII). 

Effects  of  advancing  glaciers.  —  Advancing  glaciers  may  cause 
damage  in  different  ways.  Several  cases  may  be  mentioned. 

Glacier  advance  over  territory  not  hitherto  glaciated  occasionally  re- 
sults in  the  destruction  of  forests  in  the  path  of  the  moving  ice,  but 
such  cases  are  comparatively  rare  in  modern  times,  although  it  has 
been  noticed  in  Alaska. 

In  rare  instances  a  glacier  may  hi  its  advance  cross  a  valley,  damming 
the  stream  occupying  the  latter.  There  is  then  danger  of  the  ponded 
water  becoming  suddenly  released.  Thus  Geikie1  states  that  "the 
valley  of  the  Dranse  in  Switzerland  has  several  times  suffered  from 
this  cause.  In  1818,  the  glacial  barrier  extended  across  the  valley  for 
more  than  half  a  mile,  with  a  breadth  of  600  feet  and  a  height  of  400 
feet.  The  waters  above  the  ice  dam  accumulated  hi  a  lake  containing 
800,000,000  cubic  feet.  By  a  tunnel  driven  through  the  ice  the  water 
wras  drawn  off  without  desolating  the  plains  below." 

Marginal  lakes  held  between  the  edge  of  the  glacier  and  the  moraine, 
or  rock  walls  are  not  uncommon,  and  the  change  in  position  of  the 
glacier  sometimes  permits  their  sudden  release.  There  are  many  cases 
of  damaging  floods  from  the  breaking  of  dams  of  marginal-glacier  lakes. 
At  Yaldez,  Alas.,  a  few  years  ago  such  a  flood  swept  away  many  houses, 
and  on  the  Copper  River  Railway,  in  Alaska,  a  portion  of  a  trestle  was 
swept  away.2 

An  interesting  case  of  trouble  caused  by  living  glaciers  is  found  in 
Alaska,  along  the  line  of  the  Copper  River  and  Northwestern  Rail- 
road. The  road,  which  has  its  terminus  at  Cordova,  runs  eastward 
across  the  great  delta  of  the  Copper  River,  and  here  shifting  glacial 
streams  made  railroad  building  very  difficult,  for  the  river  is  subject 
to  great  and  rapid  fluctuations  of  volume  and  load,  so  that  quick- 
sand bottom,  erosion,  deposition,  channel  shifting,  and  floating  ice, 
all  add  to  the  engineers'  problems.  Farther  up  the  line  where  the 
Xiles  Glacier  has  pushed  across  the  valley,  crowding  the  Copper 
River  to  one  side,  the  road  was  blasted  out  of  the  steep  rock  wall 

1  Textbook  of  Geology,  3rd  ed.,  1893,  p.  382. 

2  Private  communication  from  Prof.  L.  Martin. 


418  ENGINEERING  GEOLOGY 

above  the  river,  and  the  track  here  is  exposed  to  rock  and  snow  slides. 
Still  farther  up  the  route,  the  Allen  Glacier  was  found  to  project 
clear  across  the  main  valley,  and  the  engineers  decided  to  build  the 
road  on  the  glacier  itself.  They  accordingly  blasted  out  a  grade 
across  5J  miles  of  a  stagnant,  moraine-veneered,  tree-covered  ice 
mass.  Ice  lies  beneath  the  ties,  and  future  melting  of  it  will  cause 
slumping  and  repeated  grading.  If  the  glacier  begins  to  advance 
there  will  be  more  trouble.1 

During  the  Glacial  Period  the  continental  ice  sheet  of  North  America  in  several 
cases  formed  a  dam  across  valleys  occupied  by  lakes,  causing  the  water  surface  to  rise 
as  much  as  several  hundred  feet  above  its  normal  level.  A  fine  example  of  this  is  seen 
in  the  valley  of  Cayuga  Lake  in  New  York  State,  where  the  numerous  delta  terraces 
observed  at  different  levels  on  the  valley  slopes  show  the  several  levels  at  which  the 
lake  stood,  while  its  waters  were  dammed  by  the  ice  during  its  retreat  to  the  north- 
ward. Elevated  shore  lines  around  some  of  the  Great  Lakes  were  formed  when  their 
waters  formerly  stood  at  higher  levels  due  to  the  same  cause. 

Additional  trouble  may  be  caused  by  streams  fed  by  the  melting 
snow  and  ice.  During  winter,  or  cold  days  and  nights  of  summer, 
when  little  or  no  melting  takes  place,  the  streams  flowing  from  the 
swnofields  are  sometimes  of  small  volume,  but  on  warm  sunny  days 
when  the  snow  and  ice  melt  rapidly,  the  volume  of  the  streams  is  greatly 
augmented. 

Care  should  be  taken,  therefore,  to  bear  this  in  mind  in  constructing 
rail  and  wagon  roads  in  mountain  regions  where  there  is  an  abundant 
accumulation  of  snow  and  ice. 

Cases  are  known  where  roads  constructed  too  near  to  the  edge  of  a 
snowfed  stream  have  been  overflowed  regularly  on  warm  summer  days, 
and  in  some  instances  undermined  and  washed  away  in  places. 

Glacial  erosion.  —  Glaciers  like  rivers  perform  a  certain  amount 
of  erosion  which  is  so  characteristic  that  it  enables  us  to  recognize  the 
former  existence  of  the  ice,  even  though  it  has  long  since  disappeared. 
How  much  erosive  work  they  are  capable  of  doing  is  a  matter  of  dispute, 
but  it  must  vary  since  it  depends  on  the  velocity  of  movement,  amount 
of  rock  material  held  in  their  lower  layers,  the  pressure  on  the  beds, 
thickness  of  ice,  and  character  of  rock  surface. 

Erosion  may  be  accomplished  in  several  ways,  as  follows:  (1)  In 
moving  over  a  surface  not  yet  traversed  the  ice  often  removes  the 
soil  or  other  loose  materials  from  it.  (2)  Rocks  and  sand,  partly 
imprisoned  in  the  lower  part  of  the  ice,  when  rubbed  over  a  bare 
rock  surface,  and  held  down  against  it  under  great  pressure  abrade 

1  Martin,  Bull.  Amer.  Geog.  Soc.,  XLV,  p.  801,  1913;  Nat.  Geog.  Mag.,  XXII, 
p.  541,  1911;  Tarr  and  Martin,  Annals  Assoc.  Amer.  Geog.,  II,  p.  25,  1913. 


PLATE  LXVII,  FIG.  1.  —  View  of  lower  end  of  Asulkan  glacier  as  it  appeared  in 

1908.     (H.  Ries,  photo.) 


FIG.  2.  —  The  same  glacier  as  it  appeared  in   1910,  showing  how  the  end  has 
receded.     (H.  Ries,  photo.) 

(419) 


420  ENGINEERING  GEOLOGY 

the  bed  rock  more  or  less,  as  well  as  polishing,  scratching  or  grooving 
it  in  a  very  characteristic  manner. 

Glaciated  rock  surfaces  are,  therefore,  easily  recognized.  They  are 
sometimes  very  uneven,  and  hence  in  a  glaciated  region  the  bed  rock 
often  lies  at  a  variable  distance  below  the  surface,  a  fact  that  engineers 
should  remember  in  sinking  foundations. 

Erosion  is  also  performed  by  a  process  known  as  plucking,  which  is 
the  tearing  away  of  joint  blocks  by  the  advancing  ice. 

Where  glaciers  have  performed  much  erosion  the  topographic  fea- 
tures are  usually  quite  characteristic.  Thus  angular  outlines  are 
rounded  off,  and  the  cross-section  of  a  glaciated  valley  is  U-shaped 
with  a  broad  bottom  and  very  steep  sides.  A  river  valley  in  contrast 
has  a  V-shaped  cross-section,  with  projecting  spurs.  These  latter  are 
removed  by  prolonged  glaciation.  Lake  valleys  are  sometimes  deepened 
by  glacial  erosion,  as  in  the  case  of  the  Great  Lakes,  and  also  the 
Finger  Lakes  of  central  New  York. 

If  a  main  valley  is  deepened  by  glacial  erosion,  while  its  tributary  is 
less,  or  but  slightly  deepened,  the  lower  end  of  the  latter  will  be  above 
the  former  when  the  ice  disappears,  that  is,  the  tributary  will  be  dis- 
cordant as  to  grade  with  its  main  valley,  depending  upon  the  inequality 
of  deepening  in  the  two  valleys.  The  tributary  valley  is  then  known  as 
a  hanging  valley  (Plate  LXVI,  Fig.  2).  Such  valleys  are  not  uncommon 
in  some  glaciated  regions. 

Glacial  transportation.  —  Glaciers  can  transport  material  on  their 
surface,  within  their  mass  or  in  the  bottom  part  of  the  ice. 

The  material  which  is  carried  on  the  surface  consists  of  rock  frag- 
ments of  all  sizes  and  other  debris  that  has  fallen  on  to  the  ice  from 
cliffs  and  slopes  that  project  above  it.  Sometimes  the  surface  of  the 
glacier  is  so  completely  covered  by  debris,  that  the  ice  is  not  visible 
(Plate  XLVI,  Fig.  2). 

The  bottom  of  the  glacier  is  often  a  confused  mass  of  ice,  stones, 
etc.,  and  when  deposited  forms  the  ground  moraine. 

The  englacial  drift  is  either  debris  that  has  fallen  into  cracks  from 
the  surface,  or  has  collected  on  the  surface  of  the  snow,  and  become 
covered  by  subsequent  snowfalls.  It  is  protected  from  wear  by  the 
glacier  and  can  usually  be  recognized  by  its  angular  character. 

Glacial  Deposits 

Surface  moraines.  —  The  debris  which  accumulates  on  the  surface 
of  a  glacier  is  sometimes  arranged  in  belts  or  bands  which  are  called 
moraines.  If  the  debris  is  heaped  up  in  ridges  on  the  side  of  the  glacier 


GLACIAL  DEPOSITS:    THEIR  ORIGIN,   ETC.  421 

it  is  called  a  lateral  moraine.  If  in  a  parallel  position  but  some  distance 
from  the  edge  it  is  known  as  a  medial  moraine,  and  several  of  these 
may  exist  on  the  same  glacier.  Such  moraines  are  sometimes  formed 
by  the  union  of  lateral  moraines  when  two  glaciers  join. 

If  the  edge  of  the  glacier  remains  stationary  or  nearly  so  for  some 
time,  all  the  transported  material,  except  that  carried  away  by  water, 
is  dropped  as  the  ice  melts,  and  forms  a  more  or  less  hummocky  ridge 
at  the  end  of  the  glacier,  known  as  a  terminal  moraine. 

Since  ice  does  not  sort  material  as  does  water,  the  terminal  moraine, 
when  not  modified  by  escaping  glacier  waters,  is  unstratified  and  con- 
sists of  materials  of  all  sizes  from  silt  and  fine  sand  up  to  boulders 
weighing  many  tons. 

If  a  glacier  remains  stationary  for  a  considerable  period  of  time,  all 
things  being  equal,  a  moraine  of  large  size  may  be  built  up,  provided 
the  glacier  transports  much  material;  or  if  during  its  recession  the 
glacier  halts  for  a  time  at  different  points  a  number  of  terminal  moraines 
will  be  formed. 

When  a  glacier  melts  slowly  its  debris  is  deposited  as  an  irregular 
sheet,  which  constitutes  the  ground  moraine.  This  is  not  stratified  ex- 


FIG.  188.  —  Section  showing  relation  of  overwash  plain  to  a  terminal  moraine. 

cept  in  those  places  where  modified  or  formed  by  water.  It  consists 
of  fine  clay  or  sand  with  scattered  boulders,  the  latter  often  showing 
scratches  and  is  termed  till  or  boulder  clay.  Drift  is  a  general  term 
applied  to  glacial  deposits. 

Nature  of  glacial  deposits.  —  Glacial  deposits  are  usually  quite 
characteristic  in  appearance  for  several  reasons:  (1)  The  ice  does  not 
exercise  a  sorting  action,  so  that  we  find  boulders,  cobbles,  pebbles, 
sand  and  clay  forming  a  confused  mass;  (2)  the  stones  of  the  drift, 
although  worn,  are  not  rounded  like  those  transported  by  water,  but 
have  a  more  or  less  subangular  form;  and  (3)  the  stones  are  often 
striated  and  polished. 

The  moraines  of  pre-existing  glaciers  often  form  natural  dams  across 
valleys,  obstructing  the  drainage,  and  creating  lakes  that  serve  as 
sources  of  water  supply.  As  the  material  is  not  very  permeable,  little 


422  ENGINEERING   GEOLOGY 

seepage  results.  At  other  times  the  old  moraines  still  remain  as  ranges 
of  hummocky  hills  extending  across  the  country. 

Glacial-water  deposits.  —  The  water  flowing  from  a  glacier  may 
carry  vast  amounts  of  debris,  sometimes  of  considerable  coarseness, 
and  deposit  the  latter  over  the  surface  beyond  the  glacier  margin. 
If  this  is  deposited  in  valleys  it  is  called  a  valley  train,  but  if  on  a  more 
or  less  flat  surface  of  large  areal  extent,  the  term  outwash  plain  or 
frontal  apron  (Fig.  188)  is  applied  to  it.  Deposits  of  this  kind  are 
usually  distinguishable  from  ordinary  river  deposits  by  the  fact  that 
they  often  grade  into  moraines,  and  that  their  constituents  bear  evi- 
dence of  glacial  origin.  Eskers  are  long,  winding  gravel  ridges,  de- 
posited by  streams  flowing  in  channels  in  the  ice,  or  beneath  it. 
Kames  are  short  ridges  of  similar  material  piled  up  by  glacial  streams 
flowing  from  beneath  the  ice,  frequently  against  the  end  or  terminal 
moraine. 

Past  glaciation.  —  Glaciers  in  the  past  have  accomplished  similar 
work,  and  built  up  the  same  kind  of  deposits  as  existing  ones.  From 
such  evidence,  therefore,  as  glacial  erosion,  smoothed  and  striated  rock 
surfaces,  the  deposition  of  moraines  and  other  glacial  drift  including 
perched  erratics  of  foreign  rock,  and  general  characteristic  modification 
of  the  land  surface  (stream  and  interstream  areas)  by  erosion  and 
deposition,  we  can  affirm  that  all  of  Canada,  and  the  northern  part  of 
the  United  States  were  formerly  covered  by  a  vast  continental  glacier, 
which  started  from  two  or  three  centers  to  the  north  and  moved  from 
these  centers  of  dispersion,  probably  outward  in  all  directions.  In 
the  eastern  United  States  it  extended  to  the  dotted  line  indicated  on  the 
map  (Fig.  163). 

As  a  result  of  this  the  engineer  at  the  present  day  finds  himself  con- 
fronted with  a  number  of  phenomena,  which  sometimes  seem  very  per- 
plexing, but  whose  understanding  is  often  of  vital  importance  from  the 
financial  standpoint.  Some  of  these  are  discussed  below. 

Glacial  drift.  —  The  glaciated  area  of  the  United  States  and 
Canada  is  covered  with  a  more  or  less  continuous  mantle  of  drift  of 
variable  thickness,  usually  being  deepest  in  the  valley  bottoms  and 
thinnest  on  the  interstream  areas.  In  the  United  States  it  is  thickest 
in  a  broad  belt  a  little  within  the  margin  of  the  drift  area,  which  ex- 
tends from  central  New  York  throughout  Ohio,  Indiana,  Illinois, 
Iowa,  Minnesota  and  Dakota,  and  thence  northward  to  an  unknown 
limit  in  Canada  (Chamberlin  and  Salisbury). 

Over  any  region  the  thickness  of  the  drift  may  vary  within  short 
distances.  The  depth  then  to  bed  rock  may  be  quite  variable,  and 


GLACIAL  DEPOSITS:    THEIR  ORIGIN,  ETC. 


423 


the  drift  mantle  either  decreases  or  increases  the  relief  of  the  surface 
(Figs.  189  and  190). 

The  vertical  range  is  also  great,  for  in  New  York  State  it  is  found 
from  sea-level  to  nearly  5000  feet  altitude  in  the  Adirondacks. 

The  contact  between  the  drift  and  the  underlying  rock  surface  is 
usually  sharply  defined  for  the  reason  that  the  continental  glacier 


FIG.    189.  —  Section  through  glacial  drift  and  bed  rock,  showing  how  the  deposi- 
tion of  morainal  material  has  made  the  surface  more  irregular. 

removed  in  most  places  the  residual  soil,  leaving  the  fresh  and  firm 
underlying  rock. 

Many  of  the  rocks  distributed  through  the  drift  are  of  kinds  occur- 
ring many  miles  to  the  north  of  where  they  are  now  found.     Large  ice- 


FIG.  190.  —  Section  showing  how  the  deposition  of  glacial  drift  has  reduced  sur- 
face irregularities. 

transported  boulders  many  tons  in  weight  are  also  found  scattered 
over  the  drift-covered  area,  regardless  of  topography. 

Sometimes  the  drift  is  of  great  thickness  even  in  places  where  one 
might  not  expect  it.  Thus  at  Mineville,  N.  Y.,  one  of  the  mine  shafts 
sunk  on  a  hillside  passed  through  250  feet  of  drift  before  reaching  bed 
rock. 

Large  boulders  in  the  drift  are  sometimes  mistaken  for  bed  rock  in 
drilling,  especially  where  wash  borings  are  made.  In  sinking  test 
holes  along  the  line  of  the  Catskill  aqueduct  for  New  York  City  the 
drillers  on  Moodna  Creek  struck  a  glacial  boulder  at  15  feet  and  re- 
ported bed  rock,1  whereas  the  latter  was  300  feet  below  the  surface. 

Topography  of  the  drift.  —  The  drift  presents  certain  character- 
istic topographic  features,  such  as:  (1)  Depressions  without  outlets; 

1  Berkey,  N.  Y.  State  Museum,  Bull.  146,  p.  26,  1911. 


424  ENGINEERING  GEOLOGY 

(2)  knobs,  hills  and  ridges  of  similar  size  to  the  depressions,  associated 
with  them;  (3)  and  ponds  often  formed  in  the  depressions. 

The  topography  of  a  terminal  moraine  is  more  or  less  characteristic. 

"It  sometimes  constitutes  a  more  or  less  well-defined  ridge,  though 
this  is  not  its  distinctive  feature,  since  its  width  is  generally  great 
relative  to  its  height.  A  moraine  50  or  even  100  feet  high  and  a  mile 
wide  is  not  a  conspicuous  topographic  feature,  except  in  a  region  of 
unusual  flatness.  In  such  situations  terminal  moraines  sometimes 
constitute  important  drainage  divides.  The  surface  is  often  character- 
ized by  hillocks  and  hollows,  or  by  interrupted  ridges  and  troughs, 
following  one  another  in  rapid  succession,  and  without  apparent  order 
of  arrangement"  (Chamberlin  and  Salisbury). 

Glaciation  and  Engineering  Problems 

Buried  channels.  —  Many  of  the  present  streams  occupy  the 
partly  or  completely  filled  pre-Glacial  valleys.  During  the  Glacial 
Period  their  valleys  or  gorges  became  completely  clogged  with  glacial 
drift  so  that  after  the  recession  of  the  glacier  these  streams  had  to  cut 
new  channels.  Abundant  modification  of  stream  drainage  has  resulted. 

In  some  cases  a  stream  has  sunk  its  channel  through  the  thickness  of 
drift,  in  others  not,  while  in  still  others  the  deflection  to  one  side  of  its 
former  valley,  has  enabled  it  to  cut  through  into  the  underlying  hard 
rock.  Again  others  are  flowing  in  new  channels  on  the  drift  cover. 

Tunneling  and  buried  channels.  —  Tunnels  sometimes  encounter 
these  buried  channels.  For  example,  in  bringing  the  aqueduct  tunnel 
from  the  Catskill  Mountains  to  New  York  City,  a  number  of  these 
buried  channels  were  encountered  (Fig.  191),  and  it  was  necessary  to 
carry  the  water  under  these  by  inverted  siphons.  The  deepest  was 
that  of  the  Hudson  Valley  in  the  Highlands,  where  the  tunnel  had  to  be 
carried  1000  feet  below  sea-level  in  order  to  get  under  the  buried  gorge 
of  the  Hudson. 

Buried  channels  are  also  of  importance  in  connection  with  under- 
ground water  supply,  for  the  gravels  and  sands  that  sometimes  fill 
them  carry  a  sufficient  supply  of  good  water  to  be  drawn  upon  (Fig. 
156). 

In  central  New  York  some  of  the  streams  tributary  to  the  lakes  now 
occupy  post-Glacial  gorges,  while  their  buried  pre-Glacial  channels  lie 
at  the  same  level  to  one  side.  One  of  these  buried  channels  was  used 
to  conduct  a  water  pipe  from  a  reservoir  to  a  power  house  farther  down 
the  gorge.  In  another  case  it  was  noticed  during  the  construction  of 
a  reservoir  across  a  post-Glacial  valley  that  the  pre-Glacial  channel  left 


GLACIAL  DEPOSITS:    THEIR  ORIGIN,   ETC. 


425 


the  stream  a  short  distance  above  the  dam.     Some  fear  was  at  first 
felt  lest  there  might  be  leakage  from  the  reservoir  through  this  channel. 


SHAWANGUNK 


HUDSON  RIVER  S. 


FIG.  191.  —  Sections  across  Rondout  Valley,  N.  Y.,  showing  pre-Glacial  valleys  which 
have  been  filled  with  glacial  drift.     (Berkey,  N.  Y.  State  Museum,  Bull.  146.) 

It  was  found,  however,  that  the  latter  was  choked  with  rather  dense 
clay. 

Underground  water  supply.  —  The  drift  is  known  to  contain  con- 
siderable water,  which  is  drawn  upon  for  dug  and  artesian  wells  as 
discussed  elsewhere.  (See  Artesian  Water.) 

Dam  sites.  —  In  the  construction  of  dams  across  valleys  in  gla- 
ciated areas,  it  is  sometimes  necessary  to  construct  them  hi  glacial 
drift,  which  covers  the  bed  rock.  In  such  cases  the  drift  should  be 
carefully  tested  at  different  points  to  get  a  water-tight  foundation,  for 
the  reason  that  within  the  till  there  are  frequently  pockets,  lenses  or 
beds  of  sand  and  gravel  which  are  permeable  to  water.  Obviously, 
where  several  sites  are  available,  that  one  will  be  the  best,  which  con- 
tarns  the  densest  material,  thus  avoiding  the  danger  of  leakage  under, 
or  around  the  ends  of  the  dam. 


426  ENGINEERING  GEOLOGY 

In  selecting  a  dam  site  for  the  reservoir  that  is  to  supply  the  new 
Catskill  aqueduct  leading  to  New  York  City,  the  engineers  found  two 
locations  known  as  the  Olive  Bridge  (Fig.  193),  and  the  Cathedral 
gorge  or  Tongore  site  (Fig.  192),  either  of  which  seemed  possible  from 
a  topographic  standpoint.  Both,  however,  were  carefully  explored  by 
trenches,  shafts,  and  boreholes. 

In  each  case  it  was  found  that  the  bed  rock  had  an  uneven  surface, 
that  there  was  a  buried  gorge  of  Esopus  Creek,  and  that  the  glacial 
deposits  were  over  200  feet  thick  in  the  narrow  valley,  as  shown  by 
sections. 

The  Olive  Bridge  site  was  chosen  because  of:  (1)  Higher  bed  rock 
surface  throughout;  (2)  more  uniform  and  impervious  character  of 
the  drift;  (3)  more  massive  cross-section  of  the  drift  barrier  for  the 
foundations;  (4)  perfectly  tight  contacts  of  till  and  bed  rock;  and  (5) 
restriction  of  more  porous  materials  to  the  higher  levels  of  the  section. 

Quarrying  operations.  —  The  continental  glacier  has  indirectly 
affected  quarrying  operations.  ,Thus  in  the  states  lying  within  the 
glaciated  area,  the  residual  soil  and  partly-decayed  rock  have  been 
removed,  and  the  quarryman  usually  finds  sound  stone  at  bed  rock 
surface,  but  south  of  the  glaciated  region,  the  residual  soil  and  partly- 
decayed  rock  still  remain,  and  stripping  to  some  depth  is  often  necessary 
to  reach  fresh  rock. 

Water  powers.  —  Attempts  are  sometimes  made  to  show  that  the 
continental  glacier  was  an  indirect  cause  in  the  development  of  abund- 
ant water  power.  However,  this  view  may  be  a  somewhat  exaggerated 
one,  as  many  important  water  powers  exist  and  are  being  developed 
outside  of  the  glaciated  area.  It  is  true,  of  course,  that  a  considerable 
fall  is  sometimes  obtained  in  post-Glacial  valleys,  and  at  the  mouth_of 
hanging  valleys,  which  can  be  used  for  power  purposes. 

Economic  materials  in  glacial  deposits.  —  Owing  to  the  diversified 
nature  of  the  glacial  drift,  it  contains  a  variety  of  materials  of  economic 
value.  The  masses  of  clay  found  in  moraines  and  glacial-lake  basins 
can  be,  and  are  used  frequently  for  brick  manufacture.  Beds  of  sand 
and  gravel  occurring  in  the  moraines  and  modified  drift  are  employed 
for  mortar  work,  railway  ballast,  concrete,  cement  blocks,  foundry 
molds,  sand-lime  brick,  glass  manufacture,  and  filter  plants. 


References  on  glaciers 

1.  Berkey,  C.  P.  —  Geologic  Features  and  Problems  of  the  New 
York  City  (Catskill)  Aqueduct,  N.  Y.  State  Museum,  Bull.  146,  1911. 
2.  Chamberlin  &  Salisbury,  —  A  College  Textbook  of  Geology,  New 


GLACIAL  DEPOSITS:    THEIR  ORIGIN,  ETC. 


427 


Ban, 


Site 


Elev. 


FIG.  192.  —  Section  through  Tongore  dam  site,  tested  for  Catskill,  N.  Y.,  aque- 
duct.    (After  Berkey,  N.  Y.  State  Museum,  Bull.  146.) 


.  _Cenfer 


I/toe 


Olive 


Elev.600 


SCALE  OF  FEET 
,100  0   200  400  600  800 


SECTION  OF  SITE  ON  CENTER  LINE 


FIG.  193.  —  Section  through  Olive  Bridge  dam  site,  tested  for  Catskill,   N. 
aqueduct.     (After  Berkey,  N.  Y.  State  Museum,  Bull.  146.) 


428  ENGINEERING  GEOLOGY 

York,  1909.     (Henry  Holt  &  Co.)     3.   Salisbury,  R.  D.  —  New  Jersey 
Geol.  Surv.,  Final  Report,  V,  1902. 

Many  of  the  geological  surveys  of  states  lying  within  the  glaciated 
region  have  published  special  bulletins  or  reports  on  their  glacial  de- 
posits. The  United  States  Geological  Survey  has  also  issued  a  number. 
Most  of  these  are  written  from  the  purely  geologic  standpoint.  They 
may  be  of  value  to  engineers  in  the  areas  of  which  they  treat,  since 
they  give  information  regarding  the  thickness  and  character  of  the 
drift. 


CHAPTER  XI 
BUILDING   STONE 

Properties  of  Building  Stone 

Under  the  term  " building  stone"  are  included:  (1)  All  stone  used  for 
dimension  blocks  in  the  ordinary  construction  of  buildings,  dams,  dry 
docks,  retaining  walls,  etc.;  (2)  stone  used  for  purposes  of  ornamenta- 
tion; and  (3)  stone  used  for  roofing.  Stone  employed  for  flagging  and 
paving  blocks  belong  more  properly  under  the  head  of  paving  materials. 

Kinds  of  rock  used.  —  Many  different  kinds  of  rock  are  employed 
for  building  purposes,  although  the  sedimentary  ones,  because  of  their 
wider  distribution  and  lower  cost  of  quarrying,  are  more  extensively 
utilized  than  the  igneous  or  metamorphic  ones. 

Moreover  whatever  the  class  of  stone  selected  for  building  work,  it 
is  usually  only  the  more  massive  and  denser  varieties  that  are  chosen, 
although  in  some  regions  of  mild  climate  very  soft  and  porous  rocks 
may  occasionally  be  quarried.  However,  the  choice  of  such  should  be 
made  with  great  care. 

Factors  governing  the  selection  of  building  stone.  —  The  factors 
which  govern  the  choice  of  a  building  stone  are  cost,  color,  and  durability. 
The  first  of  these  is  often  given  the  greatest  weight,  the  second  forms  the 
primary  consideration  when  the  stone  is  used  for  purely  ornamental 
purposes,  while  the  last  is  given  altogether  too  little  weight  by  many 
purchasers  of  structural  materials. 

Each  of  these  factors  may  be  considered  in  more  detail. 

Cost.  The  cost  of  a  building  stone  will  depend  on:  (1)  Its  avail- 
bility,  whether  easy  of  access,  close  to  transportation  lines,  or  in  abund- 
ance and  purity;  (2)  its  workability,  whether  easy  to  extract  and  dress; 
and  (3)  location.  One  of  these  may  greatly  overbalance  the  others. 

Thus  granites  are  found  in  several  parts  o  the  United  States,  but  an 
engineer  requiring  a  granite  for  use  in  the  Southern  states,  may  select 
the  Maine  product,  not  because  granites  of  equally  good  quality  are 
wanting  in  the  South,  but  because  the  quarries  of  the  New  England 
states  are  often  not  only  well  equipped  for  quarrying  and  handling  stone, 
but  as  a  rule  have  the  advantage  of  water  transportation. 

Color.  —  This  factor  is  perhaps  of  more  importance  to  the  architect 
than  the  engineer,  and  yet  the  latter  does  not  entirely  neglect  it,  for  in 

429 


430  ENGINEERING  GEOLOGY 

engineering  work  a  light-colored  stone  is  preferred  to  a  dark  one,  because 
of  its  brighter  and  cleaner  appearance.  Aside  from  this,  however, 
lighter-colored  stones  of  igneous  character  are  more  widely  used,  because 
their  structure  is  usually  such  as  to  permit  the  extraction  of  larger  blocks. 

Beauty.  —  This  factor  is  considered  mainly  in  the  selection  of  decora- 
tive stone,  such  as  marble  and  serpentine. 

Durability.  —  Curiously  enough  this  property  which  should  be 
regarded  as  of  primary  importance  in  the  selection  of  a  building  stone, 
is  often  relegated  to  last  place.  Many  a  costly  structure  stands  as  a 
mute  witness  to  the  neglect  of  this  important  property.  The  mere  fact 
that  a  stone  is  hard  and  dense  is  no  guarantee  that  it  will  endure  the 
attacks  of  weathering  agents  for  even  a  period  of  25  years. 

Structural  features  of  building  stone.  —  Under  this  head  are 
included  jointing,  stratification,  and  cleavage.  The  discussion  of  these 
is  of  great  practical  importance  since  they  affect  the  character  of  the 
rock,  ease  of  quarrying,  and  indirectly  the  durability. 

Joints.  —  No  stone  is  free  from  joints.  In  stratified  rocks  they  are 
usually  vertical,  and  occur  in  one  or  more  systems. 

In  igneous  rocks  they  may  be  both  vertical  and  horizontal,  and  show 
their  best  development  in  granites.  In  these  the  more  pronounced  of 
the  vertical  systems  is  termed  the  rift,  and  even  if  there  is  not  a  second 
system  at  right  anglef  to  the  rift,  the  stone  may  have  a  grain  along  which 
it  splits. 

The  horizontal  jointing  usually  present  in  granite  quarries  tends  to 
break  the  rock  into  a  series  of  sheets,  which  are  not  of  uniform  thick- 
ness throughout,  because  the  horizontal  joints  usually  converge,  thus 
breaking  the  granite  into  a  series  of  flat  lenses  often  of  considerable 
horizontal  dimensions. 

Joints  may  be  both  an  advantage  and  a  disadvantage.  Their  pres- 
ence is  beneficial  in  that  they  facilitate  the  extraction  of  the  stone,  a 
matter  of  importance  in  a  hard  rock  like  granite. 

They  are  injurious  in  some  cases,  because  (1)  they  form  a  channel  of 
access  for  the  weathering  agents,  as  a  result  of  which  the  stone  may  be 
weathered  for  a  distance  of  an  inch  or  more  on  either  side  of  the  joint 
plane.  (2)  They  limit  the  size  of  the  blocks  which  can  be  extracted, 
and  sometimes  an  otherwise  good  stone  may  be  so  cracked  by  joints  as 
to  be  of  little  value  for  any  purpose  other  than  road  material. 

If  the  horizontal  joints  are  closely  spaced,  but  vertical  joints  widely 
separated,  slabs  of  large  size  can  be  obtained,  while  if  the  horizontal 
joints  are  also  far  apart,  stone  having  all  three  dimensions  large,  can  be 
extracted. 


BUILDING  STONE  431 

As  explained  under  weathering,  crystalline  rock  like  granite,  which  is 
commonly  broken  into  blocks  by  three  sets  of  joint  planes,  may  undergo 
decay  on  all  sides  of  the  blocks,  so  that  if  the  process  proceeds  far  enough 
the  upper  part  of  the  quarry  shows  a  series  of  boulders  of  fresh  rock, 
surrounded  by  residual  clay.  In  such  event  it  may  be  necessary  to 
strip  off  15  or  25  feet  before  reaching  sound  stone. 

Quarries  in  which  the  stone  is  broken  into  blocks,  often  of  irregular 
size  and  shape,  by  jointing,  are  known  as  boulder  quarries  (Plate 
LXXIII,  Fig.  2). 

Stratification.  —  The  planes  of  stratification  present  in  all  quarries 
of  sedimentary  rock  exert  an  influence  similar  to  joints.  They  facilitate 
the  extraction  of  the  stone,  but  if  too  closely  spaced,  may  make  the  stone 
so  slabby,  that  it  is  of  no  use  except  for  flagging  purposes.  They  also 
afford  more  ready  channels  of  access  for  surface  waters,  and  thus  cause 
the  stone  to  weather. 

If  the  beds  dip  at  a  high  angle,  the  water  naturally  runs  hi  more 
readily  along  the  bedding  planes,  and  not  only  discolors  or  decays  the 
stone  along  them,  but  keeps  the  quarry  wet,  and  involves  extra  cost  of 
pumping,  unless  the  quarry  be  self-draining. 

Durability  of  Building  Stone 

The  processes  of  weathering  have  been  described  in  another  chapter, 
and  need  not  be  repeated  here.  The  durability  of  a  building  stone 
depends  on  its  ability  to  successfully  resist  the  attacks  of  weathering 
agents,  and  the  factors  affecting  this  are  structure,  texture,  and  mineral 
composition. 

Structure.  —  Any  structural  weakness  facilitates  the  operation  of 
the  weathering  agents.  Thus  joint  planes,  bedding  planes,  fault  planes, 
or  irregular  fractures  produced  by  folding  or  faulting;  in  other  wrords, 
cracks  of  brecciation,  all  serve  as  pathways  for  weathering  agents 
(Plate  LXIX,  Fig.  1).  Into  these  the  surface  waters,  frost,  and  plant 
roots  can  enter,  and  if  the  stone  is  susceptible  bring  about  its  disintegra- 
tion or  decay. 

Texture.  —  A  stone  may  be  either  coarse  or  fine  (Plates  LXXI) 
and  even-grained  (Plate  LXXI,  Fig.  1),  or  it  may  be  porphyritic 
(Plate  IV,  Fig.  2).  .It  may  also  be  dense  or  porous. 

Considering  the  texture  first,  we  find  that  stones  tend  to  disintegrate 
somewhat  under  changes  of  temperature,  and  that  coarse-grained  rocks 
are  affected  more  than  fine-grained  ones,  while  those  of  porphyritic 
texture,  especially  if  coarse-grained,  are  disintegrated  more  rapidly  than 
the  finely-porphyritic  ones. 


432  ENGINEERING  GEOLOGY 

This  disintegration  is  due  in  part  at  least  to  the  different  coefficients 
of  expansion  of  the  individual  minerals. 

A  dense  stone,  other  things  being  equal,  will  break  down  less  rapidly 
than  a  porous  one,  for  the  following  reasons.  Dense  rocks  are  practi- 
cally impervious,  hence  the  weathering  agents  connot  work  their  way 
into  them.  Porous  rocks,  being  open,  absorb  water  readily,  and  if  this 
absorbed  water  freezes  in  the  pores  of  the  stone,  it  may  split  the  latter. 

Mineral  composition.  —  Since  different  minerals  show  a  different 
degree  of  resistance  to  the  attacks  of  weathering  agents,  it  follows  that 
the  rocks,  because  of  their  varying  mineral  composition,  will  also  vary 
in  their  weather-resisting  qualities,  and  that  those  containing  the  most 
susceptible  minerals  will  suffer  first  on  exposure  to  the  elements.  The 
somewhat  rapid  breaking  down  of  rocks  with  an  abundance  of  pyrite, 
or  marbles  with  much  mica,  frequently  serve  as  a  warning  that  all 
stones  will  not  endure  forever.  (See  further  in  Chapter  on  Weathering.) 

Life  of  a  building  stone.  —  The  life  of  a  building  stone  refers  to  the 
period  of  time  that  it  will  resist  the  attacks  of  weathering  agents  without 
undergoing  disintegration  or  decay.  It  may  be  influenced  by  natural 
or  artificial  causes.  The  former  include  quarry  water  and  injurious  min- 
erals; the  latter,  selection,  quarrying  and  its  position  in  the  structure. 

Quarry  water.  —  Many  stones,  especially  stratified  ones,  contain 
water  in  their  pores  when  first  quarried.  This  is  known  as  quarry  water, 
and  may  be  present  in  some  stratified  rocks,  such  as  sandstones,  in 
sufficient  quantities  to  interfere  with  quarrying  during  freezing  weather. 

The  quarry  water  usually  contains  mineral  matter  in  solution,  and 
when  the  liquid  evaporates,  as  the  stone  dries  out,  the  former  is  left 
deposited  between  the  grains,  often  in  sufficient  quantities  to  perceptibly 
harden  the  rock. 

Estimated  life  of  building  stone.  —  The  following  table  was  compiled 
some  years  ago  by  A.  A.  Julien,  and  is  based  in  part  on  observations 
made  on  building  stone  in  New  York  City. 


Kind  of  stone  . 

Life  in  years  . 

Coarse  brownstone                    

5  to    15 

Fine  laminated  brownstone  .j?  

20  to    50 

Compact  brownstone  

100  to  200 

Bluestone  (sandstone),  untried,  perhaps  centuries  

Coarse  fossiliferous  limestone 

20  to    40 

Fine  oolitic  (French)  limestone 

30  to    40 

Marble,  coarse,  dolomitic 

40 

Marble,  fine,  dolomitic  .  . 

60  to    80 

Marble,  fine  

50  to  100 

Granite  

75  to  200 

Gneiss  50  years  to  many  centuries 

BUILDING  STONE  433 

To  the  above  might  be  added  serpentine  and  cippolino  marble,  which 
in  a  severe  climate  sometimes  do  not  have  a  life  of  more  than  2  or  3 
years. 

Injurious  Minerals 

Certain  minerals  are  to  be  regarded  as  injurious  under  all  circum- 
stances, while  others  such  as  mica  can  be  considered  so  only  when 
occurring  in  abundance  in  some  rocks,  such  as  sandstones  and  marbles. 
The  effect  of  these  may  be  as  follows: 

Flint  or  chert.  —  This  term,  as  already  explained,  refers  to  the 
amorphous  or  non-crystalline  forms  of  silica  (see  under  Quartz,  Chap- 
ter I),  which  forms  concretions  in  many  limestones  (Plate  XIII,  Fig.  2). 
There  are  several  objections  to  the  presence  of  this  material. 

Firstly,  it  is  much  harder  than  the  surrounding  rock,  and  therefore, 
interferes  with  the  cutting  of  it.  Secondly,  it  is  more  resistant  to 
weather,  and  as  a  result  stands  out  in  knotty  relief  on  the  weathered 
surface  of  the  stone.  Thirdly,  the  rock  when  exposed  to  weather  is 
likely  to  split  along  the  lines  of  chert  concretions.  Cases  are  known  of 
bridge  abutments  constructed  of  cherty  limestone,  which  split  so  badly 
that  they  had  to  be  torn  down  and  replaced. 

Mica.  —  This  is  a  common  constituent  of  many  granites,  gneisses, 
sandstones,  and  marbles.  It  is  not  harmful  in  the  first,  unless  segre- 
gated into  bunches  or  knots,  in  which  case  it  renders  the  stone  unsightly. 
In  the  second  it  seldom  causes  trouble,  unless  it  becomes  so  abundant 
as  to  develop  a  schistose  structure,  thus  interfering  with  the  use  of  the 
stone  for  dimension  blocks.  In  the  third  it  does  no  harm  if  present  in 
small  quantities,  and  is  uniformly  distributed  through  the  rock;  but  if 
it  is  abundant  and  segregated  along  the  stratification  planes,  splitting 
of  the  stone  on  continued  exposure  to  frost  is  likely  to  result.  The 
trouble  has  been  sometimes  aggravated,  as  in  the  case  of  the  Connecti- 
cut brownstone,  by  setting  the  stone  on  edge,  thus  permitting  the  layers 
to  flake  off.  Many  a  brownstone  front  in  the  cities  of  the  eastern  United 
States  has  scaled  so  badly  after  15  or  20  years  exposure,  as  to  require 
the  whole  front  of  a  building  to  be  repointed  with  hammer  and  chisel. 

Mica  is  also  an  objectionable  impurity  in  many  crystalline  limestones 
or  marbles.  In  these  it  may  be  present  in  scattered  grains,  blotches  or 
bands. 

The  scattered  grains  if  few  are  not  likely  to  cause  much  trouble,  but 
in  the  other  two  cases,  the  mica  not  only  interferes  with  the  continuity 
of  the  polish,  but  often  succumbs  to  the  attacks  of  weathering  agents 
to  such  an  extent,  that  the  stone  becomes  badly  pitted  or  even  spalls  off. 


PLATE  LXVIII,   FIG.  1.  —  Weathered  sandstone,   second  story,   County  Court 
House,  Denver,  Colo.     (R.  D.  George,  photo.) 


FIG.  2.  —  Roughened  surface  of  limestone  after  some  years  of  exposure  to  weather. 

(H.  Ries,  photo.) 
(434) 


BUILDING  STONE  435 

In  some  marbles  which  are  exposed  to  the  weather  in  a  severe  climate 
this  trouble  is  likely  to  appear  within  two  or  three  years  after  the  stone 
is  placed  in  the  building. 

Pyrite.1  —  Many  building  stones  contain  at  least  small  quantities 
of  this  mineral,  which  on  exposure  to  weather  changes  through  oxida- 
tion and  hydration  to  limonite. 

A  few  small  specks  scattered  here  and  there  through  the  rock  do  little 
or  no  harm.  In  abundance,  or  if  in  large  lumps,  the  change  of  pyrite 
to  limonite,  develops  pits  in  the  stone,  and  moreover  the  limonite  set 
free  is  often  washed  down  over  the  surface  of  the  rock  causing  an  un- 
sightly stain.  Again,  in  the  decomposition  of  pyrite,  some  sulphuric 
acid  is  formed,  and  if  the  rock  contains  carbonates  these  are  attacked 
by  the  acid  set  free. 

When  the  pyrite  changes  to  iron  sulphate,  the  latter  being  easily 
soluble  is  brought  to  the  surface  by  evaporating  moisture,  and  deposited 
there  as  a  whitish  scum.  All  "whitewash"  is  not,  however,  attributable 
to  this  cause. 

In  some  stones,  as  for  example,  the  Berea  sandstone,  the  pyrite 
appears  to  be  in  a  very  finely-divided  condition,  and  evenly  distributed 
through  the  rock.  In  this  case  the  pyrite  does  not  exercise  any  injuri- 
ous influence,  but  simply  causes  a  change  of  color,  the  stone  taking  on 
a  buff  tint  as  the  pyrite  alters  to  limonite. 

It  must  not  be  understood  from  the  above  that  all  discoloration  in 
building  stone  is  due  to  pyrite,  for  it  is  not.  Take  for  instance  the  case 
of  a  building  stone  in  which  iron  is  present  in  the  form  of  ferrous  car- 
bonate. This  will  also  change  to  limonite  on  exposure  to  weather. 

In  general  we  can  say,  that  a  stone  containing  an  appreciable  quantity 
of  pyrite  is  to  be  avoided. 

Tremolite.  —  This  is  a  white  to  pale-green  variety  of  amphibole 
(see  Chapter  I),  found  in  some  magnesian,  crystalline  limestones.  It 
occurs  in  blade-like  or  silky-looking  masses,  and  on  exposure  to  weather 
tends  to  decompose  to  a  greenish-yellow  clay.  This  washes  out  leaving 
pits  on  the  surface  of  the  stone.  Tremolite  is  found  in  some  crystalline 
limestones  in  pieces  varying  from  a  fraction  of  an  inch  in  diameter  to 
patches  several  inches  across.  It  is  not  found  in  the  stone  of  all  quar- 
ries, and  even  in  those  which  do  contain  it,  all  parts  of  the  mass  do 
not  show  it.  The  product  of  a  given  quarry  might,  therefore,  at  one 
time  run  high  in  tremolite,  and  at  another  be  quite  free  from  it. 

Prolongation  of  life  of  building  stone.  —  Much  building  stone  is 
lost  due  to  careless  quarrying.  The  use  of  too  much  explosive,  or  im- 
1  This  includes  also  marcasite  and  pyrrhotite. 


PLATE  LXIX,  FIG.  1.  —  View  in  a  limestone  quarry  showing  solvent  action  of 
water  along  joint  planes.     (H.  Ries,  photo.) 


FIG.  2.  —  Weathered  outcrop  of  silicified  limestone   conglomerate.    The  silicified 

pebbles  and  quartz  veins  are  more  resistant.     (G.  van  Ingen,  photo.) 
(436) 


BUILDING  STONE 


437 


proper  placing  of  drill  holes  may  cause  shattering  of  the  stone  and 
development  of  minute  cracks.  Although  the  latter  are  often  too  small 
to  be  noticeable  to  the  naked  eye,  still  the  frost  and  other  agents  of 
weathering  will  work  their  way  into  them  and  ultimately  injure  the 
stone.  A  considerable  quantity  of  building  stone,  especially  limestone 
and  sandstone,  is  now  quarried  with  channeling  machines  thus  avoiding 
the  use  of  explosives. 

Improper  selection  often  has  much  to  do  with  the  life  of  a  stone,  and 
all  stock  should  be  carefully  examined  before  it  is  accepted. 

Stratified  rocks  should  be  set  on  bed  and  not  on  edge. 

Rocks  of  a  highly  absorbent  character  should  be  either  set  in  a  dry 
position,  or  else  coated  with  some  waterproofing  material.  Moreover 
very  porous  stones  should  not  be  used  in  a  cold  moist  climate. 

The  weather  conditions  of  the  middle  and  northern  Atlantic  states, 
with  their  frequent  extreme  changes  of  temperature  in  winter,  are 
especially  severe  on  many  building  stones. 

Even  the  careless  dressing  of  the  surface  of  a  building  stone,  may 
open  up  minute  crevices  into  which  weathering  agents  work  their  way 
quietly,  but  persistently. 

Physical  Properties 

Absorption.  —  The  absorption  of  a  building  stone  refers  to  the 
quantity  of  water  which  it  will  absorb,  and  is  usually  expressed  in 
percentage  terms  of  the  original  dry  weight.  It  shows  wide  extremes 
even  in  the  same  kind  of  rock,  but  in  general  it  is  very  low  in  igneous 
rocks  (excepting  certain  volcanic  ones)  and  metamorphic  rocks.  Lime- 
stones and  sandstones  show  variation,  but  in  general  absorption  is  low 
in  those  used  for  building  purposes.  Figures  indicating  the  range  of 
absorption  of  the  different  kinds  of  stones  are  given  under  their  respective 

heads. 

HIRSCHWALD'S  ABSORPTION  TESTS 


Percentage  by  weight. 

Percentage  of  pore  volume. 

I. 

II. 

III. 

iy. 

la. 

Ha. 

Ilia. 

S. 

Sandstone         

4.89 
6.90 
0.35 
7.51 
0.51 
22.11 
0.51 

5.66 
7.33 
0.49 
7.88 
0.55 
23.41 
0.91 

7.89 
10.80 
0.55 
19.08 
0.70 
30.25 
1.07 

9.23 
11.31 
0.59 
21.19 
0.70 
33.75 
1.25 

52.97 
61.06 
59.47 
35.46 
72.92 
65.51 
41.20 

61.30 
64.88 
84.27 
37.20 
79.16 
69.37 
57.71 

85.46 
95.48 
94.67 
90.04 
100.00 
89.64 
85.54 

0.613 
0.648 
0.831 
0.372 
0.786 
0.694 
0.728 

Sandstone 

Marble 

Limestone  

Slate 

Tuff 

Granite   

I.  Absorption  after  rapid  submersion;    II.  absorption  after  slow  submersion;    111.  submersion  under 
vacuum;    IV.  submersion  under  50  to  150  atmospheres  pressure.    Ia-IIIa  represent  the  percentage 

of  the  pore  volume  filled  by  the  water  in  each  case.    S  is  the  saturation  coefficient  and  =  j^.. 


438  ENGINEERING  GEOLOGY 

The  experiments  of  Hirschwald  (Ref.  4)  have  shown  that  a  stone 
absorbs  considerably  more  water  in  a  vacuum  or  under  strong  pressure 
than  it  does  under  normal  atmospheric  pressure. 

Relation  of  absorption  to  porosity.  —  There  is  not  necessarily  any  fixed 
relation  between  absorption  and  porosity.  The  latter  represents  the 
volume  of  pore  space  and  hence  a  stone  of  low  porosity  can  absorb 
but  little  water,  while  a  stone  of  high  porosity  may  absorb  and  hold  a 
large  quantity  of  water.  The  latter,  however,  will  depend  somewhat 
on  the  size  of  the  pores.  If  these  are  small,  the  water  is  drawn  in  by 
capillarity  and  held;  but  if  the  pores  are  large,  the  water  will  drain  off 
more  readily  if  circumstances  permit. 

It  is  probable  that  a  very  porous  stone  will  not  absorb  enough  water 
under  normal  conditions  to  completely  fill  its  pores,  and  for  this  reason 
chiefly  a  determination  of  the  porosity  of  a  stone  does  not  seem  of  great 
practical  importance,  except  in  special  cases. 

These  would  include  the  exposure  of  the  stone  to  very  moist  condi- 
tions, or  its  use  in  the  lining  of  water-carrying  tunnels,  where  the  water 
may  be  under  considerable  pressure. 

The  following  determinations  of  porosity  are  given  by  Foerster.1 

Per  cent  Per  cent 

Granite 0.04  to  0.61  Trachyte  tuff 25.07 

Syenite 1.38  Serpentine 0.56 

Diorite 25  Sandstones 6.9  to  25 . 5 

Porphyry 0.29  to  2. 75  Carrara  marble 0*22 

Basalt 1 . 28  Calcareous  tufa 32. 2 

Diabase  breccia 0. 18  Roofing  slates 0.45  to  0. 115 

Buckley's2  work  on  Wisconsin  building  stone  gives: 

Per  cent 

Granites 0 . 019  to    0. 62 

Limestones 0 . 55    to  13 . 35 

Sandstones 4. 81    to  28. 28 

and  for  Missouri  stone3  he  gives: 

Per  cent 

Granites 0. 255  to    1 . 452 

Limestones 0 . 32    to  13 . 38 

Sandstones 7,01    to  23 . 77 

The  porosity  can  be  obtained  by  the  formula: 

W  -D 


P  =  100 


W  -S 


1  Baumaterialenkunde,  I,  p.  13. 

2  Wis.  Geol.  &  Nat.  Hist.  Survey,  Bull.  IV,  p.  400,  1898. 

3  Mo.  Bur.  Geol.  &  Mines,  II,  2nd  series,  p.  317,  1904. 


BUILDING  STONE  439 

in  which 

P  =  per  cent  porosity. 

W  =  saturated  weight. 

D  =  dry  weight. 

S  =  suspended  weight  of  saturated  stone. 

Character  of  pores.  —  It  has  already  been  pointed  out  that  the  pores 
of  a  rock  may  be  either  large  or  small.  To  this  should  be  added  that 
they  may  be  comparatively  straight  or  tortuous,  and  of  varying  diam- 
eter. The  practical  significance  of  these  two  points  is  that  in  a  stone 
with  straight  pores  the  water  in  expanding  as  it  freezes  may  squeeze  out, 
and  thus  exert  less  internal  pressure  on  the  stone,  but  if  the  pores  are 
winding,  the  reverse  is  true,  and  the  stone  is  subjected  to  greater  pres- 
sure from  inside.  The  same  is  true  if  the  pores  are  constricted  at  points, 
for  the  water  finds  difficulty  in  squeezing  through  them. 

Amount  of  water  absorbed  under  different  conditions.  —  There  is  some 
question  as  to  whether  the  quantity  of  water  absorbed  in  the  laboratory 
test  is  not  greater  than  that  absorbed  when  the  stone  is  in  use. 

In  the  former  case  the  stone  is  submerged  in  water,  and  encouraged  to 
soak  up  as  much  as  possible,  indeed,  some  suggest  placing  the  submerged 
stone  under  a  vacuum,  which  would  still  further  increase  the  amount 
of  water  absorbed. 

In  use  the  stone  is  set  in  the  wall  with  one  or  at  most  two  sides  ex- 
posed to  the  rain,  and  it  is  questionable  whether  the  stone  would,  when 
so  exposed,  take  up  as  much  water  as  when  surrounded  by  water  on  all 
sides.  Of  course  if  the  stone  is  set  in  damp  soil,  or  is  exposed  to  water 
under  pressure  it  might  take  up  more. 

It  is  also  true  that  a  stone  set  in  a  cornice,  or  water  table,  or  on  a  flat 
surface  will  hold  water  or  snow  longer  than  if  placed  in  the  face  of  a 
vertical  wall. 

Crushing  strength.  —  The  crushing  strength  of  a  stone  refers  to 
its  resistance  to  pressure.  Unfortunately  it  is  a  property  to  which 
undue  importance  has  been  attached;  indeed,  in  some  cases  it  may  be 
the  only  test  made  on  a  stone.  It  can  be  safely  assumed,  as  has  been 
claimed  by  some  that  a  stone  which  "is  so  weak  as  to  be  likely  to  crush 
in  the  walls  of  a  building,  or  even  in  a  window  stool,  cap  or  pillar,  bears 
such  visible  marks  of  its  unfitness  as  to  deceive  no  one  with  more  than 
an  extremely  rudimentary  knowledge  on  the  subject." 

Few  stones  used  for  building  purposes  will,  when  tested,  show  a 
strength  under  6000  pounds  per  square  inch,  and  many,  especially 
igneous  ones,  range  as  high  as  20,000  to  30,000  pounds  per  square  inch, 
and  in  extreme  cases  40,000  pounds. 


440  ENGINEERING  GEOLOGY 

To  be  sure,  in  some  large  buildings  a  single  column  or  block  may  be 
called  upon  to  carry  a  heavy  load,  but  even  then  it  probably  does  not 
approach  the  limit  of  strength  of  the  stone. 

Buckley  has  shown  that  the  stone  at  the  base  of  the  Washington 
monument  supports  a  maximum  pressure  of  22,658  tons  per  square  foot, 
or  314.6  pounds  per  square  inch.  Allowing  a  factor  of  safety  of  twenty 
would  only  require  the  stone  at  the  base  of  the  monument  to  sustain 
6292  pounds  per  square  inch.  Even  at  the  base  of  the  tallest  buildings 
the  pressure  is  probably  not  more  than  160  pounds  per  square  inch. 

The  crushing  strength  of  a  stone  is  commonly  obtained  by  breaking 
a  cube  (usually  2  in.)  in  a  special  testing  machine.  Great  care  should 
be  taken  to  see  that  the  cubes  are  prepared  with  the  sides  smooth  and 
exactly  parallel.  In  some  cases,  instead  of  preparing  the  surface  of  the 
cube  carefully,  it  is  only  made  approximately  smooth  and  bedded  be- 
tween the  plates  of  the  machine  with  pasteboard  or  plaster  of  Paris. 

In  order  to  accurately  compare  the  crushing  strength  of  different 
building  stones,  the  conditions  under  which  the  tests  are  made  should 
be  alike  in  every  case.  The  importance  of  this  is  clearly  recognizable 
if  we  stop  for  a  moment  to  consider  the  factors  that  may  affect  the  result. 
These  may  be:  (1)  Method  of  quarrying,  whether  by  channeling  ma- 
chine or  explosive;  (2)  length  of  time  of  seasoning;  (3)  method  of  pre- 
paring cubes  for  test;  (4)  degree  of  dryness  of  stone;  (5)  temperature  of 
test  piece;  (6)  direction  of  application  of  pressure,  with  respect  to  bedding 
planes,  cleavage,  grain,  etc. ;  (7)  character  of  bearing  faces  of  machine;  (8) 
material  interposed  between  bearing  plates  of  machine  and  face  of  cube. 

These  emphasize  the  fact  that  the  crushing  test  should  be  standard- 
ized, and  all  tests  made  in  accordance  with  this  standard. 

Other  things  being  equal,  the  crushing  strength  of  a  stone  is  depend- 
ent on  the  state  of  aggregation  of  the  mineral  particles. 

In  stratified  rocks  it  depends  on  the  character  and  amount  of  the 
cementing  material,  while  in  igneous  and  metamorphic  rocks  it  is 
dependent  on  the  interlocking  of  the  mineral  grains  (Plate  VII).  This 
interknitting  of  the  minerals  produces  a  higher  average  crushing  strength 
in  the  two  last-named  classes  of  rocks. 

A  large  number  of  crushing  tests  of  building  stones  have  been  pub- 
lished (see  especially  Refs.  6,  43,  and  78),  but  those  made  by  different 
persons  are  not  always  comparable  with  safety  for  the  reason  that  the 
tests  have  not  always  been  carried  out  in  exactly  the  same  manner. 
The  following  figures  from  tests  by  Buckley  for  Missouri  and  Wiscon- 
sin, Marston  for  Iowa,  and  Parks  for  Ontario,  will  give  some  idea  of  the 
variations  which  exist  in  the  different  groups  of  stones. 


BUILDING  STONE 


441 


State  or  Province. 

Kind. 

Range,  Ibs.  per  sq.  in. 

IV'Iissouri 

Lim6ston6 

5,714  to  27,  183  on  bed 
5,774  to  25,577  on  edge 
4,371  to    9,002  on  bed 
3,933  to    9,206  on  edge 
18,236  to  19,410 
15,009  to  47,674 
6,675  to  42,787  on  bed 
7,508  to  40,453  on  edge 
4,340  to  13,669  on  bed 
1,763  to  12,566  on  edge 
2,470  to  16,435 
3,600  to  13,000 
9,539  to  31,793 
23,152  to  33,453 
12,079  to  25,018 

i 

Limestone 

f 

Sandstone  ...          

i 

Sandstone  

i 

Granite  

Wisconsin  
i 

Igneous  rocks 

Limestone  . 

^ 

< 

Limestone  .                     

Sandstone  

Iowa 

Sandstone  

Limestone 

>•>• 

Sandstone. 

Ontario1 

Sandstone  .  .  . 

Granites  and  gneisses  .... 

ii 

Crystalline  limestone  and  marbles  . 

i  Report  on  Ontario  Building  Stones  by  Parks,  Dept.  of  Mines,  Can.,  1912. 

Relative  strength  on  bed  and  on  edge.  —  The  statement  is  made  by  some 
writers  that  bedded  or  laminated  stones  will  stand  a  greater  pressure 
in  a  direction  at  right  angles  to  their  bedding  than  parallel  with  it. 
This  seems  theoretically  correct,  but  the  published  tests  do  not  always 
appear  to  confirm  it. 

The  following  data  taken  from  the  work  of  Buckley  on  Missouri  and 
Wisconsin  building  stones  indicate  no  general  law. 


Kind. 

Locality. 

Bed. 

Edge. 

Limestone  .  . 

Bowling  Green,  Mo.  . 

8,881 

6,019 

Breckenridge   Mo 

6  944 

8  036 

Carthage  Mo 

14271 

11  879 

U                         (I 

16,337 

15,396 

l(               (( 

12,741 

12  684 

Hannibal,  Mo. 

9,286 

9,915 

Kansas,  City,  Mo.  .  .  . 

13,124 

10,449 

Sandstone  

4,942 

4,143 

Sandstone 

Warrensburg  Mo 

5  911 

4  869 

Limestone 

Wauwatosa   Wis 

10  111 

13  406 

Limestone 

Wauwatosa,  Wis. 

17  647 

23  744 

Sandstone  ... 

Ashland,  Wis. 

6,244 

4747 

Sandstone  

Dunnville,  Wis.  . 

2,502 

2,944 

Relative  strength  wet  and  dry.  —  A  building  stone  should  always  be 
tested  dry,  for  the  reason  that  it  shows  a  lower  strength  when  wet. 

There  are,  unfortunately,  few  published  tests  to  show  this,  but  the 
following  figures  given  by  Watson,  Laney  and  Merrill  in  their  report  on 
North  Carolina  building  stones  (Ref.  51)  emphasize  the  difference  to 
a  marked  degree. 


442 


ENGINEERING  GEOLOGY 
CRUSHING  TESTS  OF  NORTH  CAROLINA  SANDSTONES 


Per  cent  absorp- 
tion. 

Conditions. 

Crushing  strength, 
Ibs.  per  sq.  in. 

- 

Dry..          ..\ 

10,322 

42             5 

y                  I 

11,150 

Wet                | 

6,962 

( 

Dry..              \ 

5,837 
12,250 

3.71       ..  4 

y                  I 

11,232 

I 

Wet  | 

5,637 

6,712 

The  greatest  decrease  of  strength  on  soaking  is  likely  to  be  shown  by 
those  stones  whose  cement  is  liable  to  soften  when  they  are  soaked  in 
water. 

Effect  of  intermittent  pressure.  —  Stones  usually  weaken  when  sub- 
jected to  continued  or  intermittent  pressure,  and  may  fall  considerably 
below  their  normal  ultimate  crushing  strength.  However,  great  diffi- 
culty is  experienced  in  obtaining  satisfactory  data  on  this  point,  for  the 
reason  that  it  is  difficult  to  tell  within  a  range  of  1000  to  5000  pounds, 
the  crushing  strength  of  samples  to  be  tested  (Buckley,  Ref.  43). 

Effect  of  freezing  on  crushing  strength.  —  It  is  quite  evident  that  a 
stone  which  is  saturated  with  water  and  then  subjected  to  repeated 
freezings  for  20  or  more  times  may  be  weakened  to  such  an  extent  that 
it  will  not  withstand  the  same  pressure  as  a  cube  of  fresh  stone. 

Buckley  found  that  out  of  thirty-four  sets  of  samples  of  Missouri 
stones  tested,  only  eleven  gave  an  average  crushing  strength  higher  than 
that  of  the  fresh  samples.  The  greatest  loss  does  not  appear  in  those 
showing  the  highest  porosity  as  can  be  seen  from  the  following  table : 

CRUSHING  TESTS  ON  FRESH  AND  FROZEN  SAMPLES  OF  MISSOURI  STONES 


Kind. 

Locality. 

Per  cent 
porosity. 

Average  crush- 
ing strength, 
fresh. 

Average  crush- 
ing strength 
after  freezing. 

Limestone  
i 

Bowling  Green  
Breckenridge 

10.62 

7  90 

8,881.6 

6  944  0 

11,074.0 
8  163  0 

< 

Carthage 

1  34 

14  270  6 

13  382  7 

i 

Columbia 

3  10 

9  828  5 

9  738  0 

t 

Hannibal 

5  03 

9286  3 

8975  0 

« 

Joplin 

1  13 

11,870  0 

8  111  0 

(i 

Rolla 

13  00 

8486  7 

9  323  3 

ft 

St.  Louis  

7  30 

17,095  0 

16  246  0 

Sandstone  

Curless  

22  95 

4,942  0 

5,742  0 

a 

Miami  

14.31 

7,477  6 

8,670  5 

tt 

Warrensburg 

16  77 

5  910  6 

5  097  0 

BUILDING  STONE 


443 


CRUSHING  STRENGTH  OF  WISCONSIN  STONES  BEFORE  AND  AFTER  FREEZING 


Kind  of  rock. 

Location. 

Crushing  strength, 
fresh. 

Crushing  strength, 
frozen. 

Granite 

Athelstane 

19,988 

10,619 

a 

Berlin       

24,800 

36,009 

u 

Montello  

38,244 

35,045 

Limestone   . 

Duck  Creek  

24,522 

28,392 

Sturgeon  Bay  

35,970 

20,777 

i 

Wauwatosa 

18,477 

25779 

t 

Burlington                  .... 

12,827 

7,554 

Sandstone 

Presque  Isle  

5,495 

5,930 

i 

Dunnville  

2,722 

3,464 

< 

Port  Wing  

5,329 

4,399 

Hirschwald  (Ref.  4)  states  that  in  order  to  determine  the  effect  of 
freezing  on  the  crushing  strength,  the  crushing  test  should  always  be 
made  on  the  wet  stone. 

Transverse  strength.  —  The  transverse  strength  of  a  stone  may  be 
denned  as  its  ability  to  withstand  a  bending  strain,  and  as  numerically 


FIG.  194.  —  Sandstone  broken  by  transverse  strain,  caused  by  settling 
of  the  building. 


expressed  represents  the  force  required  to  break  a  bar  1  inch  square 
resting  on  supports  1  inch  apart,  the  load  being  applied  in  the  middle. 


444 


ENGINEERING   GEOLOGY 


This  is  measured  in  terms  of  the  modulus  of  rupture,  which  is  computed 
from  the  formula: 

3  wl 


R  = 


2bd2' 


in  which 


R  =  modulus  of  rupture. 

w  =  weight  required  to  break  stone. 

I  =  distance  between  supports. 

6  =  width  of  stone. 

d  =  thickness  of  stone. 

The  importance  of  this  test  is  not  universally  recognized,  and  it  is, 
therefore,  rarely  carried  out.  Many  a  stone  used  for  a  window  sill  or 
cap  has  cracked  under  transverse  strain  because  its  modulus  of  rupture 
in  the  section  used  is  too  low.  Such  transverse  breaks  are  not  uncom- 
monly caused  by  the  settling  of  a  building  (Fig.  194.) 

It  must  be  remembered  that  the  transverse  strength  does  not  appear 
to  stand  in  any  direct  relation  to  the  crushing  strength. 

While  there  is  considerable  variation  in  the  modulus  of  rupture  shown 
by  different  stones  of  the  same  class,  the  same  kind  of  rock  will  usually 
show  a  lower  transverse  strength  when  wet  than  when  dry,  and  also 
after  exposure  to  hot  and  cold  water  baths. 

Figures  bearing  out  these  statements  are  given  below: 

RANGE  OF  TRANSVERSE  STRENGTH  OF  WISCONSIN  AND  MISSOURI  BUILDING 
STONES  (AFTER  BUCKLEY) 

Modulus  of  rupture. 


Wisconsin. 

Missouri. 

Granite 

2,324  3  to  3,909  7 

Limestone               .           

1,164  3  to  4,659  2 

851  30  to  3,311  60 

Sandstone   

362.9  to  1,324.0 

418  61  to  1,321  76 

MODULUS  OF  RUPTURE  OF  ONTARIO  STONES  (AFTER  PARKS) 


Kind. 

No.  tested. 

Range. 

Average. 

Limestones                              

33      . 

818  to  4,291 

2,224 

Sandstones     .  .             

10 

417  to  2,186 

1,283 

Crystalline  limestone  

8 

1,091  to  3,737 

1,907 

Granites 

3 

2  480  to  3  382 

BUILDING  STONE 


445 


RELATIVE  TRANSVERSE  STRENGTH  OF  STONES  IN  NATURAL  STATE,  AND  AFTER 
EXPOSURE  TO  HOT  AND  COLD  WATER  BATHS.  l 

Granites 


Modulus  of  rupture  per  sq .  in. 


Description. 

Natural 

state, 
total. 

After  exposure  to  hot  and  cold 
water  baths. 

Total. 

Loss. 

Per  cent 
of  natural 
state. 

From    Braddock    quarries,    near    Little 
Rock  Ark 

Lbs. 
1704 

2069 
1423 
1378 
1415 
2335 

Lbs. 
1244 

2027 
1230 
1053 
1083 
2002 

Lbs. 
460 

42 
193 
325 
332 
333 

From   Millbridge,    Me.,    "  White    Rock 
Mountain  " 

From  Rockville,  Stearnes  County,  Minn.  . 
Drake's  granite,  from  Sioux  Falls,  Minn.  . 
From  Branford   Conn 

From  Troy  N  H 

Means 

1721 

1440 

281 

83.7 

Marbles 


Rutland  white,  Vt. 

1202 

a291 

911 

Mountain  Dark,  Vt.                

2109 

1408 

701 

Sutherland  Falls,  Vt. 

3054 

1531 

1523 

From  St.  Joe,  Ark.   .  .             .  .      .         ... 

1615 

567 

1048 

From  De  Kalb,  St.  Lawrence  Co.,  N.Y.  .  .  . 

1144 

533 

611 

From  Kennesaw  quarry  Tate,  Ga 

1553 

605 

948 

Means 

1779 

822 

957 

46  2 

a.     Heated  in  hot-air  oven  to  402°  F. 


Limestones 


From  Isle  La  Motte,  Vt  

2493 

786 

1707 

From  Mount  Vernon,  Ky  

1434 

1076 

358 

From  Beaver,  Carroll  County,  Ark  

2860 

2247 

613 

From  Bowling  Green,  Ky.  .          

1317 

799 

518 

Blue  colored  from  Bedford   Ind 

1867 

958 

909 

Means 

1994 

1173 

821 

58  8 

Deport  on  Tests  of  Metals,  etc.,  1335,  War  Department. 


446 


ENGINEERING  GEOLOGY 


RELATIVE  TRANSVERSE  STRENGTH  OF  STONES  IN  NATURAL  STATE,  AND  AFTER 
EXPOSURE  TO  HOT  AND  COLD  WATER  BATHS.  —  (Continued) 

Sandstones 


From  Cromwell,  Conn  

2243 

1500 

743 

From    Worcester    quarry,     East    Long 
Meadow  Mass 

987 

189 

-202 

From  Kibble  quarry,  East  Long  Meadow, 

Mass.    .  . 

1273 

655 

618 

From  Cabin  Creek,  Johnson  County,  Ark. 
Quarries  near  Fort  Smith   Ark 

2442 
1761 

890 
1185 

1552 
576 



From  Olympia   Wash 

2073 

2297 

—224 

From  Chuckanut,  Wash. 

2016 

961 

1055 

From  Tenino,  Wash  .       . 

667 

323 

344 

Means.                    

1683 

1125 

558 

66  9 

Means  of  all  stones  

65.1 

Fire  resistance  (Refs.  5,  7).  —  Many  building  stones  suffer  serious 
disintegration  as  a  result  of  exposure  to  fire,  or  still  worse  the  combined 
action  of  fire  and  water,  and  the  serious  conflagrations  in  such  cities 
as  Baltimore,  San  Francisco,  etc.,  have  demonstrated  this  fact. 

This  disintegration  by  fire  may  be  due  to  unequal  stresses  set  up 
within  the  stone  by  the  outer  portion  of  a  block  becoming  highly  heated 


FIG.  195. — Effect  of  fire  on  granite  columns,  U.  S.  Public  Storehouse,  Baltimore,  Md. 

while  the  interior  is  still  comparatively  cool,  or  it  may  be  caused  by  the 
stone  first  becoming  highly  heated,  and  then  being  suddenly  cooled 
by  the  application  of  a  stream  of  cold  water.1 

1  Some  believe  that  the  crumbling  of  granite  under  heat  is  due  to  microscopic 
bubbles  in  the  quartz  grains,  which  contain  water  or  liquid  carbonic  acid  gas.  Under 
heat  these  hundreds  of  microscopic  bubbles  expand  and  burst. 


BUILDING  STONE  447 

The  best  form  of  test  to  determine  the  fire  resistance  of  a  building 
stone  consists  of  building  up  a  section  of  masonry  of  the  stone  to  be 
heated,  or  the  stone  can  be  built  up  in  an  iron  framework  which  forms 
one  movable  wall  of  a  furnace.  In  either  case  the  stone  after  being 
heated  to  about  1750°  F.,  is  cooled  down  by  a  strong  stream  of  cold 
water  from  a  hose. 

Many  stones  after  heating  to  redness  and  slow  cooling  emit  a  dull 
sound  when  struck.  Lime  rocks,  if  heated  above  850°  C.  calcine  to 
quicklime,  but  at  a  lower  temperature  they  are  less  affected  by  heating 
and  slow  cooling  than  any  other  rocks.  Granites  seem  on  the  whole 
to  have  a  lower  resistance  than  sandstones. 

Considered  as  a  class,  however,  building  stones  are  of  low  fire  resist- 
ance, especially  if  rapidly  cooled.  In  comparative  tests  they  are  often 
found  inferior  to  clay  products  of  non-vitrified  character. 

A  series  of  tests  made  by  W.  E.  McCourt  consisted  in:  (1)  Heating 
two  cubes  to  550°  C.  and  cooling  one  fast,  the  other  slow;  (2)  similar 
treatment  of  two  other  cubes  at  850° C.;  (3)  heating  for  five  minute 
intervals  in  a  strong  blast  and  cooling  for  alternate  five  minutes; 
(4)  alternately  heating  in  a  blast  for  five  minutes  and  quenching  with 
water  for  five  minutes. 

Professor  McCourt  in  summarizing  his  New  York  tests  made  the 
following  interesting  statements: 

"  At  550°  C.  (1022  F.)  most  of  the  stones  stood  up  very  well.  The  temperature 
does  not  seem  to  have  been  high  enough  to  cause  much  rupturing  of  the  samples, 
either  upon  slow  or  fast  cooling.  The  sandstones,  limestones,  marble  and  gneiss 
were  slightly  injured,  while  the  granites  seem  to  have  suffered  least." 

"  The  temperature  of  a  severe  conflagration  would  probably  be  higher  than 
550°  C.  but  there  would  be  buildings  outside  of  the  direct  action  of  the  fire  which 
might  not  be  subjected  to  this  degree  of  heat  and  in  this  zone  the  stones  would  suffer 
little  injury.  The  sandstones  might  crack  somewhat;  but,  as  the  cracking  seems 
to  be  almost  entirely  along  the  bed,  the  stability  of  the  structure  would  not  be  en- 
dangered, provided  the  stone  had  been  properly  set." 

"  The  gneiss  would  fail  badly,  especially  if  it  were  coarse-grained  and  much 
banded.  The  coarse-grained  granites  might  suffer  to  some  extent.  These,  though 
cracked  to  a  less  extent  than  the  sandstones,  would  suffer  more  damage  and  possibly 
disintegrate  if  the  heat  were  long-continued  because  the  irregular  cracks,  intensified 
by  the  crushing  and  shearing  forces  on  the  stone  incident  to  its  position  in  the  struc- 
ture, would  tend  to  break  it  down.  The  limestones  and  marble  would  be  little  in- 
jured." 

"  The  temperature  of  850°  C.  (1562°  F.)  represents  fairly  the  probable  degree  of 
heat  reached  in  a  conflagration,  though  undoubtedly  it  exceeds  that  in  some  cases. 
At  this  temperature  we  find  that  the  stones  behave  somewhat  differently  than  at  the 
lower  temperature.  All  the  cubes  tested  were  injured  to  some  degree,  but  among 
themselves  they  vary  widely  in  the  extent  of  the  damage." 


PLATE  LXX.  —  Fire  Tests  on  3-inch  cubes  of  limestone,  Newton,  N.  J.     (After  W. 

E.  McCourt.) 


216.  550°  C.,  slow  cooling. 
218.  850°  C.,  slow  cooling. 
220.  Flame  test. 


217.  550°  C.,  fast  cooling. 
219.  850°  C.,  fast  cooling. 
221.  Flame  and  water  test. 


(448) 


BUILDING  STONE  449 

"  All  the  igneous  rocks  and  the  gneiss  at  850°  C.  suffered  injury  in  varying  degrees 
and  in  various  ways.  The  coarse-grained  granites  were  damaged  the  most  by  crack- 
ing very  irregularly  around  the  individual  mineral  constituents.  Naturally,  such 
cracking  of  the  stone  in  a  building  might  cause  the  walls  to  crumble.  The  cracking 
is  due,  possibly,  to  the  coarseness  of  texture  and  the  differences  in  coefficiency  of 
expansion  of  the  various  mineral  constituents.  Some  minerals  expand  more  than 
others  and  the  strains  occasioned  thereby  will  tend  to  rupture  the  stones  more  than 
if  the  mineral  composition  is  simpler.  The  rupturing  will  be  greater,  too,  if  the  rock 
be  coarser  in  texture.  For  example,  a  granite  containing  much  plagioclase  would 
be  more  apt  to  break  into  pieces  than  one  with  little  plagioclase  for  the  reason  that 
this  mineral  expands  in  one  direction  and  contracts  in  another,  and  this  would  set 
up  stresses  of  greater  proportion  than  would  be  occasioned  in  a  stone  containing  little 
of  this  mineral.  In  the  gneisses  the  injury  seems  to  be  controlled  by  the  same  factors 
as  in  the  granites,  but  there  comes  in  here  the  added  factor  of  banding.  Those  which 
are  made  up  of  many  bands  would  be  damaged  more  severely  than  those  in  which 
the  banding  is  slight." 

"  All  the  sandstones  which  were  tested  are  fine-grained  and  rather  compact.  All 
suffered  some  injury,  though,  in  most  cases,  the  cracking  was  along  the  lamination 
planes.  In  some  cubes,  however,  transverse  cracks  were  also  developed." 

"  The  variety  of  samples  was  not  great  enough  to  warrant  any  conclusive  evidence 
toward  a  determination  of  the  controlling  factors.  It  would  seem,  however,  that  the 
more  compact  and  hard  the  stone  is  the  better  will  it  resist  extreme  heat.  The 
following  relation  of  the  percentage  of  absorption  to  the  effect  of  the  heat  is  interest- 
ing. In  a  general  way  the  greater  the  absorption,  the  greater  the  effect  of  the  heat. 
A  very  porous  sandstone  will  be  reduced  to  sand  and  a  stone  in  which  the  cement  is 
largely  limonite  or  clay  will  suffer  more  than  one  held  together  by  silica  or  lime 
carbonate." 

"  The  limestones,  up  to  the  point  where  calculation  begins  (600°-800°  C.)  were 
little  injured,  but  above  that  point  they  failed  badly,  owing  to  the  crumbling  caused 
by  the  flaking  of  the  quicklime.  The  purer  the  stone,  the  more  will  it  crumble. 
The  marble  behaves  similarly  to  the  limestone;  but,  because  of  the  coarseness  of  the 
texture,  also  cracks  considerably.  As  has  been  mentioned  before,  both  the  lime- 
stones and  marble  on  sudden  cooling  seem  to  flake  off  less  than  on  slow  cooling." 

"  The  flame  tests  cannot  be  considered  as  indicative  of  the  probable  effect  of  a 
conflagration  upon  the  general  body  of  the  stone  hi  a  building,  but  rather  as  an 
indication  of  the  effect  upon  projecting  cornices,  lintels,  pillars,  carving  and  all  thin 
edges  of  stonework.  All  the  stones  were  damaged  to  some  extent.  The  limestones 
were,  as  a  whole,  comparatively  little  injured,  while  the  marble  was  badly  damaged. 
The  tendency  seems  to  be  for  the  stone  to  split  off  in  shells  around  the  point  where 
the  greatest  heat  strikes  the  stone.  The  temperature  of  the  flame  probably  did  not 
exceed  700°  C.,  so  it  is  safe  to  say  that  in  a  conflagration  all  carved  stone  and  thin 
edges  would  suffer.  However,  outside  of  the  intense  heat,  the  limestones  would  act 
best,  while  the  other  stones  would  be  affected  in  the  order:  sandstone,  granite,  gneiss 
and  marble." 

"  After  having  been  heated  to  850°  C.,  most  of  the  stones,  as  observed  by  Buckley, 
emit  a  characteristic  ring  when  struck  with  metal,  and  when  scratched,  emit  a  sound 
similar  to  that  of  a  soft  burned  brick.  It  will  be  noted  that  hi  those  stones  in  which 
iron  is  present  hi  a  ferrous  condition  the  color  was  changed  to  a  brownish  tinge  owing 
to  the  change  of  the  iron  to  a  ferric  state.  If  the  temperature  does  not  exceed  550°  C., 


450 


ENGINEERING  GEOLOGY 


all  the  stones  will  stand  up  very  well,  but  at  the  temperature  which  is  probable  in 
a  conflagration,  in  a  general  way,  the  finer-grained  and  more  compact  the  stone  and 
the  simpler  in  mineralogical  composition  the  better  will  it  resist  the  effect  of  the 
extreme  heat.  The  order,  then,  of  the  refractoriness  of  the  New  York  stones  which 
were  tested  might  be  placed  as  sandstone,  fine-grained  granite,  limestone,  coarse- 
grained granite,  gneiss  and  marble." 

Expansion  and  contraction  of  building  stone.  —  Building  stones 
expand  when  heated  and  contract  when  cooled,  but  do  not  return  to 
their  original  length.  This  slight  increase  in  size  is  known  as  the 
permanent  swelling.  Although  it  is  a  very  small  amount  when  a  piece 
of  stone  one  foot  long  is  being  considered,  still  it  may  be  appreciable 
when  it  involves  a  mass  of  masonry  100  or  200  feet  in  length. 

The  following  averages  are  based  on  experiments  made  at  the  Water- 
town,  Mass.,  arsenal,1  the  permanent  swelling  being  for  a  bar  of  stone 
20  inches  long,  heated  and  cooled  through  a  range  of  temperature  from 
32°  F.  to  212°  F. 


Kind  of  stone. 

Inch. 

Granite     

0.004 

Marble  .         

0.009 

Limestone  

0.007 

Sandstone  

0.0047 

If  the  stones  were  set  tight,  with  no  joints,  buckling  of  the  wall  might 
follow,  but  it  is  probable  that  the  cement  joints  take  up  some  of  the 
increase  in  size.  But  even  so,  engineers  sometimes  allow  for  this 
expansion  by  putting  in  some  elastic  joints  of  asphaltic  material  or  tar 
felt.  The  practice  is  not  a  universal  one  however. 

COEFFICIENTS  OF  EXPANSION  OF  STONES,  AS  DETERMINED  IN  WATER  BATHS 


Name. 

Location. 

Original 
gaged 
length 
in  air. 

20.0033 
20.0084 
19.9989 
20.0061 
20.0034 
19.9912 
20.0019 
19.9954 
20.0052 
20.0023 
19.9951 
19.9303 

Temperature. 

Coefficient 
of 
expansion. 

Hot. 

Cold. 

Differ- 
ence. 

Differ- 
ence in 
length. 

Buff  oolitic  limestone.  . 
Limestone 

Bedford,  Ind  
Indiana  
Vermont  

Deg. 

178 
177 
203 
189.5 
183 
180 
183 
194 
192 
183 
199 
181 

Deg. 
33.5 
33.5 
34 
33.5 
33.5 
33.5 
33.5 
34 
33.5 
33.5 
33.5 
33.5 

Deg. 

144.5 
143.5 
169 
156 
149.5 
146.5 
149.5 
160 
158.5 
149.5 
165.5 
147.5 

Ins. 
0.0109 
0.0103 
0.0122 
0.0175 
0.0152 
0.0154 
0.0186 
0.0158 
0.0189 
0.0122 
0.0126 
0.0091 

0.00000375 
0.00000376 
0.00000361 
0.00000562 
0.00000501 
0.00000526 
0.00000622 
0.00000500 
0.00000596 
0.00000408 
0.00000381 
0.00000311 

Marble.  .  . 

Marble  
Red  sandstone 

Lee,  Mass  
Maryland  
Portland,  Conn... 
Ohio  
Monson,  Me  
New  York  

Red  sandstone  

Sandstone  

Slate 

Bluestone  

Granite     

Milford,  Mass  
Quincy,  Mass  
Rockport,  Mass.... 

Granite 

Granite  

1  Report  on  Tests  of  Metals,  etc.,  at  Watertown  Arsenal,  U.  S.  War  Department, 
1895,  p.  322. 


BUILDING  STONE 


451 


The  proceeding  table  gives  the  coefficients  of  expansion  of  a 
number  of  stones  as  determined  in  a  water  bath.1 

Modulus  of  elasticity.  —  This  term  is  synonymous  with  coefficient 
of  elasticity,  and  can  be  defined  as  the  weight  required  to  stretch  a  rod 
of  one  square  inch  section  to  double  its  length. 

Baker  states  that  it  is  valuable  in  determining  the  effect  of  combining 
masonry  and  metal,  of  joining  different  kinds  of  masonry,  or  of  joining 
new  masonry  to  old;  in  calculating  the  effect  of  loading  a  masonry 
arch;  in  proportioning  abutments  and  piers  of  railroad  bridges  subject 
to  shock,  etc. 

A  method  of  determining  it  consists  in  measuring  the  amount  of 
compression  which  a  2-inch  cube  of  stone  shows  for  each  increment 
of  500  to  1000  pounds  load  to  the  limit  of  its  elasticity.  The  modulus  of 
elasticity  is  then  computed  from  these  data  by  means  of  an  empirical 
formula. 

Few  determinations  have  been  made  of  this  property  of  building 
stones,  but  the  following  are  taken  from  the  report  of  Buckley  on  the 
Wisconsin  building  stones  (Ref.  78.) 


Kind. 

Locality. 

Position. 

Modulus  of  elas- 
ticity, Ibs.  per 
sq.  in. 

Granite 

Amberg 

201  000 

n 

Amberg     .  . 

951,500 

ti 

Granite  Heights.  .  .  . 

1,450,000 

« 

Montello  

1,653,000 

Limestone  

Duck  Creek  

Bed.  . 

462,800 

Burlington 

Bed 

31  500 

Burlington 

Edge 

501  300 

Fountain  City 

Bed 

171  000 

Fountain  City.  .    . 

Edge 

237,900 

Sandstone  

Presque  Isle  .... 

Bed 

114,500 

Presque  Isle  

Edge  .  . 

94,000 

Dunnville 

Bed 

103  420 



Dunnville  
Houghton  

Edge  
Bed  

145,300 
170,600 

Houghton 

Edge 

151  300 

Bass  Island 

Bed 

76,300 

Bass  Island 

Edge 

64,900 

Abrasive  resistance.  —  The  abrasive  resistance  of  a  stone  depends 
in  part  on  the  state  of  aggregation  of  the  mineral  particles  and  in  part 
on  their  individual  hardness.  Some  stones  wear  very  unevenly  because 
of  their  irregularity  in  hardness,  and  may  be  less  desirable  than  those 
which  are  uniformly  soft. 

1  Report  on  Tests  of  Metals,  etc.,  U.  S.  War  Dept.,  1890. 


452  ENGINEERING  GEOLOGY 

The  abrasive  resistance  of  a  stone  has  to  be  considered  whenever  it 
is  placed  in  a  position  where  it  is  subjected  to  rubbing  action.  Such 
situations  include  the  use  of  a  stone  for  steps,  for  paving  or  flooring 
purposes,  or  for  lining  troughs  or  tunnels  where  it  is  subjected  to  the 
abrasive  action  of  running  water  carrying  mud-  or  sand.  Moreover, 
in  dry  climates  having  sandy  soils,  which  are  frequently  transported 
by  strong  winds,  or  along  the  sea  coast  where  dune  sand  is  moved  by 
the  same  forces,  the  stone  is  subjected  to  the  grinding  action  of  a  natural 
sand  blast. 

Many  rock  outcrops  exposed  to  abrasive  action  show  a  very  irregular 
surface,  because  the  softer  minerals  have  been  worn  away,  leaving  the 
harder  ones  standing  out  in  knotty  form. 

A  test  to  determine  the  abrasive  resistance  of  a  stone  should,  therefore, 
be  made  on  those  which  are  to  be  used  for  paving,  steps,  flooring,  or 
wherever  they  have  to  stand  rubbing  action. 

Several  methods  for  determining  the  abrasive  resistance  of  stone  have 
been  suggested,  but  none  universally  adopted. 

The  common  method  consists  in  laying  the  stone  to  be  tested  on  a 
rubbing  table,  weighting  it  down,  and  applying  emery  or  some  other 
abrasive  at  a  given  rate  while  the  table  revolves. 

The  difficulty  with  this  method  lies  in  not  being  able  to  feed  the  sand 
at  a  uniform  rate,  and  in  being  sure  that  all  of  it  passes  under  the  test  piece. 

The  method  is  of  value  chiefly  for  comparative  purposes,  where 
sseveral  pieces  of  stone  are  tested  at  the  same  time. 

Gary1  endeavored  to  perfect  the  test  by  cutting  slabs  of  50  square 
centimeters  surface  parallel  with  the  bedding.  These  were  held  down 
with  a  30-kilogram  weight  and  placed  32  centimeters  from  the  center 
of  a  circular  rubbing  plate.  At  one  minute  intervals,  20  grams  of 
Naxos  emery  of  a  certain  size  were  strewn  on  the  table.  The  abrasive 
and  abraded  rock  remained  on  the  table  until  the  completion  of  110 
revolutions,  which  consumed  about  five  minutes.  No  water  was  used. 
The  loss  of  weight  of  the  stone  indicated  the  amount  of  abrasion. 

Another  method  devised  by  Gary2  which  seems  to  the  authors  to  be 
a  better  one,  involves  the  use  of  a  sand  blast.  In  the  specially  devised 
apparatus  the  sand  is  forced  through  a  six  centimeter  diameter  opening, 
under  a  dry  steam  pressure  of  3  atmospheres,  for  2  minutes.  The  stone 
to  be  tested  is  held  immediately  over  the  opening. 

The  following  figures  give  the  results  obtained  by  Gary  with  both 
methods. 

1  Baumaterialienkunde,  II,  p.  11,  1897-98. 

2  Baumaterialienkunde,  X,  p.  133,  1905. 


BUILDING  STONE 


453 


ABRASION  TESTS  MADE  BY  GARY. 


Name. 

Abrasion  on  rubbing  table. 

Abrasion  with  sand  blast  at  right 
angles  to  bedding. 

Surface, 
sq.  cm. 

Aver, 
loss, 
ccm. 

Abrasion, 
in 
sq.  cm. 

Aver, 
loss, 
ccm. 

Abrasion, 
ccm. 
sq.  cm. 

Aver, 
loss, 
ccm. 

Abrasion, 
ccm., 
sq.  cm. 

Basalt 

50 
49 

49 
48 
49 
50 
50 
50 

5.4 
9.6 

5.1 
9.6 
8.5 
10.8 
18.4 
29.7 

0.11 

0.20 
0.10 
0.20 
0.17 
0.22 
0.37 
0.59 

1.70 
6.01 
2.64 
4.01 
3.29 
4.24 
11.15 
8.02 

0.06 
0.21 
0.09 
0.14 
0.12 
0.15 
0.39 
0.28 

1.81 

7.06 
3.78 
3.26 
2.58 
4.16 
8.42 
5.90 

0.06 
0.25 
0.13 
0.12 
0.09 
0.15 
0.30 
0.21 

Basalt  lava  

Granite  

Gneiss  

Porphyry  
Graywacke  .  . 

Sandstone  .... 

Schist  

The  sand-blast  treatment  not  only  tests  the  abrasive  resistance,  but 
also  brings  out  irregularities  in  the  hardness. 

Frost  resistance.  —  A  good  building  stone  should  resist  the  action 
of  frost.  The  disintegration  by  frost  is  due  to  the  water  absorbed  by 
the  stone  freezing  within  its  pores.  This  of  course  arises  from  the  fact 
that  the  change  of  water  to  ice  is  accompanied  by  an  increase  in  volume 
of  one-eleventh,  and  the  internal  pressure  resulting  from  this  may  be 
sufficient  to  disrupt  the  stone. 

With  other  things  equal,  one  might  expect  a  stone  of  high  absorption 
to  break  more  easily  than  one  of  low  absorption.  This,  however,  is  not 
always  the  case,  for  there  are  variable  factors  which  affect  the  result. 
Among  these  may  be  mentioned  the  size,  shape,  and  distribution  of  the 
pores,  as  well  as  the  rigidity  of  the  rock. 

A  rock  of  high  porosity  may  absorb  a  high  percentage  of  water,  and 
yet  not  disintegrate  on  freezing,  because  either  the  water  drains  off 
rapidly,  or  else  if  it  should  remain  in  the  stone  is  forced  outward,  through 
the  large  pores,  when  it  freezes. 

On  the  other  hand,  a  stone  with  small  pores,  or  irregular  ones,  retains 
longer  the  water  absorbed  by  it,  and  this  on  freezing  often  exerts  suffi- 
cient internal  pressure  to  split  the  stone. 

It  must  be  remembered,  however,  that  the  extent  of  the  damage  done 
depends  on  how  completely  the  pores  are  filled. 

A  stone  soaked  under  normal  atmospheric  pressure  is  not  likely  to  be 
completely  saturated,  while  one  soaked  in  a  vacuum  will  have  its  pores 
pretty  well  filled  with  water. 

How  different  these  results  are  is  shown  in  the  following  table  in 
which  I  represents  the  number  of  times  the  stone  stood  freezing  without 


454 


ENGINEERING  GEOLOGY 


injury  after  soaking  under  normal  atmospheric  pressure,  while  II  shows 
the  number  of  times  the  stone  was  frozen  after  soaking  in  a  vacuum 
(Hirschwald,  Ref.  4.). 

EFFECT  OF  FREEZING  A  STONE  WITH  PORES  PARTIALLY  AND  COMPLETELY  FILLED 


Kind  of  rock. 

I. 

II. 

Limestone  . 

31  times,  no  effect 

5  times   broken  in  two 

Marble  . 

25  times,  no  effect 

3  times   cracked 

Sandstone.  . 

25  times,  no  effect 

8  times   spalled  off 

Tuff  

25  times,  no  effect 

14  times   many  cracks 

Coarse  granite  

Unaffected 

8  times,  mica  scales  detached 

The  splitting  of  a  stone  when  exposed  to  freezing  temperatures  is, 
however,  not  necessarily  due  solely  to  absorbed  water,  for  as  previously 
explained  it  may  be  due  to  quarry  water. 

Careful  consideration  should,  therefore,  be  given  by  the  engineer  to 
the  frost  resistance  of  a  building  stone:  (1)  By  not  quarrying  stratified 
rocks  in  cold  weather;  (2)  by  the  selection  of  a  rock  of  known  high  frost 
resistance;  and  (3)  by  not  placing  porous  or  absorbent  rocks  in  a  posi- 
tion where  they  are  sure  to  absorb  considerable  moisture. 

Laboratory  tests  made  to  determine  the  frost  resistance  should  as  far 
as  possible  simulate  the  conditions  of  use. 

Freezing  method.  —  The  most  logical  method  of  making  a  frost  test 
consists  in  thoroughly  soaking  the  stone,  and  then  exposing  it  to  a 
temperature  below  freezing,  this  being  repeated  about  20  times.  The 
stone  is  weighed  before  and  after  the  tests  and  any  loss  of  weight  meas- 
ured in  percentage  terms  of  the  original  dry  weight. 

Other  effects  of  alternate  freezing  and  thawing  may  be:  (1)  Formation 
of  cracks;  (2)  detaching  of  grains  from  surface;  and  (3)  loss  of  strength. 

The  second  type  of  loss  might  occur  in  a  laboratory  test  without 
being  accompanied  by  any  serious  disintegration  of  the  stone,  as  the 
surface  of  many  dressed  stones  is  coated  with  partly  loosened  grains. 

Buckley,  in  a  series  of  tests  made  on  Wisconsin  stone  subjected  to 
thirty-five  alternate  freezings  (outdoors)  and  thawings,  found  the 
following  losses  in  weight:  Granites  and  rhyolites,  not  over  0.05  per 
cent;  limestones,  not  over  0.03  per  cent;  and  sandstones,  not  over 
0.62  per  cent. 

A  set  of  Missouri  building  stone  tested  by  the  same  author  gave  the 
following  losses:  Limestones,  0.006-0.909  per  cent;  sandstones,  0.111- 
0.591  per  cent. 


BUILDING  STONE 


455 


Sulphate  of  soda  test.  —  An  artificial  method  consists  in  soaking  the 
stone  in  a  solution  of  sulphate  of  soda,  and  then  drying  it  out,  the  theory 
being  that  the  growth  of  the  sulphate  of  soda  crystals  in  the  pores  of  the 
rock  exerts  internal  pressure.  The  treatment  is  repeated  a  number  of 
times.1 

The  test  is  much  more  severe  than  the  ordinary  freezing  test,  and  may 
give  abnormal  losses  as  the  following  figures  taken  from  Luquer's 
experiments  will  show. 

ARTIFICIAL  AND  NATURAL  FROST  TESTS 


Stone. 

Loss  of  weigl 
10,0 

it  in  parts  per 
00. 

Sulphate. 

Freezing. 

Coarse,  crystalline  dolomitic  marble  

10.78 

3.10 

Medium,  crystalline  dolomitic  marble  

17.01 

2.30 

Fine-grained  limestone 

25  99 

2  07 

Coarse-grained  red  granite 

15  51 

1  38 

Medium-grained  red  granite  .  . 

6  55 

1  76 

Fine-grained  gray  granite      .             

5  16 

1  50+ 

Rather  fine-grained  gneiss  

6.33 

1  50+ 

Xorite,  "  Au  Sable  granite  "  

3.84 

1  50+ 

Decomposed  sandstone  

482.12 

68.74 

Very  fine-grained  sandstone 

47-65 

10  63 

Sandstone.  .  . 

145  18 

14  21 

Decomposed  sandstone 

1621  31 

25  31 

Sandstone 

57  78 

8  89 

The  structure  of  a  stone  sometimes  hastens  its  disintegration  under 
frost  action.  Thus  a  laminated  rock,  such  as  a  sandstone,  is  apt  to 
split  rather  easily  along  the  bedding  planes,  and  this  may  be  hastened 
if  the  stone  is  set  in  the  building  on  edge  instead  of  on  bed. 

Effect  of  atmospheric  gases.  —  Carbon  dioxide  and  sulphuric  acid 
gases  are  present  in  the  atmosphere  of  some  localities  in  appreciable 
quantities.  This  is  especially  true  in  the  vicinity  of  factories,  smelters, 
railroad  yards,  etc.,  where  these  acid  gases  are  being  discharged  into  the 
atmosphere  from  chimneys. 

If  moisture  is  present  this  not  only  acts  as  a  carrier  for  the  gases  but 
serves  to  aid  chemical  reaction  when  they  come  in  contact  with  the 
surface  of  the  stones  of  many  buildings. 

Another  possible  source  of  sulphuric  acid  may  be  from  the  decay  of 
pyrite  in  the  rock  itself. 

Limestones,  or  other  rocks  with  calcareous  cement,  are  most  affected 
1  Luquer,  Trans.  Amer.  Soc.  Civ.  Engrs.,  XXXIII,  Mar.,  1895,  p.  235. 


456  ENGINEERING  GEOLOGY 

by  acid  gases  of  the  atmosphere.  The  result  may  be  a  very  slow,  and 
usually  uneven  solution  of  the  stone,  which  in  the  end  causes  a  roughen- 
ing of  the  surface  or  sometimes  even  scaling  off  of  the  rock. 

Chemical  composition  of  building  stone.  —  The  chemical  analysis 
of  a  building  stone  is  usually  of  very  little  commercial  value,  for  three 
reasons,  viz.:  (1)  Many  persons  have  not  sufficient  knowledge  of 
chemistry  and  mineralogy  to  interpret  it ;  (2)  it  is  often  incomplete,  and 
does  not  indicate  the  presence  of  injurious  elements;  and  (3)  what 
information  is  obtainable  can  usually  be  obtained  more  readily  by  other 
methods,  especially  microscopic  ones. 

It  is  true,  of  course,  that  different  kinds  of  stone  show  a  more  or 
less  characteristic  chemical  composition.  Igneous  rocks  on  chemical 
analysis  show  silica,  alumina,  and  varying  proportions  of  iron  oxides, 
lime,  magnesia,  and  alkalies,  depending  on  the  rock  species. 

Limestones  if  pure  consist  solely  of  calcium  carbonate,  and  dolomites 
of  calcium  and  magnesium  carbonate,  but  if  containing  clayey  impurities 
they  show  some  silica,  alumina,  and  also  some  chemically  combined 
water. 

Sandstones  if  pure  show  little  else  but  silica.  If  clayey  they  carry 
alumina,  iron  oxide,  and  some  chemically-combined  water  in  addition. 
If  calcareous  they  may  show  several  per  cent  of  lime. 

Microscopic  examination.  —  This  consists  in  examining  a  thin 
section  of  the  rock  under  the  microscope  by  polarized  light.  It  serves 
to  indicate  the  presence  (especially  in  igneous  rocks)  of  many  accessory 
minerals  of  secondary  importance,  not  visible  to  the  naked  eye.  As  a 
rule,  however,  the  determination  of  these  is  only  of  scientific  importance, 
for  they  exert  little  or  no  effect  on  the  general  value  of  the  stone. 

The  microscopic  examination  may  also  show  incipient  weathering 
and  textural  structures  not  visible  to  the  eye  alone. 

It  is  possible  to  calculate  the  percentage  mineral  composition  of  a  rock 
from  both  the  chemical  analysis1  and  microscopic  examination,  and  if 
this  is  done  the  one  can  be  used  to  check  the  other. 

The  percentage  of  different  minerals  present  in  a  rock,  as  determined 
by  the  microscope,  is  conveniently  made  by  the  Rosiwal  method.  This 
method  was  devised  by  Rosiwal,  an  Austrian  geologist,  for  determining 
the  approximate  proportions  of  the  chief  minerals  (feldspar,  quartz, 
mica,  hornblende)  by  means  of  the  microscope. 

"  It  consists2  in  tracing  a  network  of  lines  intersecting  one  another  at  right  angles 
upon  a  polished  rock  surface,  at  intervals  so  far  distant  that  no  two  parallel  lines 

1  Kemp,  Handbook  of  Rocks. 

2  The  description  is  quoted  from  Dale,  Bull.  354,  U.  S.  Geol.  Survey. 


BUILDING  STONE  457 

will  traverse  the  same  mineral  particle.  The  total  length  of  the  lines  is  measured, 
then  the  diameters  of  all  the  particles  of  each  kind  of  mineral  are  added  separately 
and  their  proportion  to  the  total  length  of  the  lines  obtained.  The  average  size  of 
the  particles  of  each  mineral  can  be  also  calculated  from  the  same  measurements. 
Although  this  method  was  primarily  designed  for  application  to  the  coarse  and 
medium  granites,  it  can  be  extended  also  to  the  finer  ones  by  drawing  the  lines  upon 
camera  lucida  sketches  made  from  thin  sections  of  such  granites  under  polarized 
light." 

Igneous  Rocks 

Of  the  many  kinds  of  igneous  rocks,  the  granites  and  granite  gneisses 
are  more  extensively  employed  for  building  stone  than  any  others  in  the 
United  States. 

This  is  due  to  several  causes,  such  as  wider  distribution,  more  pleasing 
color,  and  greater  regularity  of  structure  such  as  jointing,  as  well  as 
greater  durability. 

The  other  plutonic  igneous  rocks  are  employed  occasionally  either 
because  they  form  a  convenient  source  for  local  use,  or  because  in  special 
cases  their  natural  beauty  may  make  them  of  value  for  ornamental 
purposes. 

Volcanic  rocks  have  a  more  restricted  use  than  the  plutonic  ones. 
Some  are  rather  soft  and  porous,  and  can,  therefore,  be  used  only  in 

mild  climates. 

Granites 

Definition.  —  The  term  granite,  as  commonly  used  by  quarrymen, 
includes  all  igneous  rocks  and  gneiss.  It  seems  best,  however,  to  use  it 
in  the  geological  sense,  which  is  more  restricted.  It  may,  therefore, 
be  denned  as  an  even-granular,  crystalline,  plutonic,  igneous  rock,  con- 
sisting of  quartz,  and  alkalic  feldspar,  with  usually  mica,  hornblende  or 
pyroxene.  There  are  also  varying  amounts  of  other  feldspars,  and  a 
large  number  of  subordinate  accessory  minerals,  few  of  which  except 
pyrite  and  garnet  are  visible  to  the  naked  eye,  or  likely  to  be  recognized 
by  any  one  not  having  a  knowledge  of  mineralogy  (see  Chapter  II  on 
Rocks). 

Properties  of  granites.  —  Since  the  granites  are  the  most  widely 
used  of  the  igneous  rocks,  their  properties  have  been  more  thoroughly 
investigated  in  this  country.  It  may  be  said,  however,  that  many 
other  plutonic  rocks  of  granitoid  texture  including  gneisses  resemble  the 
granites  in  their  absorption,  crushing  strength,  transverse  strength,  fire 
resistance,  etc. 

Specific  gravity.  —  The  average  specific  gravity  of  granite  is  about 
2.662,  which  is  equivalent  to  two  long  tons,  or  4480  pounds  per  cubic 
yard,  or  about  165  pounds  per  cubic  foot. 


PLATE  LXXI,  FIG.  1.  —  Moderately  fine-grained  granite,  Hallowell,  Me. 


FIG.  2.  —  Very  coarse-grained  granite,  St.  Cloud,  Minn. 


(458) 


BUILDING  STONE  459 

Crushing  strength.  —  The  ultimate  crushing  strength  was  found  by 
Buckley  in  Wisconsin  granites  to  vary  from  15,000  to  43,973  pounds  per 
square  inch,  but  15,000  to  30,000  pounds  would  be  the  more  usual  range. 

Texture.  —  The  texture  of  granites  is  usually  even  granular  or  grani- 
toid, but  sometimes  it  is  porphyritic.  The  granitoid  ones  may  be  fine-, 
medium-,  or  coarse-grained.  Other  things  being  equal,  a  fine-grained 
granite  is  usually  more  durable  than  a  coarse-grained  one,  and  the  lat- 
ter in  turn  longer  lived  than  a  porphyritic  one.  Finer-grained  granites 
also  lend  themselves  to  carving  for  ornamentation  better  than  the 
coarse-textured  varieties. 

Absorption.  —  Granites,  if  fresh,  always  show  a  low  absorption,  usually 
less  than  one  per  cent  when  fresh.  Their  porosity  is  consequently  small, 
and  there  is  little  danger  from  quarrying  them  in  freezing  weather. 

Elasticity.  —  This  property  is  rarely  tested.  Specimens  from  Con- 
necticut, Maine,  Minnesota  and  New  Hampshire,  showed  that  pieces 
with  a  gaged  length  of  20  inches,  and  a  diameter  of  5.5  inches  at  the 
middle,  when  placed  under  a  load  of  5000  pounds  per  square  inch, 
compressed  from  0.0108  to  0.0245  inches.  This  resulted  in  a  lateral 
expansion  of  from  0.005  to  0.007  inch,  and  gave  ratios  of  lateral  expan- 
sion to  longitudinal  compression  ranging  from  1  :  8  to  1  :  47.1 

Flexibility.  —  Granite,  in  spite  of  its  apparently  rigid  character,  is 
flexible  in  sheets  of  sufficient  thinness  and  area.  Dale  states  that  sheets 
half  an  inch  thick  and  4  feet  long,  from  a  Maine  quarry  were  flexible, 
but  suggests  that  this  flexibility  may  have  been  due  to  the  partially 
disintegrated  character  of  the  stone.2 

Fire  resistance.  —  Granite  spalls  off  badly  under  the  combined  in- 
fluence of  fire  and  water,  which  may  be  due  to  the  differential  expansion 
and  contraction  of  the  outer  and  inner  portions  of  a  block.  It  may  also 
be  due  to  the  vitreousness  of  the  quartz,  and  the  presence  of  liquids  and 
gases  contained  in  microscopic  cavities  of  the  quartz,  which  expand 
violently  on  being  heated. 

Color.  —  The  color  of  granites,  as  of  other  feldspar-bearing  igneous 
rocks,  depends  on  the  color  of  the  prevailing  mineral,  feldspar,  and  the 
proportion  of  light  and  dark  minerals. 

Pink  or  red  granites  are  not  uncommon,  and  owe  their  color  to  that 
of  the  prevailing  mineral,  feldspar.  Probably  the  most  frequent  color 
of  granite  is  some  shade  of  gray,  which  is  determined  by  the  ratio  of 
dark  to  light-colored  minerals,  and  the  light  nearly  white  color  of  the 
feldspars. 

1  Report  on  Tests  of  Metals,  etc.,  U.  S.  War  Dept,  1896,  pp.  339-348. 

2  U.  S.  Geol.  Survey,  Bull.  313,  pp.  22  and  151,  1907. 


PLATE   LXXII,    FIG.    1.  —  Port   Deposit,   Maryland,  gneissic 
granite  with  face  cut  at  right  angles  to  banding. 


(460) 


FIG.  2.  —  Port  Deposit,  Maryland,  gneissic  granite  with  face  cut 
parallel  to  the  banding. 


i-^T 

V":  *  -»v" 


PLATE  LXXIII,  FIG.  1.  —  Diorite  from  Ferris,  Calif.,  showing  contrast 
between  light  and  dark  minerals. 


FIG.  2.  —  Boulder  quarry,  Richmond,  Va.     (H.  Ries,  photo.) 


(461) 


462  ENGINEERING  GEOLOGY 

Cases  are  known  of  some  deep-pink  granites  fading  on  prolonged 
exposure  to  the  sunlight. 

Classification.  —  Granites  may  be  classified  according  to  their 
mineral  constituents,  texture,  color,  or  even  uses,  but  no  one  of  these 
is  satisfactory  as  a  basis. 

Structure  of  granites.  —  Fortunately  for  the  quarrymen  joints  are 
present  in  almost  every  granite  quarry  and  greatly  facilitate  the  extrac- 
tion of  the  stone,  but  they  vary  in  their  regularity. 

In  most  quarries  the  rock  mass  is  broken  into  sheets  or  beds  by  joints 
which  are  roughly  parallel  to  the  surface,  but  which  owing  to  their 
divergence  and  convergence  break  the  granite  into  a  series  of  flat  lenses. 

In  addition  to  these  there  are  usually  one  or  more  systems  of  vertical 
joints.  The  spacing  of  these  several  sets  of  joints  will  of  course  deter- 
mine the  size  of  block  that  can  be  extracted  from  a  given  quarry.  Mono- 
liths 50  feet  long  and  4  feet  square  are  not  difficult  to  obtain. 

When  weathering  has  taken  place  along  the  joints,  the  rounded  blocks 
of  stone,  resemble  boulders,  and  hence  the  name  boulder-quarry  (Plate 
LXXIII,  Fig.  2) .  This  feature  is  most  commonly  seen  in  the  southern 
quarries,  where  the  products  of  rock  decay  have  not  been  removed  by 
glacial  action.  Where  present  it  necessitates  at  times  the  removal  of 
much  unsound  or  partly  decayed  stone,  in  order  to  uncover  the  sound 
material.  Although  these  boulders  may  appear  to  be  fresh  interiorly, 
they  are  not  infrequently  traversed  by  minute  cracks,  which  do  not 
become  noticeable  until  the  stone  is  put  in  use.  Their  selection  is, 
therefore,  undesirable. 

The  rift  is  an  obscure  foliation,  either  vertical  (or  nearly  so)  or  hori- 
zontal, along  which  the  granite  splits  more  readily  than  in  any  other 
direction,  while  the  grain  is  a  direction  at  right  angles  to  the  rift,  along 
which  the  stone  splits  less  readily. 

Rift  and  grain  are  not  necessarily  pronounced;  indeed,  either  or  both 
may  be  poorly  developed  or  absent.  A  change  in  the  direction  of  the 
rift  is  called  the  rwi. 

The  cut  off  or  hardway  is  a  term  used  to  indicate  the  direction  along 
which  granite  must  be  channeled  because  it  will  not  split. 

Sheets  is  a  term  used  to  designate  the  division  of  granite  by  joint-like 
fractures  which  are  variously  curved  or  almost  horizontal,  and  nearly 
parallel  with  the  surface.  The  sheets  usually  become  thicker  with 
depth. 

Knots  are  segregations  varying  greatly  in  size,  but  usually  roundish 
in  outline.  They  are  made  up  chiefly  of  the  darker  minerals  and  often 
form  unsightly  spots  in  granite.  They  are  mainly  objectionable  because 


PLATE   LXXIV,  FIG.  1.  —  Granite  quarry  at  North  Jay,  Me.     (Photo  loaned  by 
Maine  and  New  Hampshire  Granite  Company.) 


FIG.  2.  —  Granite  quarry,  Hardwick,  Vt. 


(From  Ries'  Economic  Geology.) 
(463) 


464  ENGINEERING  GEOLOGY 

they  mar  the  beauty  of  the  stone,  but  in  some  plutonic  rocks  they  are  so 
numerous  and  symmetrical  in  form  as  to  be  of  ornamental  character. 
(Plate  LXXV.) 

Inclusions.  —  Many  granites  contain  angular  fragments  of  other 
rocks,  such  as  schists,  gneisses,  limestone  or  even  other  granites,  which 
become  incorporated  in  the  granite  during  its  intrusion.  Those  por- 
tions of  the  rock  containing  them  often  have  to  be  discarded. 

Dikes.  —  In  some  granite  quarries  the  stone  is  traversed  by  dikes  of 
other  igneous  rock,  such  as  diabase,  or  in  most  cases  pegmatite.  They 
are  objectionable;  because  (1)  the  stone  containing  them  is  of  no  value 
for  dimension  work;  (2)  the  rock  on  either  side  of  them  is  often  rendered 
worthless  by  shattering;  and  (3)  an  otherwise  good  stone  may  be  so 
permeated  with  small  dikes  as  to  seriously  decrease  its  usefulness. 

Uses  of  granite.  —  Granites  on  account  of  their  usually  great 
durability,  variety  of  color,  susceptibility  to  polish,  and  texture,  are 
among  the  most  widely-used  of  building  stones.  The  coarser-  and 
medium-grained  ones  are  well  adapted  to  massive  work,  such  as  the 
construction  of  large  buildings,  sea-walls,  dams,  bridge  piers,  dry 
docks,  etc. 

Distribution  of  Granites  and  Granite  Gneisses 

Granite  forms  an  important  source  of  building  stone,  somewhat 
widely  distributed  in  the  United  States,  but  probably  70  per  cent  of 
that  quarried  comes  from  the  eastern  United  States,  where  extensive 
deposits,  owing  to  their  favorable  location  for  working  and  shipment, 
together  with  their  nearness  to  large  markets,  have  been  developed 
on  an  enormous  scale.  Gneisses,  usually  of  granitic  composition,  are 
also  widely  employed  in  the  eastern  states.  Under  this  head  there 
are  also  included  certain  closely  allied  rocks  such  as  grano-diorites, 
etc. 

The  producing  areas  are:  (1)  Eastern  belt  extending  from  Maine, 
southwestward  to  northern  Alabama.  (2)  Minnesota- Wisconsin  area. 
(3)  Southwestern  area,  including  isolated  districts  in  Missouri,  Okla- 
homa and  Texas.  (4)  Cordilleran  area,  including  parts  of  Colorado, 
California  and  other  western  states.  (5)  Black  Hills  area  of  South 
Dakota. 

Eastern  crystalline  belt  (Refs.  14,  25,  28,  35,  38,  40,  45,  50,  58,  607 
66,  73) .  —  This  belt  which  extends  from  northeastern  Maine  to 
northern  Alabama  contains  a  number  of  granites  and  granite-gneisses, 
which  range  in  age  from  pre-Cambrian  to  Carboniferous,  but  are  mostly 
the  former. 


PLATE  LXXV.  —  Orbicular  gabbro  from  North  Carolina.     (After  Watson,  U.  S. 

Geol.  Survey,  Bull.,  426.) 

(465) 


466 


ENGINEERING   GEOLOGY 


Without  a  detailed  discussion,  the  following  table  may  suffice  as  a 
statement  of  the  more  important  occurrences. 


State. 

Locality. 

Kind. 

Texture. 

Color. 

Maine 

North  Jay 

Biotite-muscovite 

Fine 

Light  gray 

Hallowell 

granite 
B  iotite-musco  vi  te 

Fine 

White 

granite 

Crotch  Island 
Fox  Islands 

Biotite  granite 
Biotite  granite 

Coarse 
Coarse 

Light  gray 
Pink  gray 

New  Hampshire 

Concord 

Muscovite-biotite 

Fine-medium 

Bluish  gray 

granite 

Fitzwilliam 

Muscovite-biotite 

Fine 

Light  bluish  gray 

granite 

Marlboro 

Biotite-muscovite 

Fine 

Light  bluish  gray 

granite 

Lebanon 

Biotite-granite  gneiss 

Gneissoid  coarse 

Pink  gray 

Canaan 
Redstone 

Biotite-granite  gneiss 
Biotite  granite 

Gneissoid  coarse 
Coarse 

Light  buff  gray 
Light  pink  mottled 

Vermont 

Woodbury 

Biotite  granite 

Fine-medium 

Bluish  gray 

*Barre 

Biotite  granite 

Fine 

Shades  of  gray 

Hard  wick 

Quartz-monzonite 

Medium 

Dark  gray 

*Windsor 

Hornblende-augite 

Medium 

Olive  green 

granite 

Bethel 

Quartz  monzonite 

Medium 

Light 

Massachusetts 

Milford 

Biotite  granite 

Medium   slightly 

Pink  gray 

gneissoid 

Fall  River 
New  Bedford 

Biotite-granite  gneiss 
Biotite-m  usco  vite 

Coarse 
Coarse  sometimes 

Pink  gray 
Light  pink  gray 

Rockport 
*Quincy 

granite  gneiss 
Hornblende  -granite 
Hornblende  pyroxene 

gneissoid 
Medium  to  coarse 
Medium  to  coarse 

Gray  and  green 
Gray    or   greenish 

granite 

shades 

*Chester 

Muscovite-biotite 

Variable 

Blue  gray 

granite 

Rhode  Island 

•Westerly 

Quartz  monzonite  and 

Fine 

Pink,  blue 

biotite  granites 

Connecticut 

Stony  Creek 
Greenwich 

Biotite  granite  gneiss 
Mica-diorite  gneiss 

Coarse  gneissoid 
Coarse,  porphyritic  . 

Pink 
Blue  gray 

gneissoid 

Leete  Island 

Biotite-granite  gneiss 

Medium  gneissic 

Red  gray 

New  York 

St.  Lawrence 

Granite 

Fine  to  coarse 

Pink 

County 

New  Jersey 

Compton 

Granite 

Coarse  grained 

Pink 

Maryland 

Port  Deposit 
Woodstock 

Biotite-granite  gneiss 
Biotite  granite 

Fine,  gneissic 
Medium 

Gray 

Gray 

Baltimore 

Gneiss 

Variable 

Blue  gray 

Virginia 

Fredericksburg 

Biotite  granite 

Medium  to  fine- 

Blue gray 

grained 

Petersburg 

Biotite  granite 

Medium 

Gray 

Richmond 

Biotite  granite 

Fine  to  medium 

Gray  and  blue  gray 

North  Carolina 

Mt.  Airy 

Biotite  granite 

Medium 

Light  gray 

Salisbury 
Greystone 

Biotite  granite 
Biotite  granite 

Fine 
Fine  to  medium  , 

Pink  or  light  gray 
Gray  to  pink  gray 

gneissoid 

South  Carolina 

Columbia 

Biotite  granite 

Fine  to  coarse 

Gray 

Rion 

Biotite  granite 

Medium 

Gray 

Georgia 

Stone  Mountain 

Biotite  muscovite 

Fine  to  medium 

Light  gray 

granite 

Lexington 

Biotite  granite 

Fine 

Blue  gray 

*0glesby 

Biotite  granite 

Fine  to  medium 

Blue  gray 

Sparta 

Biotite  granite 

Medium  to  coarse 

Gray 

*  Used  also  for  monumental  purposes. 


SCALE  OF 


PLATE  LXXVI.  —  Map  showing  distribution  of  igneous  rocks  and  gneisses  in 


,     300  400  500  600 

i  the  United  States.     (After  G.  P.  Merrill,  "Stones  for  Building  and  Decoration.") 


PLATE  LXXVII,  FIG.  1.  —  General  view  of  Stone  Mountain,  Ga. 
U.  S.  Geol.  Survey,  Bull.  426.) 


(After  Watson, 


FIG.  2.  —  Quarry  of  granite  along  base  of  Stone  Mountain,  Ga.,  shows  sheeting 
following  surface.     (After  Watson,  Ibid.) 

(467) 


468 


ENGINEERING  GEOLOGY 


The  rocks  are  usually  some  shade  of  gray,  pink  being  less  frequent. 
They  are  medium-  to  fine-grained  in  texture,  and  even-granular  to 
porphyritic.  Practically  all  are  of  excellent  durability.  The  New 
England  ones  have  been  worked  the  longest  and  are  hence  more 
extensively  developed.  Indeed  many  of  the  granites  located  along 
the  coast  have  been  shipped  to  many  southern  points.  In  recent 
years,  however,  there  has  been  considerable  expansion  in  the  quarry- 
ing industry  of  the  southern  states. 

Minnesota- Wisconsin  area  (Refs.  42,  78).  —  There  are  several 
detached  areas  in  these  two  states,  which  supply  both  constructional 
and  ornamental  granites.  The  best-known  constructional  granite 
in  Wisconsin  is  the  Wausau  stone  which  is  a  coarse-grained  red  or 
gray  rock.  That  obtained  from  Amberg  is  a  fine-grained  gray 
granite.  Berlin  supplies  a  fine-grained,  grayish-black  quartz  porphyry 
utilized  chiefly  for  paving  blocks,  while  a  highly  ornamental  fine- 
grained red  granite,  of  value  for  monumental  work,  is  quarried  at 
Montello. 

In  Minnesota  medium-grained  pinkish  granite  and  a  fine-grained 
gray  or  red  syenite  is  quarried  at  St.  Cloud,  while  a  dark-red  medium  to 
coarse-grained  gray  granite  comes  from  Ortonville. 

Southwestern  area  (Refs.  19,  43,  54,  65).  —  The  several  granite 
areas  of  Missouri,  Oklahoma,  and  Texas  are  worked  on  a  small  scale, 
chiefly  because  they  are  located  in  a  region  of  limited  demand.  The 
following  types  are  mentioned: 


State. 

Locality. 

Kind. 

Texture. 

Color. 

Missouri 

Graniteville 

Biotite  granite 

Medium  to 

Red 

coarse 

Texas 

Knob  Lick 

Fine 

Gray  to  Red 

Wichita  Mts. 

Coarse  to 

Gray 

fine 

Oklahoma 

Arbuckle  Mts. 

Coarse 

Pink 

Garnet  County 

Biotite  granite 

Coarse 

Pink 

Cordilleran  area  (Refs.  6,  20,  23,  44,  75).  —  Granite  is  found  in  a 
number  of  the  Rocky  Mountain  states,  but  has  not  been  extensively 
quarried.  On  the  Pacific  coast  there  are  several  areas  of  importance 
in  California.  These  include  the  Rocklin  area  which  yields  a  gray 
biotite  granite  of  varying  texture  and  the  Raymond  area  which 
supplies  a  medium-grained,  light-gray,  biotite  granite.  Granite  has 
also  been  quarried  in  Riverside  County. 


BUILDING  STONE  469 

Plutonic  Rocks  other  than  Granite 

These  in  general  resemble  the  granites  in  their  physical  properties, 
so  that  this  topic  requires  no  further  discussion.  We  need,  therefore, 
refer  only  to  their  distribution. 

Syenite  (Ref.  19).  —  This  type  of  plutonic  rock  is  comparatively 
rare,  and  is  consequently  but  little  used  for  structural  work.  The 
most  important  occurrence  is  near  Little  Rock,  Ark.,  where  the  stone 
has  been  quarried  for  some  years.  The  rock  is  bluish-gray,  strong 
and  durable. 

Gabbro  (Refs.  37,  38,  51).  —  Gabbro  is  little  used  for  structural 
work.  This  is  due  to  its  lack  of  regular  jointing,  absence  of  pronounced 
rift  and  grain,  dark  color  and  often  great  toughness.  It  is  sometimes 
selected  for  monumental  work  because  of  its  fine  color,  and  ability  to 
take  a  good  polish. 

Gabbro  is  a  common  rock  in  the  Adirondack  Mountains  of  New 
York  state,  and  is  also  known  to  occur  in  the  vicinity  of  Baltimore  as 
well  as  farther  south  in  the  crystalline  area,  around  Lake  Superior,  and 
a  few  other  scattered  points.  The  Duluth,  Minn.,  gabbro  has  been 
used  as  a  building  stone. 

Diabase.  —  This  type  of  rock  is  more  apt  to  occur  in  dikes  than  in 
stocks  and  laccoliths.  The  most  important  occurrences  are  in  Con- 
necticut and  in  northeastern  New  Jersey  (Ref.  46)  and  the  adjoining 
parts  of  New  York.  Additional  but  smaller  areas  occur  to  the  south- 
west as  far  as  Alabama.  The  stone  from  these  is  sometimes  used 
locally  for  road  material,  and  in  rarer  cases  for  monumental  purposes. 
In  the  New  York  region  especially  the  rock  is  extensively  quarried  for 
road  material,  and  to  a  lesser  extent  for  paving  blocks.  It  is  seldom 
used  for  dimension  stone,  because  of  its  great  toughness,  abundant 
and  irregular  jointing,  as  well  as  absence  of  rift  and  grain. 

Volcanic  Rocks 

These  vary  from  very  porous  soft  materials  like  tuff  to  the  hard 
dense  basalts. 

The  use  of  basalt  as  a  building  stone  is  not  widespread,  in  spite  of 
the  extensive  areas  of  this  rock  in  the  west  and  northwest.  Its  very 
dark  color,  abundant  jointing  and  irregular  break  make  it  rather 
unfavorable  for  dimension  work. 

The  more  acid,  softer  and  usually  more  porous  volcanic  rocks,  such 
as  trachyte,  rhyolite,  and  andesite,  are  rare  in  the  east,  and  where  they 
occur  they  are  usually  metamorphosed  and  not  of  much  commercial 
value;  but  in  the  Rocky  Mountain  region,  they  are  of  common 


470  ENGINEERING  GEOLOGY 

occurrence  and  at  times  somewhat  extensively  employed  for  structural 
work. 

The  rhyolite  quarried  at  Castle  Rock  near  Denver,  Colorado,  and 
the  andesite  from  Del  Norte,  Colorado,  have  both  been  much  used. 
Their  absorption  is  often  high,  but  this  is  of  minor  importance  in  a 
dry  climate.  Their  strength  is  also  lower  than  that  of  the  plutonic 
rocks,  but  still  it  is  sufficient  for  structural  work.  The  porous  ones 
should  not,  however,  be  selected  for  any  work  where  they  are  exposed 
to  moisture,  as  in  the  construction  of  dams  or  reservoirs.  Consoli- 
dated tuffs  are  also  common  in  many  parts  of  the  Cordilleran  region  and 
for  ordinary  construction  work  give  satisfactory  results,  being  used 
at  scattered  points  from  Montana  to  Arizona.  They  are  widely  used 
in  Mexico,  and  endure  well  in  a  dry  and  mild  climate.  Many  are  soft 
enough  to  saw  and  they  are  even  more  porous  than  the  rhyolites  and 
andesites. 

Sandstones  and  Quartzites 

Sandstones  and  quartzites,  as  already  stated,  are  normally  composed 
of  grains  of  quartz  bound  together  by  some  cementing  substance. 
Other  minerals  may  be  and  often  are  present,  at  least  in  small  quanti- 
ties. These  accessory  minerals  include  feldspar,  mica,  iron  oxide, 
pyrite  or  even  tourmaline.  In  rare  cases  feldspar  predominates 
(see  further,  Chapter  II). 

Structural  features.  —  Sandstones  and  quartzites  always  show  a 
bedded  structure,  but  the  layers  are  not  necessarily  horizontal,  and 
in  regions  of  folding  may  tilt  at  a  high  angle.  The  thickness  of  the 
beds  affects  the  size  of  the  blocks  that  can  be  extracted*  while  their 
position  affects  the  cost  and  method  of  quarrying  employed.  Jointing 
is  present  in  all  sandstone  quarries,  and  the  effect  of  this  has  already 
been  explained  (p.  167). 

In  some  sandstone  formations  shaly  beds  are  not  uncommon.  If 
present  in  excess  they  cause  much  waste;  if  few  they  can  be  overlooked, 
and  be  thrown  out  as  encountered.  Of  specially  injurious  character, 
however,  are  thin  clayey  or  shaly  streaks  which  occur  in  the  sandstone 
beds,  because  these  are  liable  to  open  up  on  exposure  to  frost  and  split 
the  stone. 

Properties  of  sandstones.  —  Texture.  —  Sandstones  vary  in  text- 
ure from  very  fine-grained  ones,  through  those  of  medium  coarseness, 
to  extreme  cases  in  which  the  grains  are  quite  large,  so  by  increas- 
ing coarseness  they  pass  into  conglomerates.  On  the  other  hand  by 
increasing  fineness  and  increasing  clayey  matter  they  grade  into 
shales. 


BUILDING  STONE  471 

Hardness.  —  Since  the  hardness  of  a  rock,  as  already  explained, 
depends  in  part  on  the  state  of  aggregation  of  the  mineral  grains,  the 
hardness  of  sand  rocks  will  depend  upon  the  tightness  with  which  they 
are  cemented  together.  A  sandstone,  therefore,  although  composed 
entirely  of  quartz  grains  may  be  so  soft  that  a  lump  of  it  can  be  almost 
crushed  under  foot. 

The  cementing  material  in  sandstone  may  be  iron  oxide,  silica,  cal- 
cium carbonate  or  clay.  The  quality  and  character  of  the  cement  af- 
fects the  strength,  durability,  workability  and  even  color  of  the  stone. 
In  some  sandstones  more  than  one  kind  of  cementing  material  is  present. 

Silica  cement  is  the  most  durable,  but  if  present  in  too  great  quantity 
makes  the  stone  hard  to  work.  The  Berea  sandstone  of  Ohio  contains 
a  moderate  amount  of  siliceous  cement,  while  the  Potsdam  quartzite 
of  New  York  is  strongly  cemented  with  the  same  material. 

Iron  oxide  may  also  act  as  a  strong  binder,  but  probably  to  a  less 
degree  than  silica,  and  at  the  same  time  it  colors  the  stone. 

Calcium  carbonate,  though  giving  a  fairly  strong  cement  is  an 
undesirable  one,  for  the  reason  that  it  is  not  only  soft,  but  readily 
dissolves  in  carbonated  waters.  It  can  be  detected  usually  by  the 
fact  that  it  effervesces  when  a  drop  of  dilute  muriatic  acid  is  put  on  the 
stone.  Small  quantities  of  this  cement  are  not  harmful. 

Clay  cement  has  both  its  advantages  and  disadvantages.  It  is  not 
a  strong  cement,  and,  moreover,  serves  to  attract  moisture  to  the  stone; 
hence  an  excess  of  clay  renders  a  stone  liable  to  injury  by  freezing.  A 
small  amount  softens  the  stone  somewhat  and  makes  it  easier  to  work. 
The  clay  also  gives  the  stone  at  times  a  dull,  earthy  look  If  present 
the  clay  should  be  evenly  distributed,  and  not  concentrated  in  seams. 

Color.  —  Buff,  yellow  and  yellowish-brown  colors  are  due  to  limonite, 
and  red  or  red-brown  tints  are  caused  by  hematite,  while  bluish-gray 
and  black  are  due  to  clay  or  carbonaceous  matter. 

Uneven  distribution  of  coloring  matter  produces  a  blotchy  appear- 
ance. Many  sandstones  change  color  on  exposure  to  the  atmosphere 
due  to  oxidation  of  the  iron  compounds  contained  in  them.  This 
change  is  not  necessarily  a  sign  of  decay,  and  the  weathered  rock  may 
have  a  more  pleasing  tone  than  the  fresh  stone. 

Absorption.  —  Sandstones  show  a  wide  range  of  absorption.  Hard, 
dense  ones  like  quartzite  take  up  under  1  per  cent  of  water,  while 
porous  ones  may  take  up  10  or  11  per  cent. 

Crushing  strength.  —  Sandstones  often  show  a  crushing  strength  of 
from  9000  to  12,000  pounds  per  square  inch,  but  may  fall  below  this, 
or  run  even  considerably  higher,  especially  if  quartzitic  in  character. 


472 


ENGINEERING  GEOLOGY 


The  following  figures  give  us  some  idea  of  their  variation  in  crushing 
strength,  as  well  as  their  other  properties. 

TESTS  OF  SANDSTONE 


Locality. 

Crushing  strength. 

Transverse 
strength, 
modulus  of 
rupture. 

Absorption, 
per  cent. 

Specific 
gravity. 

Position. 

Lbs.  per 
sq.  in. 

Presque  Isle,  Wis  

Bed 

Edge 
Bed 
Edge 
Bed 
Edge 
Bed 
Edge 

6,244 
4,747 
4,549 
4,090 
2,502 
2,842 
5,498 
1,658 
12,580 
12,210 

Presque  Isle,  Wis.         ... 

Houghton,  Wis. 

574.6 

8.89 
"'i5!22'" 

Houghton,  Wis  

"'2!582'" 

Dunnville,  Wis  

Dunnville,  Wis. 

Port  Wing,  Wis  
Port  Win",  Wis  

10.33 

2.649 

Portland,  Conn. 

2073 

2.35 
2.49 
2.604 
2.16 
2.66 

East  Longmeadow,  Mass.  .  . 

'"2.08  " 
5.00 
4.00 

Potsdam,  N.  Y. 

Marquette,  Mich. 

3,800 
14,753 
11,547 

Waltonville,  Pa  
Kettle  River,  Minn. 

Berea,  O 

11,213 
5,911 
4,869 
6,309 
8,880 
8,500 
19,022 
(     5,750    ) 
\     3,270    } 
17,250 
14,812 
10,110 
9,665 

'"777!  97" 

'"7'.&" 

"'2!649'" 

Warrensburg,  Mo  
Warrensburg,  Mo. 

Bed 
Edge 

Flagstaff,  Ariz  

3.76 
3.025 
3.9 

2.346 
2.558 
2.34 

Colusa,  Cal  

Columbus,  Mont. 

Warsaw,  N  Y. 

Tenino,  Wash.  .  .  . 

Medina  N  Y. 

Bed 
Edge 
Bed 
Edge 

•  •'•  

2.0 
2.0 

}    .„ 

2.41 
2.39 

2.39 

Medina,  N.  Y  

Trinidad,  Colo. 

Trinidad,  Colo  

Durability.  —  Hard  quart zites  are  usually  of  high  durability,  and 
withstand  the  attacks  of  the  weather  for  a  long  period.  Sandstones 
of  low  absorption  and  good  hardness  also  show  a  long  life  as  a  rule,  but 
some  of  the  softer  ones  may  disintegrate  under  frost  action.  Clay 
seams,  as  already  mentioned,  are  sources  of  weakness,  and  mica  scales, 
if  abundant  along  the  bedding  planes  are  also  liable  to  cause  trouble. 
In  both  cases  this  is  likely  to  be  aggravated  if  the  stone  is  set  on  edge 
instead  of  on  bed. 

The  Connecticut  brownstone  forms  a  striking  case  in  point.  It  was 
formerly  much  used  in  many  of  the  eastern  cities,  and  in  order  to  get 
a  smooth  surface  was  rubbed  parallel  with  the  bedding.  The  stone 
was  then  set  in  the  wall  on  edge.  The  result  was  that  after  the  stone 
had  been  in  place  for  15  years  it  began  to  scale  off  badly  parallel  with 
the  bedding  planes.  Had  the  rock  been  set  in  the  building  on  bed 
much  of  this  trouble  might  have  been  avoided. 

Fire  resistance.  —  Sandstones  are  perhaps  as  little  affected  by  a 
temperature  of  1500°  F.  as  any  building  stones,  but  are  likely  to  spall 
and  crack  when  exposed  to  the  combined  action  of  fire  and  water. 


BUILDING  STONE  473 

Varieties  of  sandstone.  —  The  varietal  names  have  been  given  in 
Chapter  II. 

Distribution  of  Sandstones  and  Quartzites 

Geologic  distribution.  —  Sandstones  have  a  wide  geologic  distribu- 
tion, but  the  geologic  age  is  not  necessarily  an  index  of  quality,  al- 
though we  can  state  in  general  terms  that  those  sandstones  occurring 
in  the  older  geological  formations  are  usually  harder  and  denser  than 
those  found  in  the  younger  ones.  This  being  so  it  is  fortunate  that 
many  occurrences  of  the  second  class  are  found  in  those  parts  of  the 
United  States  where  the  climate  is  mild  or  dry. 

Geographic  distr  bution  (Refs.  33,  37,  43,  48,  54,  56,  73,  77,  78).  - 
It  may  seem  superfluous  to  discuss  the  areal  distribution  of  sandstones 
as  there  is  hardly  a  state  that  does  not  contain  deposits  of  them  that 
are  fit  for  quarrying. 

In  the  eastern  states  one  broken  belt  of  brownstone 1  extends  from 
southern  Massachusetts  southward  to  North  Carolina  and  is  quarried 
at  a  number  of  points. 

Another  belt  of  sandstones  extends  along  the  Appalachian  Mountain 
range  from  Pennsylvania  southward  to  Alabama.  These  vary  from 
Ordovician  to  Carboniferous  in  age,  and  are  quarried  at  a  number  of 
points  for  local  use.  The  Medina  sandstone  quarried  in  western 
New  York  forms  an  isolated  area  of  this  belt. 

In  the  central  states  there  are  a  number  of  sandstone  formations, 
which  are  worked  here  and  there.  Many  of  them  occur  in  the  Car- 
boniferous. The  most  important  is  the  Berea  sandstone  of  northern 
Ohio,  a  widely-used  sandstone  at  the  present  day;  the  Kettle  River 
sandstone  of  Cambrian  age  of  Minnesota  is  much  used  also. 

Many  sandstones  often  of  porous  character  are  quarried  in  the 
Cordilleran  area,  and  form  an  excellent  source  of  building  material. 

Limestones 

Structural  features.  —  Limestones  are  always  stratified,  but  the 
beds  vary  in  thickness  in  different  quarries  or  even  in  the  same  quarry. 
Those  deposits  which  show  massive  bedding  will  naturally  be  of 
greater  value  for  extracting  dimension  stone.  In  most  districts  where 
limestones  are  quarried  for  structural  work  the  beds  lie  flat  or  nearly 
so,  but  at  times  owing  to  folding  of  the  rocks  the  beds  may  be  tilted  at 
varying  angles.  Jointing  is  rarely  absent,  and  since  limestones  are  more 
soluble  in  surface  waters  than  sandstones  the  rock  along  these  joints  is 
sometimes  more  or  less  weathered  by  solution  (Plate  LXIX,  Fig.  1). 

1  The  typical  brownstone  is  a  brown  sandstone,  but  the  name  often  includes  sand- 
stones of  other  colors  occurring  in  the  same  formation. 


474  ENGINEERING  GEOLOGY 

Vertical  and  horizontal  variations  may  occur.  Thus  thick  beds 
may  alternate  with  thin  ones,  or  shaly  seams  with  limestones.  Cer- 
tain beds  may  be  of  even  character,  while  others  interbedded  with 
them  may  be  of  cherty  nature.  As  a  result  a  good  series  of  beds  occurs 
at  one  level,  while  at  a  higher  or  lower  level  the  beds  may  be  worthless. 
Again,  the  limestones  if  followed  up  along  the  strike  sometimes 
become  shaly,  or  change  in  composition. 

Bearing  these  facts  in  mind,  it  will  be  realized  that  in  searching  for 
a  quarry  site,  the  engineer  should  not  base  his  conclusions  on  one  or 
two  outcrops. 

Properties  of  limestones.  —  Texture.  —  Limestones  show  a  variable 
texture,  but  the  majority  are  fine-grained.  Those  which  are  coarse- 
grained are  either  strongly  fossiliferous  or  else  coarsely  crystalline. 
The  finer-grained  ones  split  more  evenly  and  have  better  weathering 
qualities.  The  texture  does  not  necessarily  bear  any  direct  relation 
to  the  absorption. 

Hardness.  —  Dense  limestones  are  usually  quite  hard,  while  the 
more  porous  ones  are  likely  to  be  soft.  The  French  Caen  stone  much 
used  for  decorative  work  is  a  good  example  of  a  fine-grained  porous 
one,  while  the  Coquina  rock  of  Florida  is  an  excellent  type  of  a  very 
open  coarse-grained  stone. 

Some  are  so  soft  as  to  be  readily  cut  with  a  saw.  The  Bedford 
limestone  of  Mississippian  (Lower  Carboniferous)  age,  quarried  in 
Indiana,  is  moderately  hard,  while  the  Shenandoah  group  of  limestones 
of  the  southern  states  represents  a  very  firm  hard  rock. 

Color.  —  A  pure  limestone  whether  calcitic  or  dolomitic  is  white, 
but  clayey  or  carbonaceous  impurities  tend  to  give  it  a  grayish  color 
and  the  former  may  also  make  it  grayish  or  brownish  black.  Many  of 
the  latter  fade  slightly  on  exposure  to  the  atmosphere. 

Durability  and  mineral  impurities.  —  Both  limestones  and  dolomites 
of  dense  and  massive  character,  as  well  as  those  free  from  mineral 
impurities,  are  of  good  durability,  although  not  as  long-lived  as  dense 
sandstones  and  granites. 

Limestones  weather  primarily  by  solution;  that  is  to  say,  rain  or 
surface  water  may  slowly  attack  the  rock,  but  the  solution  of  the 
surface  is  likely  to  go  on  very  unevenly.  If  certain  portions  are 
silicified,  such  as  fossils  replaced  by  silica,  or  if  quartz  veins  are  present 
in  the  rock,  these  resist  the  solvent  action  of  the  surface  waters  more 
than  the  surrounding  calcareous  parts  of  the  rock  and  are  left  standing 
out  in  relief,  giving  the  stone  a  rough  appearance  (Plate  LXIX, 
Fig.  2). 


PLATE  LXXVIII,  FIG.  1.  —  Horizontally  stratified  limestone,  Milwaukee,  Wis. 
(From  Ries,  "Building  Stones  and  Clay  Products".) 


FIG.  2.  —  Quarry  in  calcareous  tufa,  Tivoli,  Italy.     (J.  C.  Branner,  photo.) 

(475) 


476 


ENGINEERING  GEOLOGY 


Dolomites  do  not  weather  so  readily  by  solution.  Some  coarse- 
grained ones  disintegrate,  breaking  off  a  grain  at  a  time. 

Certain  mineral  impurities  interfere  with  the  value  of  the  stone. 

Pyrite  is  an  undesirable  one,  not  only  because  it  weathers  to  rusty 
limonite,  but  for  the  reason  that  in  this  change  sulphuric  acid  is  set 
free,  which  attacks  the  rock. 

Chert  (Plate  XIII,  Fig.  2)  is  another  common  impurity  in  some 
lime  rocks,  the  nodules  usually  being  strung  out  in  bands  along  the 
stratification  planes.  It  not  only  causes  the  rock  to  weather  unevenly, 
but  interferes  with  the  dressing  of  it,  in  drilling  through  it,  and  lastly 
imparts  to  the  stone  a  tendency  to  split  along  the  lines  of  the  chert 
concretions  when  exposed  to  frost  action.  Cases  are  known  where 
masonry  composed  of  cherty  limestone  has  split  so  badly  as  to  neces- 
sitate its  being  replaced  by  fresh  stone. 

Absorption.  —  The  majority  of  the  harder  limestones  have  a  very 
low  absorption,  usually  under  two  per  cent.  Some  widely-used  ones 
may  run  much  higher.  Thus  the  Bedford,  Ind.,  limestone  shows 
4  or  5  per  cent;  the  French  Caen  stone  10  to  12  per  cent;  the  Roman 
travertine  still  more. 

Fire  resistance.  —  The  resistance  of  limestone  to  fire,  at  temperatures 
below  that  required  to  convert  the  stone  into  quicklime,  is  usually 
fair,  although  lime  rock,  like  other  stones,  is  apt  to  spall  badly  under 
the  combined  attack  of  fire  and  water. 

Crushing  strength.  —  Most  hard  limestones  show  a  good  crushing 
strength,  ranging  from  9000  to  12,000  pounds  per  square  inch,  or 
sometimes  very  much  higher. 

The  table  on  page  477,  though  not  exhaustive,  shows  something  of 
the  variation  of  their  crushing  strength  and  other  properties. 

Chemical  composition.  —  For  structural  work  the  chemical  analysis 
of  a  limestone  is  of  comparatively  little  value,  but  the  following 
analyses  are  given  for  those  who  desire  to  see  the  range  in  chemical 
composition  shown  by  limestones  which  are  ordinarily  employed  for 
building  purposes. 

ANALYSES  OF  LIMESTONE 


I. 

II. 

III. 

IV. 

CaCO3.. 

97.26 

54.33 

81.43 

98.91 

MgCOs 

0  37 

39  41 

15  04 

0  58 

A1203  I 

0.49 

0.26 

0.57 

0.63 

Fe2O3  J   ' 
SiO2     

1.69 

3.96 

2.89 

0.10 

H2O 

1  50 

0  08 

I.  Bedford,  Ind.;  II.  Newburg,  N.  Y.;  III.  Spore,  O.;  IV.  Siluria,  Ala. 


BUILDING  STONE 


477 


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478  ENGINEERING  GEOLOGY 

Varieties  of  limestone  and  dolomite.  —  The  different  varietal  names 
have  been  explained  on  p.  119. 

Distribution  of  Limestones  in  the  United  States 

Limestones  are  found  in  many  states  (Plate  LXXIX,  and  Refs.  6, 
20,  30,  33,  37,  43,  46,  53,  73,  75,  78),  and  in  all  geologic  horizons  from 
Cambrian  up  to  Tertiary.  Those  found  in  the  older  geologic  horizons 
are  on  the  whole  denser  and  harder  than  those  occurring  in  the  younger 
ones.  Because  of  their  wide  distribution,  few  areas  have  attained  great 
importance. 

In  the  Atlantic  states  an  important  belt  of  hard  dense  limestones 
extends  from  New  York  to  Alabama  following  the  Great  Valley. 
This  is  known  as  the  Shenandoah  group  of  limestones,  and  has  been 
opened  at  a  number  of  points  for  structural  work. 

In  the  central  states  the  Mississippian  or  Lower  Carboniferous 
formation  carries  a  number  of  important  limestone  deposits,  of  which 
that  worked  near  Bedford,  Ind.,  is  the  best  known.  Much  is  also 
supplied  by  the  Devonian  and  Silurian  formations.  West  of  the 
Great  Plains,  however,  there  are  no  such  extensive  deposits  as  are 
known  in  the  East,  as  a  glance  at  the  map  (Plate  LXXIX)  showing 
the  distribution  of  limestone  formations  will  show. 

Marbles 

Three  types  of  rock  can  be  included  under  this  head,  viz.,  (1) 
crystalline  limestones,  or  marbles  proper;  (2)  onyx  marbles,  and  (3) 
serpentine  marbles. 

Crystalline  Limestones 

These  are  metamorphosed  limestones  or  dolomites,  occurring  in 
areas  of  metamorphic  rocks. 

Properties  of  crystalline  limestones.  —  Structure.  —  These  marbles 
are  usually  massively  bedded,  not  abundantly  jointed  and  the  beds 
show  a  variable  dip.  Owing  to  their  massive  character  they  are 
commonly  quarried  with  channeling  machines. 

Texture.  —  Marbles  vary  in  texture  from  coarse  to  fine,  and  for 
general  purposes  the  latter  are  preferred.  Some  ornamental  ones 
show  a  brecciated  structure,  which  although  it  may  add  to  their 
decorative  value  renders  them  unfit  for  exterior  use  in  a  severe  climate. 

Color.  —  The  range  of  colors  shown  by  marbles  is  very  great  and 
this  adds  to  their  ornamental  value.  Some  are  white,  others  gray  to 


127° 


119° 


SCALE  CF 
0      50    100  200         *« 

PLATE  LXXIX.  —  Map  showing  limestone  areas 


00  400  600  600 


s  in  the  United  States.     (After  U.  S.  Geol.  Survey.) 


480 


ENGINEERING  GEOLOGY 


black,  due  to  carbonaceous  matter;  still  others  may  show  varying  and 
often  beautiful  shades  of  red,  pink,  yellow,  green,  etc.,  due  to  iron 
compounds  and  micaceous  minerals. 

Mineral  composition.  —  When  pure,  marbles  are  composed  of  calcite 
or  dolomite,  or  a  mixture  of  the  two.  Other  minerals,  if  present,  are 
often  to  be  regarded  as  injurious  impurities. 

Pyrite  is  one  of  these  and  its  effect  is  the  same  as  in  limestone. 
Mica  is  another.  It  occurs  usually  in  fine  scales  which  form  blotches, 
or  wavy  bands.  In  small  amounts  it  is  not  very  harmful,  but  if 
abundant  it  lowers  the  weather-resisting  qualities  of  the  stone,  the 
mica  dropping  out  or  decaying,  and  leaving  a  pitted  surface.  Mica- 
ceous marbles  should  not,  therefore,  be  exposed  to  a  severe  climate. 

Tremolite  is  found  in  some  dolomitic  marbles,  and  its  light-colored, 
silky-lustered  grains,  when  fresh,  are  easily  recognizable.  It  weathers 
somewhat  easily  to  a  clayey  material,  so  that,  if  abundant,  the  surface 
of  the  stone  may  become  pitted  as  the  tremolite  weathers  out.  Its 
occurrence  in  a  given  marble  deposit  may  be  irregular.  Quartz  may 
occur  in  some  marbles  as  veins,  concretions  or  thin  laj^ers.  Such  stone 
should  be  rejected.  In  some  Vancouver  Island  marbles  diopside  and 
wollastonite  grains,  which  are  present  in  the  rock,  not  only  interfere 
with  the  production  of  a  good  polish  but  also  weather  out  somewhat 
easily. 

Durability.  —  What  has  been  said  regarding  the  durability  of  lime- 
stones holds  true  for  marbles,  and  to  this  should  be  added  the  fact 
that  the  presence  of  much  mica  or  a  brecciated  structure  are  addi- 
tional points  of  weakness. 

TESTS  OF  MARBLE 


Locality. 

Crushing 
strength, 
average 
crushing 
strength. 

Transverse 
strength, 
average 
modulus 
rupture. 

Specific 
gravity. 

Per  cent 
absorption. 

Weight  per 
cu.  ft. 

Colville,  Wash. 

19,000 

2.87 

0.16 

178 

South  Dover,  N.  Y. 

19,000 

2.86 

0.267 

Tate,  Ga. 

12,800 

2.71 

0.008 

169 

Marble  Hill,  Ga  

13,300 

2.73 

0.006 

171.8 

Tennessee 

16500 

0  008 

Cockeysville,  Md 

16,000 

175 

Dorset,  Vt. 

2  63 

0  58 

164  7 

Tuckahoe,  N.  Y. 

13,600 

2  80 

175 

Rutland,  Vt.  .  . 

11,892 

1202 

Rutland  Vt 

13864 

2057 

DeKalb,  N.  Y  

838 

PLATE  LXXXI,  FIG.  1.  —  Quarry  of  Vermont  Marble  Company,  Proctor,  Vt. 
(Photo  loaned  by  Vermont  Marble  Company.) 


FIG.  2.  —  Slate  quarry,  Penrhyn,  Pa.     (From  Ries'  Economic  Geology.) 

(481) 


482  ENGINEERING  GEOLOGY 

Absorption.  —  The  absorption  of  marbles  is  always  low,  usually 
under  one  per  cent. 

Crushing  and  transverse  strength.  —  A  few  tests  taken  from  different 
sources,  and  given  below  will  give  some  indication  of  these  and  other 
properties  of  marbles. 

Sonorousness.  —  The  ring  which  a  marble  emits  when  struck  with 
a  hammer  is  to  some  extent  indicative  of  its  soundness.  However, 
good  marbles  may  vary  somewhat  in  this  respect. 

Uses.  —  Marbles  are  being  used  in  increasing  quantities  for  ordinary 
structural  work,  although  many  of  the  lighter-colored  ones  soon 
become  soiled  by  dust  and  smoke.  They  are  also  widely  employed 
for  decorative  work,  because  of  their  beauty,  susceptibility  to  polish 
and  easy-working  qualities.  They  are  still  widely  used  for  monuments 
and  shafts,  especially  in  the  rural  districts,  but  are  rapidly  being 
replaced  by  granite. 

Distribution  of  Marbles  in  the  United  States 

One  belt  of  marble  extends  from  western  Vermont  (Ref.  70)  south- 
ward into  Alabama  and  this  is  an  important  one,  for  it  supplies  chiefly 
white  and  gray  marbles  which  are  quarried  in  Vermont,  Pennsylvania, 
Maryland  (Ref.  37),  Georgia  (Ref.  26)  and  Alabama.  Pink  and 
black  marbles  are  locally  known.  Variegated  marbles  of  siliceous 
character  are  obtained  in  northern  Vermont,  and  pink  and  brown  ones 
in  eastern  Tennessee  (Ref.  64). 

White  ones  are  obtained  in  Colorado,  and  white  and  gray  ones  in 
California.  Near  Carthage,  Mo.,  there  is  quarried  a  hard,  dense, 
creamy-white  limestone  that  is  sometimes  marketed  as  a  marble,  and 
takes  a  polish. 

Onyx  Marble 

Under  this  term  we  can  include  two  types  of  calcareous  rock,  the 
one  a  hot-spring  deposit  or  travertine  formed  at  the  surface,  the  other 
a  cold-water  deposit  formed  in  limestone  caves,  in  a  similar  manner 
to  stalactites  and  stalagmites.  Neither  type  of  deposit  is  extensive, 
and  the  stone  is  solely  used  for  decorative  work. 

Serpentine 

This  rock  is  occasionally  found  in  sufficiently  massive  form  to  be 
used  for  structural  and  decorative  work;  indeed  the  latter  use  is  its 
main  one.  The  main  objection  to  it  is  the  frequent  and  irregular  joint- 
ing developed  in  practically  all  quarries,  and  its  poor  weathering 
qualities,  for  on  exposure  to  weather  it  wears  irregularly,  cracks,  loses 


BUILDING  STONE 


483 


its  lustre  and  fades  in  spots.  The  impurities  that  are  often  present  in 
the  stone  are  iron  oxides,  pyrite,  hornblende,  pyroxene  and  carbonates 
of  lime  and  magnesia. 

The  colors  of  the  rock  are  often  very  beautiful.  Green  and  yellow 
predominate  in  the  purer  forms,  while  the  more  impure  ones  commonly 
exhibit  various  shades  of  black,  red  or  brown  (see  Chapter  II). 

Serpentine  deposits  (Ref.  6)  are  known  in  Massachusetts,  Vermont, 
New  York,  New  Jersey,  Pennsylvania,  Maryland  (Ref.  37),  Georgia, 
California  and  Washington. 

Ophicalcite  or  ophiolite  is  a  spotted  green  and  white  variety,  which 
consists  of  a  white  ground  of  calcite,  and  green  spots  of  serpentine. 
It  is  not  much  used  (see  Chapter  II). 

Slates 

Structural  features.  —  Slates,  as  previously  explained,  are  metamor- 
phic  rocks,  derived  usually  from  clay  or  shale  and  more  rarely  from 
very  fine-grained  igneous  rocks  (see  Metamorphic  Rocks). 

Their  commercial  value  depends  primarily  on  the  existence  of  a  well- 
defined  plane  of  splitting,  called  cleavage.     This  has  been  developed 
by  metamorphism,  through  the  rearrange- 
ment and  flattening  of  the  original  min- 
eral grains,  and  in  the  mica  slates  at  least 
by  the  development  of   mica  scales  (see 
under  Metamorphism). 

During  the  process  of  metamorphism 
many  of  the  stratification  planes  become 
sealed  up,  their  position,  however,  being 
indicated  by  dark  bands  or  ribbons.  As 
a  rule  the  slaty  cleavage  is  not  coincident 
with  the  bedding  but  may  form  any  angle 
with  it  (Figs.  68  and  196). 

The  cleavability  of  different  slates  va- 
ries, some  splitting  evenly  and  smoothly 
into  thin  layers,  while  others  do  so  with 
difficulty.  Repeated  freezing  and  thaw- 
ing has  a  disastrous  effect  on  the  cleava- 
bility, so  that  the  material  should  be  split  when  fresh  from  the  quarry 
and  before  it  has  a  chance  to  dry  out. 

False  cleavage  and  slip  cleavage  (Fig.  197)  are  terms  applied  to 
extremely  minute  applications  seen  on  the  cleavage  surfaces,  which 
are  due  to  microscopic  slips  or  faults  along  which  the  slate  breaks  easily. 


FIG.  196. —  Section  in  slate 
quarry  with  cleavage  parallel 
to  bedding,  a,  purple  slate; 
6,  unworked;  c,  and  d,  varie- 
gated; e  and/,  green;  g  and  h, 
gray-green;  i,  quartzite;  j, 
gray  with  black  patches. 
(After  Dale.) 


484 


ENGINEERING  GEOLOGY 


The  grain  (Plate  LXXX1I,  Fig.  1)  is  a  direction  along  which  the 
slate  can  be  split,  but  not  as  smoothly  as  along  the  true  cleavage. 
It  is  indicated  by  a  somewhat  indistinct  striation  on  the  cleavage 
surface  in  a  direction  nearly  parallel  to  the  cleavage  dip. 

Joints  (Plate  LXXXI,  Fig.  2)  are  found  in  slate  of  all  quarries  and 
may  traverse  the  rock  in  various  directions.  The  term  post  is  applied 
to  a  mass  of  slate  traversed  by  so  many  closely-spaced  joints  as  to 
be  worthless. 

Veins  of  calcite  or  quartz  are  not  uncommon,  and  sometimes  occupy 
the  joint  fissures.  Their  presence  renders  that  portion  of  the  slate 
in  which  they  occur  worthless. 

Properties  of  slate.  —  Since  slate  differs  in  its  occurrence,  properties 
and  uses  from  other  building  stones,  its  properties  and  the  tests  which 
can  be  applied  to  it  need  special  reference. 

Sonorousness.  —  A  piece  of  roofing  slate  when  struck  usually  emits 
a  ring  like  vitreous  china.  The  mica  slates  are  more  sonorous  than 


More  carbonate  and  quartz 
filler   grained.   pyritife>-ous 


finer  grained 


More  carbonate  and 
larger  quartz   grains 


Finer    grains    and 

finer    cleavage 

slip    cleavage    .faint 


Pyriti/crons   and  darker 
slip   cleavage   pronounced 


slip    cleavage    faint 


More  carbonate  aud  quartz 
^_      bedding 


Slaty   cleavage 
10°  to    bedding 


See  fig.  B  from    this 

.Slip  cleavage 
>v       .  50°  to     bedding 
X     ("false    cleavage") 


X  4  diam. 


FIG.  197.  —  Section  showing  relation  of  cleavage,   false    cleavage    and  bedding. 

(After  Dale.) 


the  clay  slates;  but  those  with  considerable  chlorite  may  be  deficient 
in  this  respect. 

Cleavability.  —  This  property  is  tested  by  splitting  the  slate  with  a 
thin,  broad-edged  chisel,  in  order  to  determine  the  smoothness,  thin- 
ness and  regularity  with  which  it  cleaves. 


PLATE  LXXXII,  FIG.  1.  —  Sculping  slate.     The  slab  has  been  broken  along  the 
grain,  and  as  the  one  piece  dropped,  it  broke  along  the  cleavage.     (H.  Ries,  photo.) 


FIG.  2.  —  Splitting  slate.     (H.  Ries,  photo.) 


(485) 


486  ENGINEERING  GEOLOGY 

Cross  fracture  (Sculping) .  —  This  property  is  tested  to  determine 
the  character  of  the  grain. 

Color  and  discoloration.  —  The  value  of  a  roofing  slate  depends 
somewhat  upon  its  permanence  of  color.  Information  on  this  point 
is  best  obtained  by  comparing  a  freshly-quarried  piece  with  a  weathered 
one  that  has  lain  on  the  dump  for  several  years  at  least. 

Strength.  —  It  is  important  to  determine  the  transverse  strength  of 
a  slate.  The  modulus  of  rupture  in  the  best  slates  ranges  from  7000 
to  10,000  pounds.  An  impact  test  is  sometimes  used  instead  of  the 
regular  transverse  test.  A  simple  one  devised  by  Merriam  consists 
in  dropping  a  wooden  ball  of  15.7  ounces  weight,  from  a  height  of 
9  inches,  on  to  a  piece  of  slate  6  by  7f  inches  and  0.20  to  0.28  inches 
thick.  The  blows  are  repeated  until  the  slate  breaks.  The  foot- 
pounds of  work  per  pound  of  slate  can  be  calculated  from  the  weight 
and  thickness  of  the  slate,  and  the  number  of  blows. 

Toughness  or  elasticity. — If  a  slab  of  slate  is  fastened  between  two 
supports  and  subjected  to  pressure  it  will  bend  slightly  before  break- 
ing, the  amount  of  deflection  indicating  the  degree  of  toughness  of 
the  slate. 

Abrasive  resistance.  —  This  is  of  importance  where  the  slate  is  used 
in  thick  slabs  for  paving  or  stair  treads,  but  there  is  no  standard  method 
for  determining  it. 

Corrodibility.  —  Slates  should  resist  exposure  to  acid  atmospheres. 
They  may  be  exposed  to  it  either  by  moisture  or  rain  water  with  acid 
flowing  on  the  upper  surface,  or  by  such  water  being  drawn  up  by 
capillarity  between  the  slate  slabs  on  the  roof. 

A  method  of  testing  this  consists  in  using  a  solution  of  98  parts  of 
water,  and  1  part  each  of  hydrochloric  and  sulphuric  acids.  A  weighed 
piece  of  slate  is  immersed  in  this  for  120  hours,  dried  for  40  hours, 
weighed,  the  solution  strengthened  and  the  process  repeated.  The 
loss  in  weight  indicates  the  degree  to  which  it  is  corroded. 

Mineral  impurities.  —  Pyrite  or  marcasite,  the  sulphides  of  iron,  are 
objectionable  impurities,  because  they  decompose  to  limonite  and 
leave  the  slate  pitted.  Calcium  carbonate  is  an  undesirable  consti- 
tuent, since  it  is  attacked  by  acids  of  the  atmosphere.  Clay  is  present 
in  some  slates,  and  such  will  emit  an  argillaceous  odor  if  breathed 
upon,  provided  they  contain  much  of  it.  Siderite  or  a  mixture  of 
dolomite  and  siderite  is  found  in  some  slates,  especially  the  sea-green 
ones  of  Vermont.  Upon  exposure  to  weather  the  iron  carbonate 
changes  to  limonite,  and  there  is  a  corresponding  change  in  color  of 
the  slate  from  green  to  grayish-brown. 


BUILDING  STONE 


487 


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488  ENGINEERING  GEOLOGY 

Tests  of  slate.  —  The  table  on  p.  487  gives  the  properties  of  a 
number  of  slates. 

Quarrying.  —  The  waste  in  slate  quarrying  is  very  high,  probably 
never  under  60  per  cent  and  not  infrequently  as  high  as  80  per  cent. 
The  utilization  of  the  tremendous  waste  heaps  is  still  an  unsolved 
problem,  but  several  possible  uses  have  been  suggested,  viz.:  (1)  As  a 
substitute  for  clay  or  shale  in  Portland  cement;  (2)  as  a  mineral  pig- 
ment when  ground  and  mixed  with  oil;  (3)  as  road  material;  and  (4) 
for  brick  manufacture. 

The  salable  material  taken  from  the  quarry  may  be  used  either  for 
roofing  purposes  or  millstock.  The  latter  represents  a  more  massive 
type,  which  is  cut  into  slabs  for  tubs,  sinks,  table  tops,  switchboards, 
blackboards,  stair  treads,  etc. 

Classification  of  slates.  —  The  following  classification  of  slates  has 
been  suggested  by  Dale. 

A.  Clay  slates.  —  Purple  red  of  Penrhyn,  Wales;  black  of  Martins- 
burg,  W.  Va. 

B.  Mica  slates: 

1.  Fading: 

(a)  Carbonaceous  or  graphitic  (blackish); 

Lehigh  &  Northampton  Counties,  Pa.;  Benson,  Vt, 

(b)  Chloritic  (greenish); 

"  Sea  green,"  Vermont. 

(c)  Hematitic  and  chloritic  (purplish); 
Purplish  of  Pawlet  and  Poultney,  Vt. 

2.  Unfading : 

(a)  Graphitic ; 

Peachbottom  of  Pa.  and  Md.;  Arvonia,  Va.;  North- 
field,  Vt.;  Brownville,  Monson,  Me.;  North  Blanch- 
ard,  Me.;  West  Monson,  Me. 

(b)  Hematitic  (reddish); 

Granville,  Hampton,  N.  Y.;  Polk  County,  Ark. 

(c)  Chloritic  (greenish); 

"  Unfading  green,"  Vermont. 

(d)  Hematitic  and  chloritic  (purplish); 
Purplish  of  Fair  Haven,  Vt.;  Thurston,  Md. 

Distribution  of  Slates  in  the  United  States 

Since  slates  are  of  metamorphic  origin,  they,  like  true  marbles,  are 
limited  to  those  regions  (Plate  LXXXIII)  in  which  the  rocks  are  meta- 
morphosed, due  to  mountain-making  movements  as  explained  on  p.  208. 


bC 

I 


490  ENGINEERING  GEOLOGY 

At  present  the  greater  part  of  our  supply  comes  from  the  Cambrian 
and  Silurian  horizons  of  the  Atlantic  states  (Ref .  69) .  The  slates  from 
this  area  are  chiefly  grayish-black,  but  that  portion  of  the  belt  lying 
on  the  border  between  New  York  and  Vermont  also  supplies  red,  green 
and  purple  slate.  Other  producing  states  are  California,  Arkansas 
and  Minnesota. 

References  on  Building  Stone 

General  works.  —  1.  Brown,  W.  M.,  Stone,  June,  1908  (Preservation 
methods).  —  2.  Davies,  D.  C.,  Slate  and  Slate  Quarrying,  London,  1899 
(Crosby,  Lockwood  &  Son).  —  3.  Forster,  C.  L.,  Elements  of  Mining 
and  Quarrying,  London,  1903  (Griffin  &  Co.).  —  4.  Hirshwald,  Hand- 
buch  der  bautechnischen  Gesteinspriifung,  Berlin,  1912  (Gebriider 
Borntrager).  —  5.  Humphreys,  U.  S.  Geol.  Surv.,  Bull.  370,  1909  (Fire 
tests).  —  6.  Merrill,  G.  P.,  Stones  for  Building  and  Decoration,  New 
York  (Wiley  &  Sons).  — 7.  McCourt,  W.  E.,  N.  Y.  State  Museum, 
Bull.  100,  1906  and  N.  J.  Geol.  Surv.,  Ann.  Kept.,  1906  (Fire  tests).  - 
8.  Renwick,  W.  G.,  Marble  and  Marble  Working,  London,  1909 
(Lockwood,  Crosby  &  Son).  —  9.  Ries,  H.,  Building  Stones  and  Clay 
Products,  New  York,  1912  (Wiley  &  Sons).  —  10.  Seipp,  H.,  Wet- 
terbestandigkeit  der  natiirlichen  Bausteine,  Jena,  1900  (H.  Costen- 
oble.) 

Serials.  —  11.  Stone,  a  monthly  magazine  published  in  New  York 
City.  — 12.  Baumaterialenkunde,  published  by  Stahle  u.  Friedel, 
Stuttgart. 

Areal  reports.  —  Alabama.  13.  Smith,  Eng.  &  Min.  Jour.,  LXVI, 
p.  398;  Smith,  Min.  Res.  of  Ala.,  Ala.  Geol.  Surv.,  1904.  —  14.  Watson, 
U.  S.  Geol.  Surv.,  Bull.  426,  1910  (Granites).  —  Alaska.  15.  Wright, 
U.  S.  Geol.  Surv.,  Bull.  345,  p.  116,  1908.  —  Arizona.  16.  Anon, 
Stone,  Aug.,  1911  (Marble).  —  Arkansas.  17.  Hopkins,  Ark.  Geol. 
Surv.,  Ann.  Rept.  1890,  IV,  1893  (Marbles).  —  18.  Purdue,  Ark.  Geol. 
Surv.,  1909  (Slate).  — 19.  Williams,  J.  F.,  Ark.  Geol.  Surv.,  Ann. 
Rept.,  1890,  II,  1891  (Igneous  rocks).  —  California.  20.  Aubury  and 
others,  Calif.  State  Min.  Bur.,  Bull.  38,  1906  (General).  — 21.  Eckel, 
U.  S.  Geol.  Surv.,  Bull.  225,  p.  417,  1904  (Slate).  — 22.  Dale,  U.  S. 
Geol.  Surv.,  Bull.  275,  1906  (Slate).  —  Colorado.  23.  Lakes,  Mines 
and  Minerals,  XXII,  pp.  29  and  62,  1901;  —  24.  Anon,  Stone,  XI,  p. 
213,  1895.  —  Connecticut.  25.  Dale,  U.  S.  Geol.  Surv.,  Bull.  484,  1911 
(Granites).— -Georgia.  26.  McCallie,  Ga.  Geol.  Surv.,  Bull.  1,  2nd 
ed.,  1904  (Marbles);  27.  Watson,  Ga.  Geol.  Surv.,  Bull.  9A,  1903 


BUILDING  STONE  491 

(Granites  and  gneisses).  —  28.  Watson,  U.  S.  Geol.  Surv.,  Bull.  426, 
1910  (Granites).  —  29.  Dale,  U.  S.  Geol.  Surv.,  Bull.  275,  1906  (Slate). 
-  Indiana.  30.  Hopkins,  Ind.  Geol.  and  Nat.  Hist.  Surv.,  20th  Ann. 
Kept.,  p.  188,  1896.  —  31.  Siebenthal,  U.  S.  Geol.  Surv.,  19th  Ann. 
Kept.,  VI,  p.  292,  1898  (Bedford  limestone);  also  Ind.  Geol.  and  Nat. 
Hist.  Surv.,  32nd  Ann.  Kept.,  p.  321,  1907  (Limestone).  —  32.  Thomp- 
son, Ind.  Geol.  and  Nat.  Hist.  Surv.,  17th  Ann.  Kept.,  p.  19,  1891 
(General).  —  Iowa.  33.  Beyer,  Williams  and  Marston,  la.  Geol.  Surv., 
XVII,  pp.  54  and  185,  1907.  —  Kentucky.  34.  Gardiner,  U.  S.  Geol. 
Surv.,  Bull.  430  (Bowling  Green  limestone).  —  Maine.  35.  Dale, 
U.  S.  Geol.  Surv.,  Bull.  313,  1907  (Granite). —36.  Dale,  U.S. 
Geol.  Surv.,  275,  1906  (Slate).  —  Maryland.  37.  Matthews,  Md.  Geol. 
Surv.,  II,  p.  125,  1898  (General) .  — 38.  Watson,  U.  S.  Geol.  Surv.,  Bull. 
426,  1910  (Granites).  — 39.  Dale,  Ibid.  Bull.  275,  1906  (Slate). 

—  Massachusetts.      40.    Dale,    U.   S.    Geol.   Surv.,   Bull.    354,   1908 
(Granites).  —  Michigan.     41.  Benedict,  Stone,  XVII,  p.  153.  (Bayport 
district).—  Minnesota.     42.  Burchard,  U.  S.  Geol.  Surv.,  Bull.  430. 

—  Missouri.     43.  Buckley  and  Buehler,  Mo.  Bur.  Geol.  and  Mines, 
II,    1904    (General).  — Montana.     44.   Rowe,  Univ.  of   Mont.,  Bull. 
50    (General). —  New    Hampshire.      45.    Dale,    U.S.   Geol.   Surv., 
Bull.    354,     1908     (Granites) .—  New    Jersey.      46.    Lewis,    N.    J. 
Geol.  Surv.,  Ann.  Kept,  1908,  p.  53,  1909  (General).—  New    York. 
47.  Dale,  U.  S.  Geol.  Surv.,  Bull.  275,  1906  (Slate).  — 48.  Dickinson, 
N.  Y.  State  Museum,  Bull.  61,  1903  (Bluestone).  —  49.  Smock,  Ibid, 
Bull.  3  (General).  —  North  Carolina.     50.  W^atson,  U.  S.  Geol.  Surv., 
Bull.  426,  1910  (Granites).  — 51.  Watson,  Laney  and  Merrill,  N.  C. 
Geol.  Surv.,  Bull.  2,  1906  (General).  —  Ohio.     52.  Orton,   Ohio  Geol. 
Surv.,  V,  p.  578,  1884.  —  53.  Orton  and  Peppel,  Ibid.,  4th  ser.,  Bull. 
4,    1906   (Limestones).  —  Oklahoma.     54.  Gould  and  Taylor,    Okla. 
Geol.  Surv.,  Bull.  5,  1911  (General) .  —  Oregon.     55.  Darton,  U.  S. 
Geol.    Surv.,    Bull.    387,    1909    (Limestones).  —  Pennsylvania.     56. 
Hopkins,  Penn.  State  College,  Ann.  Kept.,  1895.  —  Appendix,  1897; 
also  U.  S.  Geol.  Surv.,  18th  Ann.  Kept.,  V,  p.  1025,    1897    (Brown- 
stones).— 57.  Dale,  U.  S.  Geol.  Surv.,  Bull.  275,   1906  (Slate).  - 
Rhode  Island.     58.  Dale,  U.  S.  Geol.  Surv.,  Bull.  354,  1908  (Granites). 

-South  Carolina.  59.  Sloan,  S.  C.  Geol.  Surv.,  Ser.  IV,  Bull.  2, 
p.  162,  1908.  —  60.  W^atson,  U.  S.  Geol.  Surv.,  Bull.  426,  1910  (Gran- 
ites). —  South  Dakota.  61.  Todd,  S.  Dak.  Geol.  Surv.,  Bull.  3,  p.  81, 
1902.  —  Tennessee.  62.  Keith,  U.  S.  Geol.  Surv.,  Bull.  213,  p.  366, 
1903  (Marbles).  —  63.  See  also  G.  P.  Merrill's  book  mentioned  above 
64.  Gordon,  C.  H.,  Tenn.  Geol.  Surv.,  Bull.  2D,  1911.  —  Texas.  65. 


492  ENGINEERING  GEOLOGY 

Burchard,  U.  S.  Geol.,  Bull.  430  F.  —  Vermont.  —  66.  Dale,  U.  S. 
Geol.  Surv.,  Bull.  404,  1909  (Granites).  —  67.  Perkins,  Report  of 
State  Geologist  on  Mineral  Industries  of  Vermont,  1899-1900,  1903- 
1904,  1907-1908,  and  Report  on  Marble,  Slate  and  Granite  Industries, 
1898. —  68.  Ries,  U.  S.  Geol.  Surv.,  18th  Ann.  Rept.  (Marbles).— 
69.  Dale,  U.  S.  Geol.  Surv.,  Bull.  275,  1906  (Slate).  — 70.  Ibid.,  Bull. 
521,  1912  (Marbles).  —  71.  Bristol,  Rept.  State  Geologist,  1911-12 
(High-tension  testing  slate  and  marble) .  —  72.  Baker  and  Davidson, 
Ibid.  (Strength  and  weathering  qualities  slate).  —  Virginia.  73. 
Watson,  Min.  Res.  of  Va.,  Lynchburg,  1907  (General),  and  U.  S.  Geol. 
Surv.,  Bull.  426,  1910  (Granites).  —  74.  Dale,  U.  S.  Geol.  Surv., 
Bull.  275,  1906  (Slate).  —  Washington.  75.  Shedd,  Wash.  Geol.  Surv., 
II,  1902  (General).  —  West  Virginia.  76.  Dale,  U.  S.  Geol.  Surv., 
Bull.  275,  1906  (Slate).  —  77.  Grimsley,  W.  Va.,  Geol.  Surv.,  Ill,  1905 
(Limestones),  and  IV,  p.  355,  1909  (Sandstones).  —  Wisconsin.  78. 
Buckley,  Wis.  Geol.  and  Nat.  Hist.  Surv.,  Bull.  IV,  1898  (General). 
—  Wyoming.  79.  Knight,  Eng.  &  Min.  Jour.,  LXVI,  p.  546,  1898. 


CHAPTER  XII 


LIMES,  CEMENTS,  AND  PLASTER 

Limes  and  Calcareous  Cements 

The  limes  and  calcareous  cements  form  an  important  class  of 
economic  products  obtained  from  limestones  of  varying  composition 
by  heating  them  to  different  temperatures.  The  limes  are  produced 
from  limestones  low  in  clayey  impurities;  the  cements  from  limestones 
high  in  clayey  substances.  In  the  burning  of  the  former  calcium 
hydrate  is  formed,  in  the  latter  complex  silicates  and  aluminates. 

Composition  of  limestones.  —  The  subjoined  table  gives  a  sufficient 
number  of  analyses  to  indicate  how  these  rocks  vary  in  their  composi- 
tion. 

ANALYSES  OF  LIMESTONE 


I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

IX. 

X. 

XI. 

SiO,... 

72 

0  06 

3  83 

0  08 

5  5 

7.60 

6  22 

28.72 

15.37 

AM),... 

} 

1.5 

0.80 

2.31 

0.25 

1.3 

0.75 

(    1.70 

12.28 

9.13 

cS?'::: 

MgO 

56.00 

30.44 
21  73 

54.28 
0  8 

55.00 

52.16 
0  14 

30.46 
21  48 

28.2 
20  2 

50.05 
0  30 

47.86 
0  04 

25.54 
1  10 

25.50 
12  35 

C02.... 
HjO 

44.00 

47.83 

44.0 

43.22 

J41.64 

47.58 

44.3 

41.30 

42.11 

'  24.40 

(34.20 
(    1  20 

SO3 

6  05 

0  20 

1  53 

n.d. 

Total.. 

100.00 

100.00 

101.30 

99.13 

100.28 

99.85 

99.5 

100.00 

100.99 

98.79 

100.00 

I.  Calcite;  II.  Dolomite;  III.  Pure  limestone,  Smith's  Basin,  N.  Y.;  IV.  Bog  lime,  Newaygo,  Mich; 
V.  Chalk,  Western  P.  C.  Co.,  Yankton,  S.  D;  VI.  Dolomite,  Canaan,  Conn;  VII.  Magnesian  lime- 
stone, Oxford  Furnace,  Sussex  County,  N.  J.;  VIII.  Hydraulic  limestone,  Malain,  France;  IX.  Im- 
pure bog  lime,  Montezuma,  N.  Y.;  X.  Natural  cement  rock,  Cumberland,  Md.;  XI.  Natural 
cement  rock,  Rondout. 

From  this  table  it  will  be  seen  that  limestones  range  from  rocks 
composed  almost  entirely  of  calcium  carbonate  or  of  calcium  and 
magnesium  carbonates,  to  others  which  are  high  in  clayey  and  siliceous 
impurities.  The  presence  of  such  impurities  not  only  gives  the  rock 
an  earthy  appearance,  but  at  times  even  a  shaly  structure. 

It  must  not  be  assumed,  however,  that  even  marked  differences  in 
chemical  composition  can  always  be  detected  with  the  naked  eye, 
for  in  many  cases  they  cannot.  If  then  it  is  necessary  to  sample  a 
quarry  for  analysis,  it  should  be  done  thoroughly  and  systematically. 

493 


494  ENGINEERING  GEOLOGY 

Changes  in  burning.  —  If  a  limestone  is  calcined  to  a  temperature 
of  900°  C.  it  loses  all  of  its  carbon  dioxide,  as  shown  by  the  following 
equation: 

CaCO3  +  heat=  CaO  +CO2. 

This  is  the  temperature  of  decarbonation,  and  the  rock  after  heating 
to  this  point  is  porous  and  if  low  in  impurities  will  slake  when  mixed 
with  water.  Heated  still  higher  the  rock  will  clinker  or  fuse  incip- 
iently,  provided  there  are  clayey  impurities  present.  Moreover,  the 
temperature  of  clinkering  depends  on  the  amount  and  nature  of  these 
impurities.  The  presence  of  such  clayey  impurities  not  only  inter- 
feres with  the  slaking  qualities  of  the  burned  product,  but  is  responsible 
for  the  property  of  setting  to  a  hard  mass  when  the  properly-burned 
material  is  ground  and  mixed  with  water.  This  latter  type  of  product 
represents  hydraulic  cement. 

Lime 

Limestone  free  from  or  containing  but  a  small  percentage  of  clayey 
impurities  is  by  decarbonation  changed  to  quicklime,  a  substance 
which  has  a  high  affinity  for  water,  and  which,  when  mixed  with  it 
slakes,  forming  a  hydrate  of  lime.  Thus: 

CaO  +  H20  =  Ca(OH)2. 

The  heat  required  for  burning  the  lime  depends  somewhat  on  the 
character  of  the  stone,  but  decarbonation  takes  place  at  about  900°  C. 
and  the  stone  may  be  heated  as  high  as  about  1200°  C.,  although  at 
this  temperature  impurities  if  present  cause  incipient  vitrification  on 
the  outside  of  the  lump  and  retard  slaking.  Since  some  vitrification 
may  occur  even  below  this  temperature  the  lower  the  heat  at  which 
the  lime  is  burned  the  better.  The  presence  of  steam  in  the  kiln 
lowers  the  decarbonation  temperature  to  790°  C.,  and  for  this  reason 
wood  gives  better  lime  than  coal,  because  it  contains  more  moisture 
which  changes  to  steam  in  the  kiln. 

The  classification  of  limes  adopted  by  the  National  Lime  Manu- 
facturers' Association  is  as  follows: 

Per  cent  of  Magnesia 

High-calcium  lime 0-5 

Magnesian.  lime 5-25 

Dolomitic  lime 25-45 

Super-dolomitic  lime over  45 

Lime  in  slaking,  as  said  before,  combines  chemically  with  the  water, 
this  reaction  being  accompanied  by  the  generation  of  heat  and  an 
increase  in  volume.  Slaked  lime  sets  on  exposure  to  the  air,  due  to 
the  evaporation  of  the  excess  of  water,  and  the  reversion  of  the  calcium 


LIMES,   CEMENT  AND  PLASTER  495 

hydrate  to  calcium  carbonate  by  absorption  of  carbon  dioxide  from 
the  atmosphere.     Thus: 

Ca(OH)2  +  CO2  =  CaC03  +  H2O. 

Dolomitic  limes  will  in  general  slake  more  slowly,  take  up  less  water, 
generate  less  heat,  expand  less,  set  more  slowly  and  shrink  less  than 
high-calcium  limes.  An  underburned  calcite  lime  resembles  a  dolo- 
mitic  lime  in  some  respects.  An  overburned  lime  reacts  more  slowly 
than  a  normally-burned  one. 

Magnesian  limes  work  more  smoothly,  but  set  more  slowly  than 
high-calcium  limes,  as  well  as  being  stronger.  But,  after  all,  experi- 
ence in  mixing  plays  an  important  role  in  the  production  of  successful 
results. 

Hydrated  lime  is  a  product  prepared  by  adding  just  enough  water 
to  accomplish  complete  slaking,  the  heat  generated  evaporating  the 
excess  of  water  and  leaving  the  product  dry.  It  consists  of  calcium 
hydrate  and  magnesium  oxide  (if  the  latter  is  present).  It  usually 
saves  the  time  required  for  slaking. 

Hydraulic  or  Silicate  Cements 

With  an  increase  in  clayey  and  siliceous  impurities,  the  burned  rock 
shows  a  decrease  in  its  slaking  qualities  and  develops  hydraulic  prop- 
erties, or  sets  when  ground  and  mixed  with  water.  This  product 
is  the  hydraulic  cement,  whose  setting  properties  are  due  to  the  forma- 
tion of  new  compounds  during  manufacture  or  when  mixed  with  water. 
The  new  compounds  formed  in  burning  are  probably  solid  solutions  of 
aluminates  and  silicates  of  lime. 

Hydraulic  cements  can  be  divided  into  the  following  classes:  Hy- 
draulic limes,  natural  cements,  Portland  cements,  Puzzolan  cements 
and  Collos  cement.  These  four  classes  differ  in  regard  to  the  raw 
materials  used,  method  of  manufacture  and  properties  of  the  finished 
product. 

Hydraulic  Limes 

These  are  formed  by  burning  a  siliceous  or  argillaceous  limestone 
to  a  temperature  not  much  above  that  of  decarbonation.  Owing  to 
the  high  percentage  of  calcium  carbonate  in  the  rock,  considerable  free 
lime  appears  in  the  finished  product. 

The  burned  product,  therefore,  not  only  has  hydraulic  properties, 
but  it  will  also  slake  on  the  addition  of  water.  As  a  result  of  the  latter 
property  it  is  self-pulverizing,  because  the  swelling  incident  to  slaking 
disintegrates  the  mass. 


496 


ENGINEERING  GEOLOGY 


The  following  analyses  give  the  composition  of  some  limestones 
used  for  making  hydraulic  lime. 

ANALYSES  OF  HYDRAULIC  LIMESTONE 


I. 

II. 

III. 

IV. 

SiO2 

14.30 

11.03 

7.60 

17.00 

A12O3                                   

0.70 

3.75 

0.75 

1.00 

Fe2O3               

0.80 

5.07 

CaO      

46.50 

43.02 

50.05 

44.80 

MgO  

undet. 

1.34 

0.30 

0.71 

CO2  I  

36.54 

35.27 

H2O  \ 

41.30 

35.99 

I.  Teil,  France.    11.   Hausbergen,  Germany.    III.  Malain,  France.    IV.  Senonches,  France. 

In  the  best  types  of  hydraulic  limestones,  silica  varies  between  13 
and  17  per  cent,  while  alumina  and  iron  oxide  together  rarely  exceed 
3  per  cent. 

Hydraulic  limes  generally  have  a  yellow  color,  a  specific  gravity  of 
about  2.9,  and  slake  and  set  slowly,  but  have  little  strength  unless 
mixed  with  sand.  They  are  of  little  importance  in  the  United 
States,  although  small  quantities  have  in  the  last  few  years  been 
produced  in  Maryland,  Georgia,  and  New  York.  They  are,  how- 
ever, of  much  importance  in  Europe. 

The  following  table  gives  the  composition  of :  (I)  A  hydraulic  lime- 
stone, (II)  hydraulic  lime,  before  slaking,  and  (III)  hydraulic  lime  after 
slaking. 

ANALYSES  OF  HYDRAULIC  LIME 


I. 

II. 

in. 

SiO2 

13  20 

21  20 

19  08 

CaO 

78  80 

70  92 

CO2.  . 

\     86.80    1 

0  00 

0  00 

H2O 

0  00 

0  00 

10  00 

Grappier  cement  is  a  product  obtained  by  finely  grinding  lumps  of 
underburned  and  overburned  material.  It  may  approximate  Portland 
cement  in  its  properties,  provided  it  contains  enough  lime  silicate. 
Lafarge  cement,  known  also  as  a  "  non-staining  "  cement,  is  of  the 
grappier  type. 

Feebly  hydraulic  limes  include  products  whose  cementation  index 
ranges  between  0.30  and  0.70.  They  contain  considerable  free  lime 
and  are  of  low  strength,  but  are  used  in  England. 


LIMES,   CEMENT  AND  PLASTER 


497 


They  also  form  the  basis  of  Selenitic  lime  or  Scott's  cement,  which  is 
made  of  a  mixture  of  hydraulic  lime  and  plaster  of  Paris.  These 
cements  show  a  higher  strength  than  the  hydraulic  lime  proper. 

Natural  Cements 

These  are  made  from  a  clayey  limestone  containing  from  15  to  40 
per  cent  of  clayey  impurities,  by  burning  it  at  a  temperature  of  dull 
redness,  or  just  high  enough  to  cause  some  incipient  fusion. 

They  show  a  variable  and  sometimes  high  percentage  of  magnesia, 
which  is  not  considered  injurious  as  in  Portland  cement  materials. 

The  following  analyses  give  the  composition  of  natural  cement 
rocks  from  a  number  of  localities. 

ANALYSES  OF  NATURAL  CEMENT  ROCKS 


I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

IX. 

SiO2... 

14.15 

15.21 

21.80 

24.74 

12.14 

18.52 

10.66 

18.34 

17.56 

A1,O3... 

6.37 

4.07 

3.70 

16.74 

4.62 

6.34 

4  35  I 

(    1  41 

Fek>3 

2  35 

1  44 

3  10 

6  30 

1  84 

2  63 

1  47  } 

7.49 

I    3  03 

CaD 

26  32 

33  99 

35  00 

23  41 

22  66 

25  31 

27  20 

37  60 

25  50 

MgO 

12.10 

7.57 

3.50 

4.09 

16  84 

12  13 

16  77 

1  38 

15  45 

NajO,  KjO 

0  18 

6  18 

3  52 

undet. 

SO, 

1  81 

2  22 

0  13 

0  90 

C02.  .  . 

34.70 

35.03 

33.00 

22.90 

39.07 

33.31 

38  81 

31  06 

37  05 

H26 

2  03 

undet. 

1  53 

3  94 

Cementation  index 

1  11 

1  68 

3  15 

0  88 

1  43 

0  71 

1  49 

I.  Utica,  111.;  II.  Louisville,  Ky.,  district;  III.  Fort  Scott,  Ka3.;  IV.  Cumberland,  Md.;  V.  Man- 
kato,  Minn.;  VI.  Roaendale,  N.  Y.;  VII.  Central  New  York;  VIII.  Coplay,  Pa.;  IX.  Milwaukee,  Wia. 

Natural  cements  then  are  made  from  the  natural  rock.  After 
burning  and  grinding  they  are  usually  yellow  to  brown  in  color,  and 
have  a  specific  gravity  of  2.7  to  3.1.  They  set  rapidly  and  do  not 
develop  as  high  a  tensile  strength  as  the  Portlands. 

Argillaceous  limestones  suited  to  natural-cement  manufacture  are 
widely  distributed,  and  occur  interstratified  with  other  clayey  and 
calcareous  rocks  which  may  have  no  hydraulic  value.  Owing  to  the 
low  price  of  natural  cement,  the  material  must  be  exceptionally  well 
located  to  be  workable,  and  such  deposits  are  few  in  number.  But 
aside  from  this  the  consumption  has  been  decreasing  in  recent  years, 
because  Portland  cement  is  regarded  as  more  desirable. 

Portland  Cement 

Portland  cement  is  the  product  obtained  by  burning  to  incipient 
fusion  a  finely-ground  artificial  mixture,  consisting  essentially  of  lime, 
silica,  alumina  and  some  iron  oxide,  these  substances  being  present  in 
definite  proportions.  The  finely-ground  burned  product  is  the  cement. 

The  combinations  of  raw  materials  used  in  the  United  States  are: 


498 


ENGINEERING  GEOLOGY 


Marl  (bog-lime)  and  clay;  limestone  and  clay  or  shale;  chalk  and  clay; 
high-calcium  limestone  and  argillaceous  limestone  (natural-cement 
rock). 

The  ratio  of  lime  to  silica,  alumina  and  iron  oxide  combined  in  the 
finished  cement  will  be  not  less  than  1.6  to  1  or  more  than  2.3  to  1 
(Eckel). 

Raw  materials  used.  —  Clay.  —  The  cJay  (or  shale)  used  in  Portland- 
cement  manufacture  is  usually  of  the  transported  type  and,  therefore, 
often  somewhat  impure.  It  should  be  as  free  as  possible  from  gravel 
and  sand,  calcareous  fragments,  or  gypsum  and  pyrite  nodules.  The 
silica  should  be  not  less  than  55  per  cent,  and  preferably  from  60  to  70 
per  cent.  The  ratio  of  (A12O3  +  Fe2O3)  to  SiO2  should  be  about  1  :  3. 
Magnesia  and  alkalies  should  not  exceed  3  per  cent  if  possible. 

The  clays  employed  are  either  non-calcareous  or  calcareous.  Fire 
clays  are  undesirable,  because:  (1)  On  account  of  their  high-alumina 
content  they  produce  a  very  quick-setting  cement,  and  (2)  on  account 
of  their  low  percentage  of  fluxing  impurities  the  vitrification  tempera- 
ture of  the  clinker  becomes  too  high  for  practical  operating  purposes 
(Bleininger).  Number  2  fire  clays1  can  be  used  if  no  others  are  avail- 
able. 

The  hardness  of  the  clay  affects  the  expense  of  grinding,  and  its 
texture  and  uniformity  affect  the  uniformity  of  the  mixture. 

White-burning  residual  clays  are  sometimes  used  for  making  white 
cement,  but  slate  is  rarely  employed  in  Portland  cement  manufacture. 

The  following  analyses  give  the  composition  of  some  of  the  argilla- 
ceous materials  employed. 

ANALYSES  OF  CLAYS  USED  IN  PORTLAND  CEMENT  MANUFACTURE 


I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

SiO2  
A12O3 

53.21 
15  91 

74.29 
12  06 

61.92 
16.58 

55.27 
10.20 

39.23 
12.13 

61.15 
18.47 

58.20 
18.83 

Fe2O3  
CaO  .  . 

7.25 
1.89 

4.92 
0.41 

7.84 
2.01 

3.40 
9.12 

2.79 
21.61 

5.05 
0.98 

5.78 
4.35 

MgO 

0  99 

0  68 

1  58 

5.73 

2.69 

2.26 

3.51 

Na^O,  K2O  

SO3 

2.21 
0.97 

2.56 
undet. 

3.64 
tr. 

undet. 
undet. 

1.69 
undet. 

undet. 
0.91 

3.20 
0.49* 

C02 

undet. 

undet. 

undet.  ) 

(      0.60 

H2O  

17.21 

undet. 

undet. 

undet.  ) 

19.84 

7.02 

{      4.07 

so,-* 

99.64 
2  29 

94.92 
4  39 

93.57 
2  53 

83.72 
4  06 

99.98 
2  63 

93.84 
2  6 

99.03 
2  36 

A12O3  +  Fe2O3  

I.  Pacific  Portland  Cement  Co.,  Suisun,  Calif.,  (Eckel);  II.  Bedford,  Ind.;  III.  Smith's  Landing, 
N.  Y.  (Eckel);  IV.  Syracuse,  Ind.;  V.  Owen  Sound,  Ontario,  Can.;  1  to  5.  Are  clays;  VI.  Shale, 
Coldwater,  Mich;  VII.  Slate,  Rockmart,  Ga. 

*  Sulphur. 

1  Those  fusing  about  cone  27,  but  still  the  term  is  rather  loosely  used. 


LIMES,   CEMENT  AND  PLASTER 


499 


Limestone.  —  The  limestones  used  in  Portland  cement  manufacture 
vary  in  hardness,  texture  and  chemical  composition. 

The  following  tabulation  is  given  by  Eckel  to  show  the  variation  in 
lime  rocks,  and  their  possible  gradation  into  clay  rocks. 


Material. 

Hard. 

Soft. 

Unconsolidated  . 

Calcareous  (CaCO3  over 
75  per  cent) 

Argillo-calcareous 
(CaCO3,  40  to  75  per 
cent) 

Argillaceous  (CaCO3,  less 
than  40  per  cent) 

Pure  hard  limestone 

Hard  clayey  limestone 
(cement  rock) 

Slate 

Pure,  soft,  limestone 
or  chalk 

Soft  limestone  or  clayey 
chalk 

Shale 

Pure  bog-lime  (incorrectly 
termed  marl) 

Marl  (often  called  clayy 
marl) 

Clay 

It  will  be  seen  from  the  above  that  no  hard  line  of  separation  exists 
between  adjoining  members,  and  those  of  the  lowest  division  are  given 
to  show  the  possible  transition  of  the  lime  rocks  into  the  clay  rocks. 

All  of  the  lime  rocks  are  comparatively  fine-grained  in  texture, 
except  some  of  the  fossiliferous  chalks  and  hard  limestones,  and  some 
of  the  crystalline  limestones. 

The  substances  which  may  be  regarded  as  undesirable  impurities 
either  under  all  or  certain  conditions  are  magnesia,  silica,  iron,  alkalies 
and  sulphur. 

Magnesia  is  regarded  by  many  as  an  inert  or  harmful  constituent, 
and  should  be  so  low,  that  the  MgO  content  of  the  finished  cement 
will  not  exceed  3  per  cent.  Since  the  magnesia  hydrates  more  slowly 
than  the  lime,  dolomitic  Portlands  show  two  periods  of  hydration. 

Silica,  if  present  in  a  finely-divided  form,  either  free  or  combined, 
and  in  the  proper  quantity  to  bring  the  silica,  alumina-iron  ratio  within 
the  proper  limits,  does  no  harm.  If,  however,  it  is  in  the  form  of  chert 
concretions  (Plate  XIII,  Fig.  2),  the  silica  does  not  flux  easily  with  the 
lime,  and  such  limestones  should  be  avoided.  Coarse  grains  of 
silicates  such  as  are  found  in  marbles  are  likewise  undesirable. 

Iron  in  the  form  of  pyrite  should  be  avoided  if  present  in  amounts 
of  over  2  or  3  per  cent. 

The  sulphur  may  also  be  combined  with  calcium  in  the  form  of 
gypsum.  In  either  case  over  1J  per  cent  of  sulphur  is  not  wanted. 

Sulphur  compounds  are  undesirable  for  two  reasons,  viz. :  (1)  They 
form  compounds  of  lower  oxidation,  which  will  on  hydration  of  the 
cement,  oxidize  to  sulphates  with  an  increase  in  volume;  (2)  if  oxid- 
ized in  the  kiln  the  sulphur  may  take  lime  away  from  the  silica. 

It  is  important  to  remember  that  the  composition  of  a  limestone 


500  ENGINEERING  GEOLOGY 

cannot  be  judged  from  its  appearance,  and  before  utilizing  a  deposit 
of  lime  rock  for  Portland  cement  manufacture  careful  analysis  should 
be  made  of  the  fresh  rock  from  the  different  beds  in  the  deposit. 
Before  opening  a  quarry  the  areal  extent  of  the  beds  should  be  deter- 
mined and  should  be  carefully  sampled  at  close  intervals  for  analysis, 
especially  in  regions  like  the  eastern  Great  Valley,  where  beds  of 
high-calcium  rocks  grade  frequently  and  rapidly  into  high-magnesium 
beds. 

Marl  deposits  often  contain  irregular  streaks  of  muck  or  peaty 
matter. 

The  cement.  —  Finely-ground  Portland  cement  is  blue  to  gray  in 
color,  and  has  a  specific  gravity  of  from  3  to  3.25.  It  is  stronger 
than  natural  cement  and  sets  more  slowly. 

Calculation  of  Portland  cement  mixture.  —  Given  a  clay  and  lime- 
stone of  known  composition,  we  can  calculate  with  the  aid  of  the  cemen- 
tation index,  the  number  of  parts  of  each  that  will  be  required  as 
follows : 

Operation  I.  —  Multiply  the  percentage  of  silica  in  the  clayey 
material  by  2.8,  the  percentage  of  alumina  by  1.1,  and  the  percentage 
of  iron  oxide  by  0.7.  Add  the  products.  Subtract  from  the  sum  thus 
obtained  the  percentage  of  calcium  oxide  in  the  clayey  material,  plus 
1.4  times  the  percentage  of  magnesia  and  call  the  result  n. 

Operation  II.  —  Multiply  the  percentage  of  silica  in  the  calcareous 
material  by  2.8,  the  percentage  of  alumina  by  1.1,  and  the  percentage 
of  iron  oxide  by  0.7.  Add  the  products  and  subtract  the  sum  from  the 
percentage  of  calcium  oxide  plus  1.4  times  the  percentage  of  magnesia 
in  the  calcareous  material,  calling  the  result  m. 

7?  * 

Operation  III.  —    -  =  parts  of  limestone  to  be  used  for  each  part 

of  clay  by  weight. 

For  safety  the  amount  of  limestone  required  should  be  reduced 
by  about  10  per  cent. 

Burning  changes  in  cements.  —  Natural-cement  rock  is  burned  in 
a  vertical  kiln,  similar  to  that  used  for  burning  lime.  The  chemically- 
combined  water  passes  off  at  about  500°  or  600°  C. ;  the  carbon  dioxide 
about  800°  or  900°  C.,  or  if  magnesium  carbonate  is  present,  some 
decarbonation  occurs  at  a  lower  temperature.  Combination  between 
the  lime  or  magnesia  and  clayey  impurities  probably  begins  as  low  as 
1000°  C. 

Portland  cement  mixtures  are  now  usually  burned  in  rotary  kilns, 
at  a  much  higher  temperature  than  natural  cement. 


LIMES,   CEMENT  AND  PLASTER  501 

The  chemical  changes  are  complex,  and  probably  only  partly  under- 
stood. 

There  have  been  several  views  expressed  regarding  the  cause  of 
setting.  A  plausible  theory  is  that  the  basic  calcium  silicate  is  de- 
composed, setting  free  lime  hydrate,  forming  possibly  a  monocalcium 
silicate  and  some  colloidal  products. 

Economic  considerations.  —  In  determining  the  value  of  a  deposit 
for  Portland  cement  manufacture,  a  number  of  factors  have  to  be 
considered,  such  as:  (1)  Chemical  composition  of  the  material;  (2) 
physical  characters;  (3)  quantity  of  rock  available;  and  (4)  location 
of  deposit  with  respect  to  (a)  transportation  routes,  (6)  fuel  supplies, 
and  (c)  market.  The  first  of  these  has  already  been  referred  to.  The 
second  affects  the  cost  of  quarrying  and  crushing. 

With  regard  to  the  third,  it  has  been  calculated  that  a  plant  running 
on  dry  material,  such  as  limestone  and  shale,  will  use  approximately 
20,000  tons  of  raw  material  per  year  per  kiln.  Of  this  about  15,000 
tons  are  limestone  and  5000  tons  are  shale  or  their  equivalents  (bog 
lime  and  clay,  etc.).  If  the  limestone  is  taken  at  160  pounds  per  cubic 
foot,  one  kiln  will  require  about  190,000  cubic  feet  of  limestone  per  year. 
Chalk  may  run  as  low  as  110  pounds  per  cubic  foot.  Eckel  states 
that  a  cubic  yard  of  bog  lime  in  the  lake  yields  900  pounds  of  dry  bog 
lime. 

Assuming  the  clay  to  run  about  125  pounds  per  cubic  foot  dry,  each 
kiln  will  take  about  80,000  cubic  feet  per  year.  Shale  weighs  about 
140  pounds  per  cubic  foot. 

Eckel  states  that  for  each  kiln  of  a  proposed  plant  there  should  be 
in  sight  at  least  3,800,000  cubic  feet  of  limestone  and  1,600,000  cubic 
feet  of  clay  or  shale  (Ref.  1). 

Puzzolan  Cements 

This  term  in  its  broadest  sense  includes  all  natural  or  artificial 
materials,  which  when  mixed  with  lime,  yield  a  hydraulic  cement 
without  the  aid  of  heat.  The  most  important  type  made  from  natural 
materials  is  a  mixture  of  volcanic  ash  and  lime.  An  important  type 
made  from  artificial  materials,  and  of  greater  importance  commercially 
is  the  slag  cement,  which  consists  of  a  mixture  of  blast-furnace  slag  and 
lime,  both  of  which  are  finely  pulverized  before,  during  and  after  the 
mixing.  There  are  several  factories  in  the  United  States  making  slag 
cement,  but  none  making  the  natural  puzzolan. 

The  following  table  gives  the  composition  of  some  volcanic  ash 
deposits  and  slag  used  in  this  type  of  cement. 


PLATE  LXXXIV,  FIG.  1.  —  Quarry  in  natural  cement  rock,  Milwaukee,  Wis. 

(H.  Hies,  photo.) 


FIG.  2.  —  Shell  marl  outcrop  along  James  River,  Va.     Used  in  Portland  cement 

manufacture.     (T.  L.  Watson,  photo.) 
(502) 


LIMES,   CEMENT  AND  PLASTER 
ANALYSES  OF  VOLCANIC  ASH  AND  SLAG 


503 


£ 

II. 

III. 

IV. 

V. 

VI. 

SiO2 

44  5 

56.31 

47.9 

46.25 

51.08 

34  30 

A12O3 

15.0 

15.23  I 

(20.71 

16.30 

14.76 

Fe2O3 

12.0 

7.11  ( 

34.2 

1    5.48 

11.13 

CaO 

8.8 

1.74 

8.2 

2.15 

5.46 

48.11 

MgO  

4.7 

1.36 

3.9 

1.00 

1.50 

2.66 

K2O 

1  4 

6.54  ) 

Na2O 

4.0 

2.84  J 

2.6 

6.30 

6.21 

H2O 

9.2 

6.12 

3.2 

9.25 

7.64 

I.  Puzzolana,    Civita  Vecchia,  Italy;    II.  Tuff,    Monte  Nuova;    III.  Puzzolana,    Auvergne  Mountains, 
France;    IV.  Trass,  Rhine  district,  Germany;    V.  Average  of  31  analyses  of  Puzzolanic  material; 
CaO 


VI.  Slag,  Chicago,  111., 


SiO2 


Slag  cements  differ  from  Portland  cements  in  their  lighter  color 
(bluish-white  to  lilac),  lower  specific  gravity  (2.7-2.9),  and  slower  set. 
They  do  not  always  show  sufficient  strength  to  pass  the  Portland 
specifications.  They  are  also  noticeably  deficient  in  abrasive  resistance. 

The  production  of  slag  cement  in  the  United  States  has  shown  a 
falling  off  in  recent  years. 

Cement  tests.  —  The  tests  which  are  usually  applied  to  natural, 
Portland  and  slag  cements  are  those  to  determine:  (1)  Fineness;  (2) 
specific  gravity;  (3)  soundness;  (4)  time  of  setting;  and  (5)  tensile 
strength  both  alone  (neat)  and  mixed  with  sand  (mortar.) 

Collos  Cement 

The  manufacture  of  this  type  of  cement  by  a  patented  process  has 
recently  begun  in  the  United  States.  It  is  made  by  pouring  slag  of 
the  proper  composition  on  a  rapidly-revolving  corrugated  cylinder, 
which  scatters  it  in  fine  particles.  The  molten  particles  are  sprayed 
with  a  weak  solution  of  one  or  more  of  the  soluble  salts  of  the  alkaline 
earths,  magnesium  sulphate  being  generally  used.  The  material  is 
then  ground.  Collos  cement,  differs  from  both  Portland  and  puzzolan 
cements.  Its  gravity  is  about  2.88  and  it  is  non-staining. 


Cementation  Index 

The  cementation  index  is  a  formula  used  to  express  quantitatively 
the  relation  between  the  composition  and  hydraulic  value  of  a  cem- 
enting material.  It  cannot,  however,  be  employed  as  the  sole  basis 
of  classification,  since  the  properties  of  a  cement  depend  both  on  its 


504  ENGINEERING  GEOLOGY 

composition  and  conditions  of  manufacture,  such  as  temperature  of 
burning. 

The  formula  for  calculating  the  cementation  index  is  as  follows: 

(2.8X  per  cent  SiO2)  +  (1.1  X  per  cent  A1203)  +  (0.7  X  per  cent  Fe203) 
(per  cent  CaO)  +  (1.4  X  per  cent  MgO) 

The  cementation  indices  for  the  several  classes  of  cements  are  as 
follows : 

Eminently  hydraulic  limes    0.70-1.10 
Feebly  hydraulic  limes  0.30-0.70 

1.-1.15  Natural  Portlands 


Natural  cements  1.00-2.00 


Portland  cement  1.00-1.20 


1.15-1.6     Most    U.    S.    & 

Roman 
1.66-2.0  Low  lime 


It  will  be  seen  from  this  that  the  several  classes  may  overlap  some- 
what. 

Distribution  of  Lime  and  Cement  Materials  in  the  United  States 

Limestone  for  lime.  —  Limestones  of  suitable  composition  for  mak- 
ing lime  are  so  widely  distributed  that  no  particular  regions  or 
states  require  special  mention.  A  glance  at  the  map  showing  distri- 
bution of  limestones  (Plate  LXXIX)  will  emphasize  this  point. 

Natural  cement  rocks.  —  Argillaceous  limestones  suitable  for  nat- 
ural cement  are  found  at  a  number  of  points.  In  some  districts  only 
one  bed  of  cement  rock  is  present,  in  others  two  or  three.  The  rock 
worked  in  the  different  districts  does  not  all  come  from  the  same 
geological  formation,  nor  do  the  beds  lie  equally  accessible.  Thus  in 
some  districts  they  are  flat  (Milwaukee),  (Plate  LXXXIV,  Fig.  1)  while 
in  others  they  are  strongly  folded,  with  steep  dips,  and  have  to  be 
worked  by  underground  methods  (Rosendale,  N.  Y.,  and  Cumber- 
land, Md.) 

Among  the  important  districts  may  be  mentioned  those  of  Rosen- 
dale  and  the  Lehigh  Valley  region  in  Pennsylvania;  Akron,  N.  Y.; 
Cumberland,  Md.;  Milwaukee,  Wis.;  Louisville,  Ky.;  and  Utica,  111. 

Portland  cement  materials.  —  Clay  and  limestone  in  one  form  or 
another  are  so  widely  distributed  in  the  United  States  that  Portland 
cement  manufacture  would  be  possible  at  many  localities.  Economic 
conditions,  however,  render  it  in  many  cases  impracticable,  even 
though  suitable  raw  materials  are  present. 


LIMES,   CEMENT  AND  PLASTER  505 

The  most  important  region  at  present  lies  in  the  Lehigh  Valley 
district  of  eastern  Pennsylvania,  where  a  mixture  of  cement  rock  and 
high-grade  limestone  is  used.  In  the  central  states,  Ohio,  Michigan, 
and  Indiana,  and  even  parts  of  New  York,  much  Portland  cement  is 
made  from  a  mixture  of  bog  lime  and  clay.  In  the  Virginia  Coastal 
Plain  a  mixture  of  marl  (Plate  LXXXIV)  and  clay  is  used.  The  scat- 
tered plants  in  other  states  run  chiefly  on  limestone  and  clay  or  shale. 
The  map  (Plate  LXXXV)  shows  the  distribution  of  cement  plants. 

Puzzolan  cement  materials.  —  Deposits  of  volcanic  ash  are  abun- 
dant in  many  western  states,  but  the  material  is  not  utilized  for  cement 
manufacture.  In  the  construction  of  the  Los  Angeles  aqueduct  the 
experiment  was  successfully  tried  of  mixing  Portland  cement  and 
volcanic  ash. 

Production  of  cement  materials.  —  Brief  reference  to  the  production 
of  Portland  and  natural  cement  shows  the  relative  importance  of  these 
materials  in  engineering  and  general  work  of  construction.  The  Port- 
land cement  output  has  increased  from  42,000  barrels  in  1880  valued 
at  $126,000  to  82,438,096  barrels  in  1912  valued  at  $67,022,172.  On 
the  other  hand  the  natural  cement  showed  an  output  of  2,440,000 
barrels  in  1880,  rose  to  a  maximum  of  9,868,179  barrrels  in  1899,  and 
gave  only  821,231  barrels  in  1912. 

And  yet  with  the  phenomenal  increase  of  Portland  cement  there 
has  been  a  more  or  less  steady  drop  in  price  which  brought  it  down 
from  $2.50  in  1881  to  81.3  cents  in  1912.  Natural  cement  was  valued 
at  45  cents  per  barrel  in  1912. 

Gypsum  Plasters 

Gypsum,  the  hydrous  sulphate  of  calcium,  is  widely  used  for  the 
manufacture  of  plaster  of  Paris,  cement  plaster,  wall  plaster;  in  agri- 
culture, as  a  retarder  of  Portland  cement,  and  to  a  lesser  extent  in  other 
industries. 

Properties  and  occurrence.  —  The  properties  of  gypsum  and  its 
varieties  have  been  discussed  in  Chapter  I  on  Minerals,  while  the 
manner  of  its  occurrence  has  been  described  in  Chapter  II  on  Rocks. 
These  facts  need  therefore  not  be  repeated  here. 

Of  the  three  main  types  of  occurrence  there  described,  the  "rock" 
gypsum  is  the  most  important  commercially.  Gypsite  or  gypsum 
earth  is  of  importance  in  Kansas,  and  some  other  states  of  the  Great 
Plains  region,  but  the  gypsum  sands  although  occurring  in  abundance 
in  some  parts  of  New  Mexico  are  not  utilized  at  present. 

Anhydrite  (see  Chapter  I)  is  found  in  small  amounts  in  most  gypsum 


506 


ENGINEERING  GEOLOGY 


og 

I 


LIMES,   CEMENT  AND  PLASTER 


507 


deposits,  but  in  some  it  is  abundant  and  forms  large  masses,  whose 
shape,  size  and  relations  to  the  associated  gypsum  vary.  The  work- 
men usually  recognize  it  readily  by  its  slightly  greater  hardness. 

In  the  gypsum  deposits  of  Virginia,  New  Brunswick  and  Nova 
Scotia,  for  example,  anhydrite  is  especially  abundant. 

If  the  anhydrite  is  more  or  less  intimately  mixed  with  the  gypsum, 
and  is  present  in  large  amounts  the  material  is  not  marketable,  but  if 
it  occurs  in  isolated  masses  or  beds,  it  can  be  left  in  the  quarry,  or 
thrown  out  in  working  the  deposit. 

Anhydrite  on  exposure  to  the  weather  may  change  into  gypsum, 
and  under  conditions  of  extreme  aridity  the  reverse  process  sometimes 
takes  place. 

Chemical  composition.  —  The  following  table  gives  the  composi- 
tion of  gypsum  deposits  from  a  number  of  localities: 

ANALYSES  OF  GYPSUM 


Pure 

gyp- 
sum. 

Dillon, 
Kan. 

Ala- 
baster, 
Mich. 

Grand 
Rapids, 
Mich. 

Salt- 
ville, 
Va. 

3ypsite, 
Vlarlow, 
Okla. 

Gyp- 
site, 
Burns, 
Kan. 

Gyp- 
site, 
Salina, 
Kan. 

Gyp- 
site, 
Dillon, 
Kan. 

CaSO4.. 

79.10 
20.90 

78.40 

19.96 
0.35 
0.12 
0.56 
0.57 

78.51 

20.96 
0.05 
0.08 

"Q'.U 

76.26 

20.84 
tr. 
0.54 
n.d. 
n.d. 

72.06 
21.30 
1.68 
1.95 

59.46 

16.59 
10.67 
0.60 
10.21 
1.10 

67.91 

17.72 
2.31 
0.37 
11.71 
0.52 

34.38 
8.50 
34.35 
4.11 
8.14 
10.52 

56.58 
15.16 
17.10 
2.04 
7.71 
1.24 

H2O  

SiO2 

A12O3  and  Fe2O3 

CaCO3 

MgCOa  

100.00 

99.96 

99.71 

97.64 

96.99 

1 

98.63 

100.53 

100.00 

99.83 

Onondaga, 
NTYT 

Fort  Dodge, 
la. 

Sandusky,  O. 

CaSO4 

|     73.92   | 

>       4.64 
21.44 

73.44 

20.76 
0.65 

78.73 
19.70 

JO.  91 
(0.60 

H2O 

SiO2                                               

A12O3                                   

Fe2O3                            

CaCO3           

MgO 

0.54 

Chemistry  of  gypsum-calcination.  —  When  pure  gypsum  is  heated 
to  a  temperature  of  between  250°  F.  and  400°  F.  it  loses  about  three- 
fourths  of  its  water  of  combination  and  the  calcined  product  is  known 
as  plaster  of  Paris,  which  when  mixed  with  water,  takes  up  in  chemical 
combination  as  much  as  it  lost,  and  sets  to  a  hard  mass. 


508 


ENGINEERING  GEOLOGY 


Kinds  of  plaster.  —  If  the  gypsum  contains  a  considerable  quan- 
tity of  impurities,  the  latter  retard  the  setting,  and  such  slow-setting 
plasters  are  termed  cement-plasters.  They  are  of  value  for  structural 
work.  Gypsum  calcined  above  400°  F.  is  termed  dead-burnt  plaster, 
because  it  appears  to  have  no  setting  properties,  but  if  it  is  heated 
to  about  900°  F.,  and  finely  ground,  it  sets,  with  great  slowness  to  a 
hard  product  known  as  flooring  plaster  (German  estrich-gyps) .  Stucco 
is  another  name  for  plaster  of  Paris.  Keene's  cement  is  a  product  ob- 
tained by  calcining  pure  gypsum  at  a  red  heat,  immersing  it  in  an 
alum  bath,  drying  and  calcining  again. 

Mack's  cement  is  a  dehydrated  gypsum  which  is  mixed  with  0.4  per 
cent  of  sodium  sulphate  or  potassium  sulphate.  It  forms  a  quick, 
hard  and  durable  set,  and  is  used  for  flooring,  or  for  covering  wire 
mesh  on  walls  or  ceilings. 

Neat  or  pure  plaster  may  develop  a  strength  of  over  400  Ibs.  per 
sq.  in.  at  the  end  of  4  weeks,  while  one  of  plaster,  to  one,  two  and  three 
of  sand,  gave  respectively  about  350,  200  and  130  Ibs.  per  sq.  in.  for 
the  same  period. 


FIG.  198.  —  Map  showing  distribution  of  gypsum  in  the  United  States.     (From 
Ries'  Economic  Geology.) 

Distribution  of  gypsum.  —  Rock  gypsum  is  quarried  in  a  number 
of  states,  but  New  York,  Virginia,  Ohio,  Michigan,  Kansas  and  Iowa 
are  important  producers. 


LIMES,   CEMENT  AND  PLASTER  509 

The  deposits  are  not  restricted  to  any  one  geological  horizon,  but 
in  the  United  States  range  from  Silurian  to  Tertiary  in  age. 

Gypsite  is  dug  in  some  quantity  in  Kansas,  as  well  as  in  Wyom- 
ing, Oklahoma  and  Texas. 

The  general  distribution  of  gypsum  in  the  United  States  is  shown 
on  the  map  (Fig.  198). 

Much  high-grade  gypsum  is  exported  from  Nova  Scotia  and  New 
Brunswick  to  the  United  States. 


References  on  Limes  and  Calcareous  Cements 

General.  —  1.  Eckel,  Cements,  Limes  and  Plasters,  New  York,  1907 
(Wiley  &  Sons).— 2.  Humphrey,  U.  S.  Geol.  Surv.,  Bulls.,  331  and 
344  (Tests) .  — 3.  Eno,  Ohio  Geol.  Surv.,  4th  ser.,  Bull.,  2, 1904  (Uses).— 
4.  Bleininger,  Ibid.,  Bull.,  3,  1904  (Manufacture). 

Areal.  —  Eckel  and  others,  U.  S.  Geol.  Surv.,  Bull.,  522,  1913,  con- 
tains summary  of  most  of  the  literature  on  limestones  and  cement 
materials  of  the  United  States. 

Many  State  Geological  Surveys  have  issued  special  reports  dealing 
with  lime  and  cement  materials,  among  them:  Alabama,  Georgia, 
Illinois,  Indiana,  Iowa,  Maryland,  Michigan,  New  Jersey,  Ohio, 
Pennsylvania,  South  Dakota,  Tennessee,  Virginia  and  West  Virginia. 

References  on  Gypsum 

1.  Adams  and  others,  U.  S.  Geol.  Surv.,  Bull.,  223,  1904  (United 
States).  —  2.  Eckel,  Cements,  Limes  and  Plaster,  New  York,  1907. 
(Wiley  &  Sons).  The  Geological  Surveys  of  Iowa,  Kansas.  Michigan, 
New  York  and  Virginia  have  published  special  papers  or  reports  on 
their  gypsum  deposits,  and  some  of  these  contain  considerable  matter 
on  the  technology  of  gypsum  plasters. 


CHAPTER  XIII 
CLAY  AND  CLAY  PRODUCTS 

As  a  definition  of  this  material  (clay)  has  been  given  (p.  94),  there 
is  no  need  of  repeating  it  here.  Its  commercial  value  depends 
primarily  upon  the  fact  that  it  possesses  two  very  important  physical 
.properties.  These  are:  (1)  Plasticity,  by  virtue  of  which  it  forms  a 
pasty  mass  when  wet,  thus  permitting  it  to  be  molded  into  a  diversity 
of  shapes,  which  it  retains  when  dry;  and  (2)  hardening  under  fire, 
which  operates  to  make  the  given  form  permanent. 

A  number  of  different  products  are  made  from  clay,  some  of  which 
are  of  importance  to  the  engineer.  These  include  building  and  paving 
brick,  sewer  pipe,  railroad  ballast,  road  metal  and  puddle.  Clay  is 
also  an  essential  ingredient  of  Portland  cement,  but  this  is  taken  up 
in  Chapter  XII. 

Properties  of  Clay 

These  are  of  two  kinds,  physical  and  chemical  and  since  they  exercise 
an  important  influence  on  the  behavior  of  the  clay,  and  indirectly  its 
uses,  they  will  be  described,  remarking  in  advance,  however,  that  the 
physical  properties  are  the  more  important. 

Physical  Properties 

These  include  plasticity,  tensile  strength,  air  and  fire  shrinkage, 
fusibility  and  specific  gravity. 

Plasticity.  —  This,  as  defined  above,  is  an  exceedingly  important 
property,  and  clays  vary  from  exceedingly  plastic  or  "  fat  "  ones,  to 
those  of  low  plasticity  which  are  termed  "  lean,"  and  are  often  sandy. 
The  plasticity  affects  the  behavior  of  the  clay  in  molding.  Some  clays 
are  very  sticky  and  hard  to  mix,  and  such  may  also  on  account  of  their 
high  plasticity  work  badly  in  certain  types  of  brick-molding  machines. 
Deficient  plasticity  is  also  bad,  and  may  cause  the  clay  to  tear  in  the 
molding  process.  Manufacturers  of  clay  products  often  use  a  mixture 
of  two  clays  or  of  clay  and  sand,  in  order  to  get  a  mass  of  the  proper 
consistency. 

The  amount  of  water  required  to  work  up  a  clay  to  its  maximum 
plasticity  varies.  In  lean  clays  it  may  not  be  more  than  15  per  cent, 

510 


CLAY  AND  CLAY  PRODUCTS  511 

while  in  very  plastic  ones  it  often  rises  to  30  or  35  per  cent.  This  water 
must  be  eliminated  in  drying. 

Tensile  strength.  —  This  is  the  resistance  which  a  mass  of  air-dried 
clay  offers  to  rupture.  Tests  show  that  it  varies  from  15  or  20  pounds 
per  square  inch  up  to  600  pounds  per  square  inch,  but  many  common- 
brick  clays  have  a  tensile  strength  of  from  100  to  200  pounds.  The 
practical  importance  of  the  tensile  strength  is  that  it  enables  the  clay 
to  withstand  the  shocks  and  strains  of  handling  during  manufacture 
and  before  it  is  burned.  It  does  not  stand  in  any  direct  relation  to 
plasticity,  nor  to  a  clay's  tendency  to  crack  in  air  drying. 

Shrinkage  is  of  two  kinds  —  air  shrinkage  and  fire  shrinkage.  The 
former  takes  place  while  the  clay  is  drying  after  being  molded,  and  is 
due  to  the  evaporation  of  the  water,  and  the  gathering  together  and 
shrinkage  of  the  clay  particles.  The  latter  occurs  during  firing,  and 
is  due  to  a  compacting  of  the  mass  as  the  particles  soften  and  fuse 
together  under  the  action  of  heat.  Both  are  variable. 

In  the  manufacture  of  most  clay  products  an  average  total  shrinkage  of  about  8 
or  9  per  cent  is  commonly  desired,  and  excessive  shrinkage  is  likely  to  cause  cracking 
or  warping  of  the  product.  The  shrinkage  may  be  reduced  by  the  addition  of  sand, 
or  ground  brick.  A  mixture  of  clays  sometimes  produces  the  desired  effect. 

Since  clays  show  a  variable  shrinkage,  the  size  of  brick  made  from  different  ones 
will  not  be  the  same.  Even  hi  the  same  kiln  of  bricks,  however,  a  difference  in  size 
is  sometimes  observable,  because  those  which  are  harder-burned  have  shrunk  more. 

Fusibility.  —  This  is  one  of  the  most  important  properties  of  clay. 
When  subjected  to  a  rising  temperature,  clays  soften  slowly  and  hence 
fusion  takes  place  gradually.  Indeed,  it  is  possible  to  recognize  three 
stages,  which  may  be  termed  respectively,  incipient  fusion,  vitrifica- 
tion and  viscosity.  It  is  somewhat  difficult  at  times  to  exactly  locate 
each  of  these,  so  gradual  is  the  change,  but  the  recognition  of  them  is 
of  considerable  practical  importance.  They  might  be  defined  some- 
what as  follows: 

Incipient  fusion  is  the  point  at  which  the  clay  grains  have  become 
sufficiently  soft  in  part  at  least  to  make  the  mass  stick  together.  The 
clay  body  is  still  very  porous  and  can  be  scratched  with  a  knife,  and  it 
is  not,  therefore,  "  steel  hard." 

Vitrification  represents  a  further  degree  of  heating,  sufficient  to 
cause  enough  softening  of  the  grains,  and  fluxing  between  them  to 
weld  the  whole  together  into  a  dense,  practically  non-absorbent  mass. 
The  clay  body  still  holds  its  shape,  however. 

Viscosity  is  the  stage  at  which  the  clay  has  become  so  soft  due  to 
extensive  fluxing,  that  it  no  longer  holds  its  shape. 


11 


- 


CLAY  AND  CLAY  PRODUCTS  513 

Comparison  of  different  clays  shows  us:  (1)  That  the  temperature 
of  incipient  fusion  is  not  the  same  in  all.  In  the  lower  grades  of  clay, 
that  is  in  those  having  a  high  percentage  of  fluxing  impurities  it  may 
begin  about  1000°  C.,  while  in  refractory  clays  it  may  not  occur  until 
a  considerably  higher  temperature  is  attained;  (2)  the  three  stages 
of  fusion  are  not  equi-spaced,  nor  is  the  temperature  interval  between 
the  first  and  third,  the  same  in  all  clays.  Thus  in  calcareous  clays 
the  temperature  interval  between  the  extreme  points,  is  very  small, 
possibly  not  more  than  50°  C.,  while  in  others  it  may  be  quite  large. 

The  practical  bearing  of  these  facts  is  this:  In  burning  a  kiln  full  of  ware,  say  one 
containing  100,000  brick,  it  is  impossible  to  control  the  temperature  within  a  few 
degrees,  so  that  if  the  ware  is  to  be  vitrified  we  must  have  a  sufficiently  large  tem- 
perature interval  between  vitrification  and  viscosity,  to  permit  reaching  the  former 
point  without  danger  of  running  on  to  the  latter,  and  melting  down  the  entire  con- 
tents of  the  kiln. 

The  approximate  extent  of  fusion  is  often  indicated  by  the  absorption.  Common 
brick,  which  are  usually  burned  to  incipient  fusion  or  a  little  beyond,  show  an  ab- 
sorption of  10  to  25  per  cent,  while  paving  brick  which  are  vitrified  or  nearly  so  have 
a  very  low  absorption. 

If  a  brick,  therefore,  is  exposed  to  rising  temperature  its  fire  shrinkage  and  den- 
sity reach  a  maximum  at  vitrification,  beyond  which  it  begins  to  swell,  and  even 
gets  more  porous  due  to  the  development  of  a  vesicular  structure.  The  color  also 
deepens  with  increasing  temperature. 

Color.  —  Raw  or  unburned  clays  are  white  if  free  from  iron  or 
carbonaceous  matter.  They  are  often  colored  yellow,  brown,  red  or 
even  green  by  iron  oxides,  and  gray  or  black  by  carbonaceous  matter. 

Burned  clays  are  white  if  free  from  iron  oxide,  but  if  the  latter  is 
present  they  will  usually  be  buff  or  red  depending  on  the  quantity 
present  and  the  evenness  of  its  distribution  in  the  clay.  An  excess  of 
lime  over  iron  counteracts  the  latter,  and  a  cream  or  buff  product 
results,  which  turns  greenish  or  yellowish-green  on  vitrification.  The 
carbon,  unless  burned  out,  may  affect  the  color  of  the  burned  ware. 
It  should  be  emphasized  here  that  color  is  not  a  safe  basis  of  comparison 
for  bricks  made  from  different  clays,  though  many  engineers  seem  not 
to  be  aware  of  this  fact. 

Specific  gravity.  —  There  is  some  difference  of  opinion  as  to  the 
method  of  determining  the  specific  gravity  of  a  clay.  Some  believe 
that  it  should  be  determined  in  powdered  form  and  this  may  be  called 
the  true  specific  gravity.  Others  consider  that  it  should  be  deter- 
mined by  coating  a  lump  of  the  clay  with  paraffme,  and  weighing  this 
in  air  and  water;  but  many  term  this  the  apparent  specific  gravity. 
It  is  sometimes  urged  that  the  latter  method  enables  us  to  calculate 


514  ENGINEERING  GEOLOGY 

the  weight  of  the  clay  per  cubic  foot,  but  since  the  water  content  of 
different  clays  is  not  always  the  same,  the  method  can  hardly  be 
considered  accurate.  When  one  is  dealing  with  soft  clays,  125  pounds 
per  cubic  foot  can  be  taken  as  the  approximate  weight,  while  135  to  140 
pounds  should  be  allowed  for  shales. 

Chemical  Properties 

The  number  of  common  elements  which  have  been  found  in  clays 
is  great,  and  even  some  of  the  rarer  ones  have  been  noted;  but  in  most 
clays  the  number  of  elements  present  is  usually  small,  being  commonly 
confined  to  those  determined  in  the  ordinary  chemical  analysis,  which 
shows  their  existence  in  the  clay,  but  not  always  the  state  of  chemical 
combination.  The  common  constituents  of  a  clay  are  silica,  alumina, 
ferric  or  ferrous  oxide,  lime,  magnesia,  alkalies,  titanic  acid  and 
combined  water.  Carbon  dioxide  is  always  found  in  calcareous  clays. 
Carbonaceous  matter  and  sulphur  trioxide  are  usually  present  only 
in  small  amounts. 

The  effect  of  these  ingredients  may  be  briefly  stated  as  follows: 

Silica  is  most  often  present  in  the  form  of  quartz  grains,  but  it  may  also  be  con- 
tained in  grains  of  undecomposed  silicate  minerals.  It  aids  in  lowering  the  plasticity 
and  shrinkage  and  helps  to  increase  the  refractoriness  at  low  temperatures.  A  clay 
high  in  silica  (70  to  80  per  cent)  is  usually  sandy.  Alumina,  which  is  most  abundant 
in  white  clays,  is  a  refractory  ingredient.  Contrary  to  many  statements  which  have 
appeared  in  print  it  stands  in  no  direct  relation  to  the  plasticity.  Iron  oxide,  as 
already  explained,  acts  as  a  coloring  agent.  If  the  clay  is  burned  in  an  oxidizing 
atmosphere  ferric  compounds  are  formed,  but  if  the  kiln  atmosphere  has- a  deficiency 
of  oxygen,  or  if  there  are  other  substances  present  which  have  a  greater  affinity  for 
oxygen,  ferrous  compounds  result.  Lime,  magnesia  and  alkalies  as  well  as  iron 
oxide  are  fluxing  impurities,  which  promote  the  fusion  of  the  clay.  In  a  clay  of  low 
heat  resistance  the  combined  percentage  of  these  fluxes  is  high,  while  in  a  refractory 
or  fire  clay,  it  is  small.  Titanic  acid  though  rarely  exceeding  1  or  2  per  cent,  is  seldom 
absent,  and  acts  as  a  flux  at  high  temperatures.  Vanadium  compounds  are  the 
probable  cause  of  a  greenish-yellow  stain  which  develops  on  some  buff  bricks  after 
they  come  from  the  kiln. 

Chemically  combined  water  and  carbonaceous  matter  pass  off  at  a  temperature 
of  dull  redness,  the  former  between  450°  and  600°  C,  and  the  latter  between  800° 
and  900°  C.  Their  loss  leaves  the  clay  temporarily  porous  until  fire  shrinkage  sets 
in. 

Many  a  brick  made  from  carbonaceous  clay  is  ruined,  simply  because  the  manu- 
facturer does  not  realize  that  the  fire  shrinkage  should  not  be  allowed  to  begin  until 
the  carbon  is  driven  off.  If  allowed  to  remain  in  the  brick  after  it  is  dense,  the  carbon 
robs  the  iron  of  part  of  its  oxygen,  reducing  it  to  ferrous  oxide.  This  unites  readily 
with  the  silica  in  the  clay  forming  an  easily  fusible  ferrous  silicate,  which  colors  the 
center  of  the  brick  bluish-black.  But  as  the  heat  rises  gases  are  evolved  by  the  car- 
bon which  in  their  effort  to  escape  bloat  the  brick.  Sulphur  if  present  in  the  clay, 


CLAY  AND  CLAY  PRODUCTS 


515 


and  not  driven  off  in  burning  is  likewise  a  cause  of  black  coring  and  premature  swell- 
ing. Such  defects  in  a  brick,  therefore,  may  be  due  to  carbonaceous  matter  and  less 
often  sulphur  in  the  clay,  and  improper  burning.  Some  brick-makers  think  black 
coring  is  due  to  setting  the  brick  too  moist,  but  this  is  only  indirectly  so. 

The  following  analyses  show  how  clays  vary  in  their  chemical  com- 
position, but  it  must  be  stated  emphatically  that  a  chemical  analysis 
is  usually  valueless  for  judging  the  commercial  value  of  a  clay,  as 
regards  its  use  for  burned  clay  wares. 

ANALYSES  SHOWING  VARIATION  IN  COMPOSITION  OF  CLAYS 


I. 

II. 

III. 

IV. 

V. 

Silica  (SiOz)  . 

46  3 

45  78 

57  62 

59.92 

68.62 

Alumina  (Al2Oa)  

39.8 

36  46 

24  00 

27.56 

14.98 

Ferric  oxide  (Fe2Os)  

0  28 

1  9 

1.03 

4.16 

Ferrous  oxide  (FeO)  

1.08 

1.2 

Lime  (CaO) 

0  50 

0  7 

tr. 

1  48 

Magnesia  (MgO) 

0  04 

0  3 

tr. 

1  09 

Potash  (K2O) 

0  5 

Soda  (Na2O)  .  . 

|   0.25  | 

0  2 

!•   0.64 

3.36 

Titanic  oxide  (TiO2)  

Water  (H2O)  

13.9 

13.40 

10  5 

9.70 

3.55 

Moisture  

2.05 

2.7 

1.12 

2.78 

Carbon  dioxide  (CO2) 

Sulphur  trioxide  (SO3) 

0  35 

Organic  matter.  .  . 

Manganous  oxide  (MnO) 

0  64 

Total  

100.00 

99  84 

99  97 

99.97 

100.66 

VI. 

VII. 

VIII. 

IX. 

X. 

Silica  (SiO2)  .    .    . 

82  45 

54  64 

38  07 

90  00 

47  92 

Alumina  (Al2Os)  

10  92 

14  62 

9  46 

4  60 

14  40 

Ferric  oxide  (FeaOs)  

1  08 

5  69 

2  70 

1  44 

3  60 

Ferrous  oxide  (FeO)  

Lime  (CaO) 

0  22 

5  16 

15  84 

0  10 

12  30 

Magnesia  (MgO) 

0  96 

2  90 

8  50 

0  10 

1  08 

Potash  (K2O) 

5  89 

2  76 

tr 

1  20 

Soda  (XaoO)      . 

tr 

1  50 

Titanic  oxide  (TiO2)  

1  00 

0  70 

1  22 

Water  (H,O)  

2  4 

3  74 

3  49 

3  04 

4  85 

Moisture  

0  85 

Carbon  dioxide  (CO2)  

4  80 

20  46 

9.50 

Sulphur  trioxide  (SO3)  

1.44 

Organic  matter.  .  . 

1  34 

Manganous  oxide  (MnO)  



0.76 

Total  

99.03 

99  05 

100  28 

99  98 

100  35 

I.  Kaolinite;  II.  Washed  kaolin,  Webster,  N.  C.;  III.  Plastic  fire  clay,  St.  Louis,  Mo.;  IV.  Flint  fire 
clay,  Salineville,  O. ;  V.  Loess  clay,  Guthrie  Center,  la. :  VI.  Pressed-brick  clay,  Rusk,  Tex. ; 
VII.  Brick  shale,  Mason  City,  la.;  VIII.  Calcareous  brick  clay,  Milwaukee,  Wis. ;  IX.  Sandy  brick 
clay,  Colmesneil,  Tex. ;  X.  Blue  clay-shale,  Ferris,  Tex. 


516 


ENGINEERING  GEOLOGY 


Occurrence  of  Clay 


Classification  of  clay  deposits.  —  Two  important  classes  of  clays 
are:  (1)  Residual,  and  (2)  transported. 

Residual  clays.  —  Residual  clays  are  derived  from  many  different 
kinds  of  rocks  by  weathering  processes  (see  Chapter  IV.)  The  deposit 
thus  formed  will  be  found  overlying  the  parent  rock  and  often  grading 
downward  into  it.  From  its  method  of  origin  and  position  it  is  termed 
a  residual  clay. 

Residual  clays  are  formed  from  feldspathic  rocks  by  the  decompo- 
sition of  the  silicates  in  them,  such  as  feldspar,  which  breaks  down  to 
a  clayey  mass. 

They  are  derived  from  shales  by  simple  disintegration  of  the 
mass,  and  from  limestones  by  a  process  of  solution.  In  the  latter 


FIG.  199.  —  Section  showing  passage  of  the  fully-formed  residual  clay  on  the  surface 
into  the  solid  bed  rock  below.  A,  clay;  B,  clay  and  partly-decomposed  rock; 
C,  bed-rock  below,  passing  upward  into  rock  fragments  with  a  little  clay.  (After 
Ries,  Clays,  Occurrence,  Properties  and  Uses.) 

case  the  carbonates  are  dissolved  out,  and  the  residual  clay  represents 
the  clayey  impurities  which  are  left  behind.  In  this  type  the  under- 
lying surface  of  the  limestone  may  be  exceedingly  uneven,  chimneys 
of  the  unaltered  rock  extending  up  into  the  clay.  This  peculiarity 
makes  it  not  only  diffipult  to  estimate  the  tonnage  or  volume  of  such 
a  deposit  without  first  making  an  excessive  number  of  borings,  but 
in  addition  these  irregular  rock  chimneys  often  preclude  the  use  of 
cheap  methods  of  excavation,  such  as  steam  shoveling. 

The  extent  of  a  deposit  of  residual  clay  will  depend  primarily  on 
the  extent  of  the  parent  rock.  Its  depth  will  depend  on  that  to  which 
weathering  processes  have  penetrated  the  rock,  and  upon  the  degree 


CLAY  AND   CLAY  PRODUCTS  517 

to  which  the  land  surface  has  been  worn  down  by  rain  wash.  It  will 
therefore  be  thicker  on  flat  or  gently-sloping  surfaces  than  on  steep 
ones.  Residual  clays  are  moreover  rare  or  absent  in  glaciated  regions. 

The  majority  of  residual  clays  are  colored  by  iron  oxide,  only  those 
derived  from  iron-free  rocks  being  as  a  rule  white. 

Transported  clays.  —  With  the  erosion  of  the  land  surface  the  par- 
ticles of  a  residual  clay  become  washed  away  to  lakes,  seas  or  the 
ocean,  or  other  places,  where  they  settle  down  in  the  quiet  water  as  a 
fine  aluminous  sediment,  forming  deposits  of  sedimentary  clay.  Such 
deposits  are  often  of  great  thickness  and  vast  extent. 

With  the  accumulation  sometimes  of  many  feet  of  other  sediments 
on  top  of  them,  they  become  consolidated  by  pressure  and  some- 
times additionally  by  the  deposit  of  a  cement  around  the  grains. 
Consolidated  clay  is  termed  shale,  and  where  the  consolidation  is  due 
to  pressure  alone  it  breaks  down  easily  when  ground,  and  forms  a 
plastic  mass  when  mixed  with  water. 

Residual  materials  have  in  some  instances  been  transported  by 
glacial  action,  or  even  wind,  to  form  clayey  deposits. 

The  following  are  the  most  important  types  of  transported  clays: 

Marine  clays.  —  Clay  deposits  laid  down  on  the  ocean  bottom. 
Since  their  deposition  they  have  often  been  elevated  to  form  dry  land 
in  all  the  continents,  and  in  many  cases  have  been  consolidated,  but 
elsewhere,  as  in  the  Atlantic  and  Gulf  Coastal  plains,  they  have  re- 
mained unconsolidated. 

Estuarine  clays.  — These  are  formed  in  estuaries  or  arms  of  the 
sea.  The  areas  are  long  and  narrow,  as  in  the  case  of  the  Hudson 
River  Valley  deposits,  and  thin  out  towards  the  valley  walls,  where 
they  rest  on  bed  rock,  glacial  drift  or  other  sediments. 

Floodplain  clays.  —  These  originate  by  the  deposition  of  clayey 
sediment  during  periods  of  flood,  on  the  lowlands  bordering  a  river. 
Such  deposits  are  of  variable  thickness,  sometimes  very  sandy,  or  of 
alternating  layers  of  sand  and  sandy  clay.  They  usually  thin  out 
towards  the  valley  walls. 

Lake  clays.  —  Clay  deposits  in  lakes,  ponds,  and  swamps  are  in- 
cluded under  this  type.  They  vary  from  very  plastic  to  very  sandy 
material.  The  deposits  are  usually  basin-shaped,  of  varying  depth, 
and  are  common  in  many  regions. 

Glacial  clays,  often  called  till  or  boulder  clay,  consist  of  a  mixture 
of  rock  flour  (the  result  of  glacial  grinding)  together  with  residual  and 
transported  clays  eroded  by  glacial  action.  Glacial  clays  are  often 
stony,  tough,  dense,  and  commonly  unstratified.  They  form  a  mantle 


PLATE  LXXXVII,  FIG.  1.  —  Deposit  of  stony  glacial  clay.    (After  Ries,  N.  J. 
Geol.  Survey,  Fin.  Kept.,  VI.,  p.  128.) 


FIG,  2.  —  Stratified  marine  clay,  from  Athens,  Tex.    Shows  gently  dipping  layers. 

(H.  Ries,  photo.) 
(518) 


CLAY  AND  CLAY  PRODUCTS  519 

of  variable  thickness,  immediately  underlying  the  surface  in  many 
regions  formerly  occupied  by  glaciers,  hence  they  are  common  in  the 
northern  states.  Glacial  clays  are  used  for  brickmaking. 

Uses  of  Clay 

Kinds  of  clay.  —  Many  kinds  of  clay  are  known  by  special  names, 
which  in  some  cases  indicate  their  use,  but  in  others  refer  to  certain 
physical  properties.  Those  of  interest  or  importance  to  engineers  are 
mentioned  below. 

Adobe.  A  sandy,  often  calcareous,  clay  used  in  the  west  and  southwest  for  making 
sun-dried  brick.  Brick  clay.  Any  common  clay  suitable  for  making  ordinary  brick. 
Fire  clay.  A  clay  capable  of  resisting  a  high  degree  of  heat.  The  term  is  applied  to 
many  clays,  having  no  right  to  it.  Gumbo.  A  very  sticky,  highly  plastic  clay,  of 
dark  color,  occurring  abundantly  in  the  central,  west-central,  and  southern  states. 
It  is  used  for  making  railroad  ballast  and  road  material.  Kaolin.  A  white-burning 
residual  clay.  Loess.  A  sandy,  calcareous,  clay,  covering  thousands  of  square  miles 
in  the  Great  Plains  region.  Paving-brick  clay.  One  capable  of  being  molded  in  a 
machine,  and  burning  to  a  vitrified  body  at  a  moderate  temperature.  Pressed-brick 
clay.  Any  clay  capable  of  being  used  for  the  manufacture  of  pressed  brick,  but 
usually  a  No.  2  fire  clay.  Sewer-pipe  clay.  A  term  applicable  to  any  clay  that  can 
be  used  for  the  manufacture  of  sewer  pipe. 

Engineering  Uses  of  Clay 

These  have  been  already  named,  and  may  be  taken  up  briefly. 
Since  the  character  of  the  product  is  affected  not  only  by  the  nature 
of  the  raw  material,  but  by  the  method  of  manufacture  as  well,  these 
points  should  be  given  some  attention  in  the  discussion. 

The  use  of  clays  for  brick.  —  It  may  be  stated  as  a  general  propo- 
sition that  the  higher  grades  of  brick  are  usually  made  of  the  better 
grades  of  clay. 

Clays  for  common  brick.  — -  The  clays  and  shales  selected  are  com- 
monly of  low  grade,  and  mostly  red-burning,  but  calcareous  clays  yield- 
ing a  cream-colored  product  are  worked  in  some  regions  where  they 
abound,  as  in  parts  of  Wisconsin,  Michigan,  Illinois,  etc.  The  main 
requisites  are  that  the  clay  shall  mold  easily,  and  burn  hard  at  as  low 
a  temperature  as  possible.  Unfortunately  but  little  care  is  often  used 
in  the  selection  of  clay  for  common  brick,  and  the  product  shows  it. 
Lime  pebbles  if  present  should  be  crushed  or  screened  out,  otherwise 
they  are  sure  to  cause  cracking  and  bursting  of  the  brick. 

Clays  for  pressed  brick.  —  These  are  made  of  red-burning  clays  or 
shales,  cream-burning  calcareous  clays,  or  buff-burning  No.  2  fire  clays. 
The  last  named  are  most  used. 


PLATE  LXXXVIII,  FIG.  1.  —  Section    showing    fire  clay  underlying  coal  seam. 
The  upper  clay  above  coal  is  of  impure  character. 


FIG.  2.  —  Shale  used  for  paving  blocks,  Veedersburg,  Ind.     (After  Blatchley,  29th 

Ann.  Kept.,  Ind.  Dept.  Geol.  and  Nat.  Res.,  p.  80.) 
(520) 


CLAY  AND  CLAY  PRODUCTS  521 

Clays  for  paving  brick.  —  These  are  made  either  from  red-burning 
clays  or  shales  which  burn  easily  to  a  vitrified  body,  or  else  from  a  low- 
grade  fire  clay,  which  gives  a  buff-colored  ware.  Both  types  of  ma- 
terial are  capable  of  yielding  excellent  results. 

Methods  of  manufacture.  —  These  may  be  briefly  taken  up  in  order 
to  point  out  their  influence  on  the  character  of  the  product,  and  some 
other  details  of  importance  to  the  engineer.  The  steps  in  the  process 
are  essentially  similar  for  all  classes  of  brick,  the  difference  being  chiefly 
in  the  raw  material,  and  care  used  in  manufacture. 

The  manufacture  of  brick  can,  therefore,  be  resolved  into  the 
following  steps:  Preparation,  molding,  drying,  and  burning. 

Preparation.  —  Shales  and  tough  clays  require  a  preliminary  dis- 
integration to  facilitate  their  admixture  with  water,  or  sand,  or  even 
other  clays,  and  weathering  is  sometimes  resorted  to  as  a  means  of 
partial  disintegration.  In  the  mixing  or  tempering  which  follows,  the 
water  must  be  thoroughly  incorporated  into  the  clay,  for  imperfect 
tempering  often  leads  to  warping  or  splitting  of  the  brick,  because 
lumps  of  unslaked  clay  are  left  in  the  mass.  Pebbles  not  previously 
removed  by  crushing  or  special  machinery  cause  similar  trouble. 

Tempering  is  now  done  largely  by  machines  such  as  the  pug  mill  or  wet  pan.  The 
former  is  simply  a  horizontal  trough  with  blades  on  a  revolving  shaft,  which  cut  and 
mix  the  clay.  The  latter  is  a  revolving  pan  with  two  large  mullers,  underneath 
which  the  charge  of  wet  clay  has  to  pass. 

Molding.  —  Three  common  methods  of  molding  are  in  vogue,  known 
respectively  as  the  soft-mud,  stiff-mud,  and  dry-press  process.  Each 
may  be  said  to  have  its  limitations. 

Soft-mud  process.  —  Soft-mud  brick  are  made  in  a  machine,  in  which 
the  soft  wet  clay  is  forced  into  wooden  molds.  The  latter  usually  have 
six  compartments,  and  are  sanded  to  prevent  the  wet  clay  from  sticking 
to  the  wooden  surface.  A  soft  mud  brick  has:  (1)  A  homogeneous 
structure;  (2)  five  sanded  surfaces  from  contact  with  the  interior  face 
of  the  mold,  and  a  sixth  rough  one,  caused  by  striking  the  excess  of 
clay  off  the  top  of  the  mold  as  it  conies  from  the  machine.  (3)  They 
lack  very  sharp  corners  and  straight  edges.  (4)  Their  fracture  may 
show  more  pebbly  particles  than  bricks  made  by  the  other  processes. 

A  soft-mud  machine  operated  by  steam  power  will  commonly  turn 
out  from  25,000  to  30,000  bricks  per  day.  The  process  is  adapted  to 
a  wide  range  of  clays. 

Stiff-mud  process.  —  In  this  method  the  raw  material  is  tempered 
to  a  stiff  paste,  and  forced  from  the  machine  through  a  die  of  rectangu- 
lar cross  section,  thus  giving  a  bar  of  clay,  which  is  cut  into  bricks  by 


522  ENGINEERING  GEOLOGY 

a  properly-constructed  wire-cutting  device.  The  bricks  are  termed 
either  end  cut  or  side  cut,  depending  on  whether  the  area  of  the  cross 
section  of  the  bar  of  clay  corresponds  to  the  end  or  side  of  a  brick. 

A  stiff-mud  brick  can  be  easily  recognized  by  the  four  smooth  sur- 
faces, which  represent  those  portions  of  the  bar  in  contact  with  the 
interior  :surface  of  the  lubricated  die,  and  the  two  cut  faces  showing 
the  tearing  action  of  the  cutting  wires. 

Too  much  friction  between  clay  and  die  may  cause  a  tearing  of  the  clay,  especially 
on  the  edges  of  the  bar,  resulting  in  the  production  of  serrations,  like  saw  teeth. 
Stiff-mud  brick  sometimes  show  a  laminated  or  shelly  structure  on  a  section  parallel 
to  the  cut  face,  produced  by  the  twisting  action  of  the  auger  that  forces  the  clay 
through  the  die.  It  is  not  observable  in  all  brick,  but  apt  to  be  especially  pronounced 
in  very  plastic  clays,  in  fact  at  times  so  much  so,  as  to  make  some  other  molding 
process  more  desirable.  Brick  makers  often  fail  to  realize  that  each  clay  is  a  prob- 
lem by  itself,  and  that  small  changes  in  the  construction  of  a  given  stiff-mud  machine 
may  turn  success  to  failure. 

The  stiff-mud  process  while  one  of  high  capacity,  60,000  or  even  100,000  bricks 
per  day  being  turned  out  by  one  machine,  is  not  adapted  to  all  kinds  of  clays,  those 
of  medium  plasticity  giving  perhaps  the  best  results,  so  that  defective  brick  are 
sometimes  the  fault  of  the  clay  and  not  the  process. 

The  laminations  are  regarded  by  some  as  a  structural  weakness,  and  the  bricks 
often  show  a  tendency  to  spall  off,  when  exposed  to  fire  and  water.  Paving  brick 
are  commonly  made  by  this  process,  and  repressed  as  described  below. 

Dry-press  process.  —  This  process  is  generally  used  for  front  brick, 
and  sometimes  for  common  brick,  but  very  rarely  for  pavers. 

The  clay,  containing  not  more  than  12  to  15  per  cent  moisture,  is 
disintegrated,  screened,  and  then  pressed  in  steel  molds  in  a  specially- 
constructed,  powerful  press. 

The  advantages  claimed  for  this  process  are  that  in  one  operation 
we  get  a  brick  with  sharp  edges  and  smooth  faces.  If  the  clay  does 
not  disintegrate  readily,  or  is  insufficiently  screened,  the  brick  show 
a  granular  structure.  Dry-pressed  brick  if  hard-burned  are  just  as 
strong  as  others,  but  if  not  hard-burned,  they  frequently  show  a  higher 
absorption.  In  other  words  a  clay  molded  dry-press  must  usually  be 
burned  harder  to  get  a  given  density  and  hardness,  than  if  it  were 
molded  by  another  process. 

Repressing.  —  Many  soft-mud  and  stiff-mud  brick  after  molding 
are  repressed  in  steel  molds,  the  main  object  being  to  smooth  the 
surface  and  straighten  the  edges  or  imprint  some  design  or  markings 
on  the  surface.  In  some  cases  the  brick  is  slightly  smaller  and  even 
stronger  (as  shown  by  tests).  Repressing  may  also  give  the  brick  a 
tough  exterior  skin,  which  strengthens  their  resistance  to  disintegrating 
influences.  The  following  tests  show  some  effects  of  repressing. 


CLAY  AND  CLAY  PRODUCTS  523 


Strength. 

Not  repressed. 

Repressed. 

Crushing  strength,  pounds  per  square  inch.  '.  .  .  . 

3107 

4304 

Transverse  strength,  modulus  of  rupture  

440 

.  613 

Absorption 

12  0% 

9  75% 

Drying.  —  Bricks  made  by  either  the  soft-mud  or  stiff-mud  process 
have  to  be  freed  from  most  of  their  water  before  they  can  be  burned. 
Where  the  drying  is  done  by  solar  heat  in  the  open,  the  yard  can  only 
produce  during  warm  weather,  but  where  it  is  done  by  artificial  heat 
as  in  tunnels,  the  yard  can  be  operated  throughout  the  entire  year. 
Some  clays  have  to  be  dried  with  great  care  to  prevent  cracking, 
others  do  not. 

Burning.  —  The  temperature  required  for  burning  brick  varies  with 
the  clay,  the  density,  and  degree  of  hardness  and  color  desired,  the 
same  clay  yielding  different  results  when  fired  at  different  tempera- 
tures. Common  bricks  are  usually  fired  at  a  red  heat,  sometimes  not 
much  above  1000°  C.  Pressed  brick  made  of  No.  2  fire  clays  are 
commonly  burned  at  about  1250°  C.,  while  paving  brick  may  be  burned 
from  as  low  as  1175°  to  1250°  C.,  or  possibly  even  a  little  higher. 
Even  though  all  the  preceding  stages  of  the  process  have  been  carried 
out  properly,  the  ware  may  be  ruined  if  not  properly  burned.  The 
kilns  used  should  be  briefly  referred  to. 

Common  brick  are  often  burned  in  scove  kilns.  These  simply  represent  a  rectan- 
gular pile  of  brick  set  30  to  50  courses  high,  with  arches  left  running  through  the 
bottom  of  the  pile  about  every  three  feet.  The  mass  is  enclosed  in  a  temporary  wall 
which  is  smeared  over  with  wet  clay.  Fires  are  built  in  the  arches  and  the  heat  grad- 
ually works  up  through  the  kiln.  Such  a  kiln  is  only  adapted  to  common  bricks;  its 
action  is  not  always  uniform,  consequently  care  should  be  taken  in  selecting  samples 
from  it  for  testing.  The  hardest-burned  bricks  are  near  the  arches,  the  under-burned 
ones  usually  near  the  top  and  corners,  but  still  local  cold  spots  may  give  "  pale  " 
bricks  right  in  the  center  of  the  kiln. 

In  the  permanent  kilns  —  the  type  used  for  paving  and  pressed  brick  —  there 
are  permanent  walls,  roofs,  and  fire  boxes.  The  kiln  is  better  controlled  and  we  can 
expect  a  more  uniform  product.  There  may  be  certain  differences,  however,  de- 
pending on  the  direction  of  the  draft.  In  up-draft  kilns,  the  heat  enters  at  the  bot- 
tom and  passes  out  at  the  top,  consequently  the  hardest-burned  bricks  may  be  looked 
for  in  the  lower  hah"  of  the  kiln  if  the  burning  is  not  uniform,  whereas  in  down-draft 
kilns,  the  heat  enters  at  the  top,  and  the  reverse  conditions  may  obtain. 

Properties  of  bricks.  —  The  average  of  a  number  of  tests  made  on 
bricks  molded  by  each  of  three  methods  shows  that  if  properly  burned 
there  is  not  much  difference  in  the  range  of  crushing  and  transverse 


524  ENGINEERING  GEOLOGY 

strength  of  the  several  kinds.  Dry-press  brick  often  show  a  higher 
absorption. 

The  tests  which  can  be  applied  to  brick  are:  (1)  Crushing  test; 
(2)  transverse  test;  (3)  absorption  test  and  porosity;  (4)  abrasion 
test;  (5)  frost  resistance;  (6)  fire  resistance;  (7)  permeability.  All 
of  these  are  rarely  carried  out,  but  usually  only  1  and  3  for  structural 
brick,  and  1,  3,  and  4  for  paving  brick. 

Sewer  pipe.  —  This  class  of  ware  is  made  from  a  clay  or  shale,  or 
mixture  of  two.  or  more  kinds  of  these  materials,  whose  physical  prop- 
erties are  such  that  they  will  either  burn  to  a  vitrified  body,  or  one  of 
low  absorption,  and  also  take  a  salt  glaze.  In  some  sewer-pipe  mix- 
tures a  fire  clay  is  used  as  one  of  the  ingredients. 

Sewer-pipe  clays  are  thoroughly  ground  if  necessary,  well-mixed, 
and  then  molded  in  a  special  form  of  press.  After  drying  carefully 
in  drying  rooms,  they  are  burned  in  down-draft  kilns.  The  glaze  is 
obtained  by  throwing  salt  into  the  fires,  and  the  sodium  vapors  pass- 
ing through  the  kiln  unite  with  the  clay  to  form  a  glaze.  A  poor  glaze 
may  be  due  to  the  clay,  excess  of  soluble  salts  in  the  same,  or  too  low 
temperature  of  burning. 

The  flaws  which  sewer-pipe  may  show  and  their  causes  are:  (1)  Blisters,  due  to 
air  imprisoned  in  the  clay  during  molding;  (2)  surface  pimpling,  due  probably  to 
the  texture  of  the  body  and  treatment  during  firing,  but  which  can  usually  be  pre- 
vented bv  finer  grinding  and  slower  burning;  (3)  warping  and  cracking  often 
caused  by  uneven  mixing,  uneven  heating,  or  inability  of  the  pipes  to  stand  the 
weight  of  those  set  on  them,  when  red  hot ;  and  (4)  fine  cracks  which  may  develop  in 
the  drying  and  open  still  further  in  the  burning. 

Sewer  pipe  may  be  tested  for  their  strength  to  resist  crushing,  bursting,  and 
impact  under  various  practical  conditions;  their  resistance  to  abrasion  by  sand  or 
gravel;  resistance  to  corrosion  by  acids,  alkalies,  steam,  and  gases;  and  their 
permeability. 

Railroad  ballast.  —  The  use  of  burned  clay  for  ballasting  rail- 
roads is  of  importance  in  those  areas  where  stone  is  wanting  or 
if  used  has  to  be  hauled  a  long  distance.  The  type  of  clay  much 
used  in  the  central  and  western  states  is  a  very  plastic  sticky  clay 
commonly  known  as  gumbo,  and  which  is  often  found  as  extensive 
surface  deposits. 

The  method  of  use  consists  in  locating  a  deposit  near  the  line  of  rail- 
road, and  running  a  spur  track  out  on  to  it.  A  trench  6  or  8  feet  deep 
is  dug  in  the  clay  along  the  track,  and  the  clay  so  excavated  is  piled 
up  on  the  opposite  side.  A  wood  fire  is  started  in  the  bottom  of  the 
trench  and  on  this  are  piled  alternating  layers  of  clay  and  fuel  (either 


CLAY  AND   CLAY  PRODUCTS  525 

coal  or  wood).  The  heat  supplied  by  the  burning  fuel  bakes  the  clay 
lumps,  the  larger  ones  becoming  broken  up  also  by  the  rapid  heating. 
These  hard  lumps  of  clay  can  be  utilized  for  ballast. 

The  cost  of  manufacture  approximates  50  to  60  cents  delivered  on 
the  cars,  and  the  product  is  sometimes  hauled  as  far  as  100  miles. 
Burned  clay  is  said  to  make  a  fairly  satisfactory  ballast,  being  easy 
on  the  rolling  stock,  and  keeping  well  drained.  If  the  clay  is  under- 
burned  it  disintegrates  somewhat  rapidly.  The  making  of  burned 
clay  ballast  is  not  always  practicable  in  thickly-settled  regions, 
since  the  abundant  smoke  developed  in  the  burning  is  regarded  as 
a  nuisance. 

Road  material.  —  In  some  regions  where  the  surface  clay  is  of 
gumbo-like  nature,  and  no  cheap  material  is  available  for  highway 
construction,  the  burned  gumbo  is  used  for  top  dressing  on  the  wagon 
roads.  Indeed,  sometimes  the  upper  few  inches  of  the  road  bed  itself 
are  plowed  up,  gathered  into  heaps,  burned,  and  then  spread  on  the 
road  again. 

Puddle.  —  This  term  is  applicable  to  any  clay  that  can  be  used  to 
form  a  waterproof  lining  or  backing  to  a  reservoir  or  other  water- 
retaining  embankment  or  wall.  The  two  main  requisites  are  that  the 
clay  shall  be  water-tight  and  dry  without  cracking.  If  the  clay  is  too 
plastic  it  has  to  be  made  leaner  by  adding  sand  or  gravel. 


Distribution  of  Clays  in  the  United  States 

Clays  have  a  wider  distribution  than  most  other  rocks,  being  found 
in  all  formations  from  the  oldest  to  the  youngest. 

Both  white  and  colored  residual  clays  are  derived  from  the  older 
crystalline  rocks,  and  are  of  widespread  occurrence  in  the  Piedmont 
region  of  the  southern  states.  Deposits  of  shale  as  well  as  fire  clay  are 
abundant  and  important  in  the  coal-measures  formations  of  the  east- 
ern and  central  states  where  they  form  the  basis  of  an  extensive  paving 
and  fire-brick  industry. 

In  the  Coastal  Plain  region  of  the  Atlantic  and  Gulf  coast  states, 
clays  suitable  for  fire  brick,  pressed  brick,  stoneware,  and  terra  cotta 
are  obtained  from  the  Cretaceous  and  Tertiary  deposits.  Somewhat 
similar  uses  are  open  to  the  clays  of  these  formations  found  in  parts 
of  the  Great  Plains,  in  the  eastern  foothills  of  the  Rocky  Mountains, 
and  along  the  Pacific  Coast. 

The  surface  clays  of  recent  origin  are,  however,  the  most  wide- 
spread and  are  used  everywhere  for  brick  and  tile. 


526  ENGINEERING  GEOLOGY 

References  on  Clay 

Technology  and  properties.  (1)  Bourry,  E.,  A  Treatise  on  Ce- 
ramic Industries,  Translation  by  A.  B.  Searle,  New  York,  1911  (Van 
Nostrand  &  Co.);  (2)  Merrill,  G.  P.,  Rocks,  Rock  Weathering  and 
Soils,  New  York,  1906  (Macmillan  Co.);  (3)  Ries,  Clays,  Occurrence, 
Properties  and  Uses,  New  York,  1908  (Wiley  &  Sons) ;  (4)  Wheeler, 
Vitrified  Paving  Brick,  Indianapolis,  1895  (Clayworker  Pub.  Co.); 
(5)  Transactions  American  Ceramic  Society,  Columbus,  0.,  Vols.  I  to 
XV  have  appeared;  contain  many  excellent  papers. 

Areal  reports.  Reference  No.  3  summarizes  the  literature  dealing 
with  the  distribution  of  clay  in  the  United  States.  In  addition,  the 
Geological  Surveys  of  Alabama,  Connecticut,  Georgia,  Illinois,  In- 
diana, Iowa,  Maryland,  Mississippi,  Missouri,  New  Jersey,  New 
York,  North  Carolina,  North  Dakota,  Ohio,  Oklahoma,  South  Car- 
olina, Texas,  Virginia,  Washington,  West  Virginia,  and  Wisconsin  have 
published  special  reports  on  the  clay  deposits  of  their  respective  states. 
Scattered  papers  are  contained  in  the  reports  of  the  United  States 
Geological  Survey,  and  the  Canadian  Geological  Survey  is  issuing 
a  series  of  special  bulletins  on  Canadian  clay  deposits. 


CHAPTER  XIV 
COAL   SERJES 

Kinds  of  Coal 

UNDER  this  heading  are  included  a  number  of  substances  consisting 
chiefly  of  a  mixture  of  fixed  carbon,  volatile  hydrocarbons  (as  well  as 
some  other  volatile  matter),  sulphur  and  ash. 

It  is  generally  admitted  that  all  the  members  of  the  coal  series  are 
of  vegetable  origin,  as  will  be  explained  later,  and  that  they  probably 
form  a  lineal  succession,  represented  by  the  following  members:  Peat, 
lignite,  subbituminous,  bituminous,  semibituminous,  semianthracite 
and  anthracite.  The  properties  of  these  are  as  follows: 

Peat.  —  This  is  a  surface  deposit,  representing  the  first  stage  in 
coal  formation,  and  is  formed  by  the  growth  and  decay  of  grasses, 
bog  moss  and  other  plants  in  moist  places. 

A  section  in  a  peat  bog  from  the  top  downward  may  show:  (1)  A 
layer  of  living  plants;  (2)  a  layer  of  dead  plant  roots,  stems  and  leaves, 


FIG.  200.  —  Diagram  showing  how  plants  fill  depressions  from  the  sides  and  top, 
to  form  a  peat  deposit:  (1)  Zone  of  Chara  and  floating  aquatic  plants.  (2)  Zone 
of  Potamogetons.  (3)  Zone  of  water  lilies.  (4)  Floating  sedge  mat.  (5) 
Advance  plants  of  conifers  and  shrubs.  (6)  Shrub  and  Sphagnum  zone.  (7) 
Zone  of  tamarack  and  spruce.  (8)  Marginal  fosse.  (After  Davis,  Mich.  Geol. 
Survey,  Ann.  Rep.  for  1906.) 

whose  structure  is  clearly  recognizable  and  which  grades  into  (3)  a 
layer  of  fully-formed  peat;  a  dense,  brownish-black  mass  of  more  or 
less  jelly-like  or  cheesy  character,  in  which  the  vegetable  structure 
is  often  indistinct.  The  following  analyses  show  the  difference  in 
composition  of  the  different  layers  in  a  peat  bog.  They  also  indicate 
that  in  the  passage  from  vegetable  matter  to  peat  the  hydrogen  and 
oxygen  diminish,  while  the  carbon  increases  in  proportion. 

527 


COAL  SERIES  529 

ANALYSES  OF  DIFFERENT  LAYERS  OF  A  PEAT  Boo 


Material. 

Carbon. 

Hydrogen. 

Oxygen. 

Nitrogen. 

Sphagnum1 

49  88 

6  54 

42  42 

1  16 

Porous,  light-brown  peat  

50.86 

5.80 

42.57 

0  77 

Porous,  red-brown  peat 

53  51 

5  90 

40 

59 

Heavy  brown  peat  

56  43 

5  32 

38 

25 

Heavy  black  peat  

59.70 

5.70 

33  04 

1  56 

1  The  fact  that  sphagnum  occurs  on  the  surface  is  not  necessarily  an  indication  that  it  was  the  only 
peat-forming  plant  present. 

Peat  as  taken  from  the  bog  contains  much  moisture  —  often  as 
much  as  90  per  cent  —  and  has  to  be  dried.  It  is  then  porous  and 
light  in  weight,  burns  readily  with  a  long,  smoky  flame  and  with  a 
lower  heating  power  than  higher  grades  of  coal  (see  Refs.  31-36). 


FIG.  201.  —  Peaty  deposit  with  cypress  stumps  covered  by  sandy  clays  due 
sinking  of  land  below  sea  level.     Chesapeake  Bay,  Maryland.     (H.  Ries,  photo.) 


Lignite.  —  This  substance,  also  called  brown  coal,  represents  the 
second  stage  in  coal  formation.  It  is  often  brown  in  color,  woody 
in  texture  and  has  a  brown  streak.  It  burns  readily  with  a  long, 
smoky  flame,  but  in  its  raw  condition  is  less  valuable  than  the  higher 


530  ENGINEERING  GEOLOGY 

grades  of  coal,  partly  because  of  its  lower  heating  power.  Most  lig- 
nite contains  a  relatively  high  amount  of  moisture,  and  the  drying 
out  of  this  on  exposure  to  the  air  causes  the  material  to  disintegrate. 
On  this  account  it  should  not  be  stored  for  long  periods  or  hauled  a 
great  distance  to  market. 

Lignite  deposits  are  found  only  in  the  more  recent  geological  for- 
mations (such  as  the  Cretaceous  and  Tertiary)  interstratified  with 
shales  and  sandstones,  often  of  only  partially  consolidated  character. 

Subbituminous  coal  or  black  lignite.  —  A  grade  intermediate  be- 
tween lignite  and  bituminous  and  not  always  distinguishable  from 
one  or  the  other  on  sight.  It  is  usually  black  and  sometimes  has  a 
fairly  bright  luster.  Campbell  has  claimed  that  subbituminous  coal 
can  be  distinguished  from  bituminous  on  the  basis  of  weathering, 
because  the  former  checks  irregularly  in  drying  and  splits  parallel 
with  the  bedding  on  weathering,  while  bituminous  coal  shows  a  co- 
lumnar cleavage  (Plate  XCI).  The  differentiation  of  subbituminous 
coal  from  lignite  is  suggested  on  the  basis  of  color,  the  former  being 
black,  the  latter  brown.  The  term  subbituminous  is  widely  used 
in  the  United  States,  but  it  is  not  officially  recognized  in  Canada,  so 
that  in  the  latter  country  some  subbituminous  coals  are  known  as 
lignites  to  the  dissatisfaction  of  the  producers.  Subbituminous  coal 
occurs  under  similar  conditions  to  lignite  and  in  formations  of  the 
same  age. 

Bituminous  coal.  —  This  represents  the  fourth  stage  in  coal  for- 
mation. It  has  greater  density  than  the  lignites  or  subbituminous 
coals,  is  deep  black  in  color,  comparatively  brittle  and  breaks  with 
a  cubical  or  sometimes  conchoidal  fracture. 

Bituminous  coal  burns  readily,  with  a  smoky  flame  of  yellow  color, 
but  with  greater  heating  power  usually  than  the  other  grades  already 
mentioned.  It  does  not  disintegrate  as  readily  on  exposure  to  air  as 
lignite.  Most  of  the  bituminous  coal  found  in  the  United  States  lies 
in  the  formations  of  earlier  geologic  age  (Carboniferous)  than  the 
lignite;  but  where  the  two  occur  in  the  same  formation  as  in  parts 
of  the  Northwest  and  West,  the  lignite  is  in  horizontal  strata,  while 
the  bituminous  is  associated  with  at  least  slightly  folded  ones.  This 
suggests  that  the  folding  bears  some  relation  to  the  character  of  the 
coal. 

Many  bituminous  coals  when  freed  of  their  volatile  constituents  by 
heating  to  redness  in  an  oven,  cake  to  a  hard  mass  called  coke.  All 
bituminous  coals  do  not  exhibit  this  property,  and  the  discussion  of 
it  will  be  taken  up  later. 


PLATE  XC,  FIG.  1.  —  Outcrop  of  lignite,  Williston,  N.  Dak.     (Photo  by  Wilder, 
from  Hies'  Economic  Geology.) 


FIG.  2.  —  Culm  pile  in  Pennsylvania  anthracite  region. 


(531) 


532  ENGINEERING  GEOLOGY 

Cannel  coal.  —  This  *is  a  compact  variety  of  non-coking  bitumi- 
nous coal,  with  a  dull  luster  and  conchoidal  fracture.  Owing  to  its 
unusually  high  percentage  of  volatile  matter,  upon  which  its  chief 
value  depends,  cannel  coal  ignites  easily,  burning  with  a  yellow  flame, 
and  when  heated  tends  to  decrepitate. 

Following  are  two  analyses  of  cannel  coal,  I  from  Cumberland  Gap 
field,  Kentucky,  and  II  from  Cannelburg,  Indiana: 

ANALYSES  OF  CANNEL  COAL 


Constituents. 

I 

II 

Moisture 

1  00 

1  47 

Volatile  matter 

51  60 

49  08 

Fixed  carbon.  ... 

40  40 

26  35 

Ash  ...               

7.00 

23  10 

Sulphur  
Fuel  ratio  

0.739 

0.78 

1.48 
0.53 

Semibituminous  coal.  —  This  is  a  term  which  was  proposed  by 
H.  D.  Rogers  of  the  Pennsylvania  Geological  Survey  as  early  as  1858 
to  apply  to  those  grades  above  bituminous,  whose  volatile  matter 
was  between  12  and  18  per  cent,  and  still  later  in  1879  Fraser  of  the 
same  organization  included  under  it  coals  whose  fuel  ratios  ranged 
from  8  to  5.1 

Semianthracite  coal.  —  This  was  another  term  proposed  by  Rogers 
at  the  same  time  as  the  preceding  one,  to  include  coals  between  bitu- 
minous and  anthracite  having  less  than  10  per  cent  volatile  matter. 
Frazer  later  included  under  it  those  coals  whose  fuel  ratios  ranged 
from  12  to  8. 

The  retention  of  both  terms  seems  perhaps  unfortunate,  but  they 
persist  to  the  present  day,  and  are  sometimes  no  doubt  rather  loosely 
used.  Possibly  the  disagreement  among  different  people  as  to  what 
shall  be  included  under  these  terms  may  partly  be  responsible  for  the 
confusion. 

Anthracite  coal.  —  This  variety  of  coal  is  black,  hard  and  brittle, 
with  high  luster  and  conchoidal  fracture.  It  has  a  lower  percentage 
of  volatile  matter  and  a  higher  percentage  of  fixed  carbon  than  any 
of  the  other  varieties.  On  this  account  it  ignites  much  less  readily 
and  burns  with  a  short  flame,  but  gives  great  heat. 

The  geological  distribution  is  more  restricted  than  that  of  bitu- 

1  The  fuel  ratio  is  the  ratio  of  the  fixed  carbon  to  the  volatile  matter. 


COAL  SERIES  533 

minous  coal.  It  occurs  usually  in  areas  of  somewhat  strongly  folded 
rocks  (northeastern  Pennsylvania),  and  is  also  found  in  certain  areas 
where  beds  of  bituminous  coal  have  been  converted  into  anthracite 
by  the  near  approach  of  intrusive  masses  of  igneous  rock  (Crested 
Butte,  Colo.;  Cerillos,  N.  Mex.,  etc.). 

Composition  of  Coal 

The  composition  of  coal  may  be  expressed  in  either  the  elementary 
or  the  proximate  form.  In  the  first  there  is  given  the  percentage  of 
carbon,  hydrogen,  oxygen,  nitrogen,  without  reference  to  its  mode  of 
combination.  In  the  latter  an  attempt  is  made  to  show  the  form  in 
which  the  elements  are  combined.  This  is  the  form  commonly  em- 
ployed as  it  is  considered  to  be  of  greater  practical  value. 

Below  are  given  the  proximate  and  ultimate  composition  of  two 
coals,  the  ash  and  sulphur  being  common  to  both. 


Moisture                 

Lignite, 
North  Dakota. 

36  13 

Bituminous  coal, 
Pennsylvania. 

3  51 

Proximate 

Volatile  matter 

29  28 

16  82 

analysis 

Fixed  carbon  

29  55 

73  04 

(Ash. 

5  04 

6  63 

(  Sulphur 

0  59 

0  94 

Hydrogen  

6.60 

4  56 

Ultimate 

Carbon 

42  00 

80  70 

analysis 

Nitrogen 

0  73 

1  26 

Oxveen.  . 

45.04 

5.91 

Proximate  analysis  of  coal.  —  The  proximate  analysis,  though 
apparently  a  simple  operation,  needs  to  be  carefully  carried  out  to 
prevent  variable  results.  The  constituents  of  the  coal  are  grouped  as 
moisture,  volatile  matter,  fixed  carbon,  ash  and  sulphur. 

The  moisture  can  be  driven  off  at  100°  C.,  and  is  usually  highest  in 
peat  and  lignite.  The  volatile  matter  was  formerly  termed  the  vol- 
atile hydrocarbons,  but  it  is  now  clear  that  other  substances  also  are 
driven  off  at  a  red  heat,  and  that  the  volatile  matter  of  coals  differs 
greatly  in  its  character.1 

Thus  the  coals  of  the  younger  geological  formations  of  the  West 
have  a  large  proportion  of  carbon  dioxide,  carbon  monoxide  and  water, 
and  a  correspondingly  small  proportion  of  hydrocarbons  and  tarry 
vapors. 

On  the  other  hand  the  bituminous  coals  of  the  Appalachian  region 

1  See  Bull.  1,  U.  S.  Bureau  of  Mines,  Washington. 


534 


ENGINEERING  GEOLOGY 


yield  volatile  matter  containing  much  tarry  vapor  and  hydrocarbon 
compounds,  which  are  hard  to  burn  completely  without  an  excess  of 
air  and  high  temperature. 

The  western  coals  give  up  their  volatile  matter  more  easily  at  mod- 
erate temperatures  than  the  eastern  ones.  The  volatile  matter  pro- 
duced at  medium  temperatures  is  rich  in  higher  hydrocarbons  of  the 
methane  (CH4-marsh  gas)  series,  such  as  ethane  and  propane,  which 
contain  a  larger  proportion  of  carbon  than  is  present  in  methane. 

These  facts  help  to  explain  the  difficulty  of  burning  Pittsburg  coal,  for  example, 
without  smoke,  the  low  efficiency  usually  obtained  in  burning  high-volatile  western 
coals,  the  advantage  of  a  preheated  auxiliary  air  supply  introduced  over  a  fuel  bed, 
and  the  advantage  of  a  furnace  and  boiler  setting  adapted  to  the  type  of  fuel  used. 
They  bear  directly  also  on  the  question  of  steaming  "capacity"  of  coals  for  locomo- 
tives, the  designing  and  operation  of  gas  producers  for  high-volatile  fuels,  and  the 
operation  of  coke  ovens  and  gas  retorts. 

The  results  of  tests  by  the  U.  S.  Bureau  of  Mines  show  that  the  inert,  non-com- 
bustible material  is  present  in  volatile  products  of  different  kinds  of  coal  in  amounts 
ranging  from  1  to  15  per  cent. 

The  following  table  gives  the  percentage  of  volatile  matter  and  coke  yield  in  some 
eastern  and  western  coals: 

VOLATILE  MATTER  AND  COKE  YIELD  OF  COALS 


Coals. 

Va. 

Penn. 

111. 

Wyo. 

Wyo., 
air- 
dried. 

Utah. 

Wyo. 

No.  of  tests,  averages  

2 

6 

2 

4 

2 

2 

2 

Coke,  per  cent 

79  1 

71  4 

63  1 

44  7 

53  0 

58  6 

63  9 

Tar,  per  cent  

7.2 

11.3 

11.9 

7.1 

5.5 

12  3 

10  3 

Water,  per  cent                               

1  3 

4  9 

10  7 

27  5 

19  0 

11  8 

10  0 

12  9 

23  8 

25  3 

27  2 

26  7 

26  3 

26  3 

CO2,  per  cent  
H2S,  per  cent 

0.44 
0  07 

0.72 
0  25 

1.2 

0  46 

8.14 
0  08 

8.41 
0  11 

3.13 
0  24 

2.13 
0  30 

9700 

8140 

8400 

7830 

8170' 

7620 

7940 

Comp.  of  gas  (6) 

1  4 

3  2 

3  0 

2  2 

2  6 

5  7 

5  5 

CO  
CH4,  C2H6,  etc. 

3.2 

26  4 

5.1 

27  8 

7.4 

(c)  26  3 

19.5 
18  1 

21.4 

(c)  22  6 

14.9 
27  2 

12.3 
25  4 

67  8 

61  0 

(c)  56  8 

54  0 

(c)  49  3 

47  8 

53  1 

N  
Total  volatile  products  (without  moisture)  . 
Water  of  constitution  :  

1.2 
19.7 
0.1 

2.9 
27.4 
3.7 

6.5 
29.8 
3.6 

6.2 
33.3 
5.5 

4.1 
35.5 
7.5 

4.4 
38.5 
8.9 

3.7 
32.4 
6.3 

Inert  volatile  matter  (d)  

0.7 

4.7 

5.1 

14.0 

16.3 

12.4 

8.8 

(a)  Calculated  to  dry  basis  at  0°  C.  and  760  mm.  pressure,  free  of  air  and  CO2. 

(6)  Calculated  to  CO2  and  O  free  basis. 

(c)  H  calculated. 

(d)  Sum  of  ammonia,  CO2  and  water  of  constitution. 

The  fixed  carbon  of  the  coal  burns  with  difficulty  and  is  highest  in 
the  anthracite  variety. 

The  value  of  a  coal  for  fuel  purposes  is  determined  mainly  by  the 
relative  amount  of  its  different  constituents.  Thus  both  the  fixed 
carbon  and  volatile  hydrocarbons  represent  heating  elements  of  the 


COAL  SERIES 


535 


coal,  the  former  being  the  stronger.  The  fuel  ratio  is  the  ratio  of  the 
fixed  carbon  to  the  volatile  hydrocarbons.  Anthracite  has  a  higher 
fuel  ratio  than  lignite.  The  free-burning  character  of  a  coal  is  due  to 
a  goodly  percentage  of  volatile  hydrocarbons. 

Moisture  is  a  non-essential  constituent  of  coal,  for  it  not  only  dis- 
places just  so  much  combustible  matter,  but  requires  heat  for  its 
evaporation,  and  when  present  in  large  amounts  often  causes  coal 
to  disintegrate  while  drying  out.  It  ranges  from  perhaps  1  per  cent 
in  anthracite  to  20  or  30  per  cent  in  lignite. 

The  following  table  gives  the  analyses  of  a  number  of  coals  from 
different  parts  of  the  United  States,  and  will  serve  to  show  how  they 
vary  in  composition: 

ANALYSES  OF  COALS 


Locality. 

Proximate                                       Ultimate 

Calories. 

B.T.U. 

Moisture. 

Volatile  matter. 

Fixed  carbon. 

i—  ' 

J3 

• 

<5 

*•                  .> 

! 

p 

OQ 

>> 

K 

j 

2 

j 

Peat. 
Halifax,  Mass  

49.80 
13.19 

32.64 
13.40 
19.13 

18.51 
13.49 
8.13 
34.89 

1.17 
5.13 
1.46 

2.87 
2.35 
0.54 
0.924 

1.28 

2.77 
0.52 

2.07 
0.73 

2.08 

2.80 
3.25 

27.27 
56.83 

29.19 
42.75 
35.36 

35.33 
37.11 
34.82 
43.48 

17.83 
32.68 
40.14 
34.51 
14.30 
19.86 
35.97 

12.82 
14.69 
12.11 

9.81 
10.55 

7.27 

1.16 
3.65 

10.88 
24.30 

26.75 
29.00 
32.54 

30.67 
43.03 
37.83 
13.56 

68.12 
47.46 
50.50 
54.31 
71.40 
74.61 
58.44 

73.69 
73.47 
58.60 

78.82 
69.92 

74.32 

88.21 
87.72 

12.05 
5.68 

11.42 
14.85 
12.97 

15.49 
6.37 
19.22 
8.07 

12.88 
14.73 
7.90 
8.31 
11.95 
4.99 
4.09 

12.21 
9.07 

28.77 

9.30 

18.80 

16.33 

7.83 
5.38 

0.34 
0.49 

3.54 

1.04 
0.65 

3.05 
0.58 
1.30 
1.33 

1.27 
4.45 
3.50 
1.36 
3.30 
0.344 
0.579 

2.01 
2.79 
0.55 

1.74 
0.66 

0.77 

0.80 
0.94 

Orlando,  Fla     

6.06 

6.15 
5.57 
5.60 

5.93 
5.75 
5.05 
6.41 

4.00 
4.88 
5.09 

51.18 

39.53 
52.06 
48.51 

47.34 
61.13 
56.71 
41.66 

75.68 
60.51 
74.44 

2.56 

0.49 
0.95 
0.91 

0.66 
1.22 
0.98 
0.56 

1.47 
1.23 
1.37 

34.03 

38.87 
25.53 
31.36 

27.53 
24.95 
16.74 
41.97 

4.70 
14.20 
7.70 

4961 

3872 
5199 
4714 

4726 
5995 
5668 
3880 

7450 
6199 
7700 

6,970 
9,358 

Lignite. 
Lehigh,  Stark  Co.,  N.  Dak.  .  . 
Crockett,  Tex  
Lester  Ark 

Subbituminous. 
Tesla,  Cal  

'16,791 
10,202 
6,984 

13,410 
11,158 
13,860 

Lafayette  Colo. 

Gallup,  N.  Mex  

Bituminous. 
Huntington,  Ark  
Coffeen,  111. 

Clarksburg,  W.  Va  

Clarion  County,  Pa  
Johnstown,  Pa.      .        ... 

4.22 

75.16 

1.13 

4.24 

7382 

13,288 

Pocahontas  steam  coal,  Va.  . 
Coking  coal,  Wise  Co.,  Va.... 

Semibituminous. 
Coal  Hill,  Ark. 

3.74 
4.02 
3.33 

3.62 
3.60 

2.81 

1.89 
3.50 

77.29 
78.71 
62.36 

80.28 
72.23 

75.21 

84.36 
84.53 

1.39 
1.46 
0.66 

1.47 
0.69 

0.80 

0.63 
1.53 

3.36 
3.95 
3.97 

3.59 
4.02 

4.08 

4.40 
4.12 

7448 
7652 
6002 

7612 
6929 

6929 

7388 
7795 

13,406 
13,774 

13,703 

Paris,  Ark  

Gary,  W.  Va.  (bony  layer)... 
Semianthractie. 
Russellville,  Ark 

Blacksburg,  Va  

Anthracite. 
Scranton,  Pa.  (culm)  
Mammoth  seam,  St.Nicholas, 
Schuylkill  Co.,  Pa 

13,298 
14,031 

Crested  Butte,  Colo 

536  ENGINEERING  GEOLOGY 

The  ash  represents  non-combustible  mineral  matter  and  bears  no 
direct  relation  to  the  kind  of  coal;  and  the  same  is  true  of  sulphur, 
which  is  present  as  an  ingredient  of  pyrite  or  gypsum.  Ash  also  dis- 
places combustible  matter,  but  otherwise,  in  most  cases,  it  is  an  inert 
impurity.  The  clinkering  of  coal  is  commonly  due  to  a  high  per- 
centage of  fusible  impurities  in  the  ash,  and  for  metallurgical  and 
other  work  the  composition  of  the  ash  is  sometimes  considered. 

Sulphur  is  an  objectionable  impurity  in  steaming  coals  on  account 
of  its  corrosive  action  on  the  boiler  tubes.  It  is  also  undesirable  in 
coals  to  be  used  for  metallurgical  purposes  and  gas  manufacture. 

Structural  Features  of  Coal  Beds 

Outcrops.  —  The  outcrop  of  a  coal  bed  is  usually  easily  recognized 
on  account  of  its  color  and  coaly  character  (Plates  XC  and  XCVII). 
Coal  weathers  easily,  however,  and  unless  the  exposure  is  a  somewhat 
fresh  one,  the  material  is  disintegrated,  the  wash  from  it  mingling  with 
the  soil,  and  if  the  outcropping  bed  is  on  a  hillside,  often  extending 
some  feet  down  the  slope.  This  weathered  outcrop  is  termed  the 
smut  or  blossom  by  coal  miners. 

In  areas  where  the  beds  have  been  tilted,  or  the  slopes  are  steep, 
and  where  there  is  no  covering  of  foreign  material,  the  coal  outcrops 
can  often  be  easily  traced,  but  in  regions  where  the  dip  is  flat  or  nearly 
so,  and  the  surface  level,  the  search  for  coal  is  often  attended  with 
difficulty,  which  is  increased  if  the  country  is  covered  with  glacial 
drift  or  other  superficial  deposits  of  unconsolidated  character.  In 
such  cases  boring  or  pitting  is  commonly  resorted  to. 

The  number  of  coal  beds  present  in  any  given  region  varies,  and 
sometimes  the  number  is  large.  Thus  in  the  Pennsylvania  section  as 
many  as  20  beds  are  known,  and  in  Alabama  at  least  55  have  been 
counted,  but  all  are  not  workable.  But  in  any  series  of  beds  all  are 
not  necessarily  sufficiently  thick  or  of  good  enough  quality  to  be 
workable;  indeed  a  bed  which  is  workable  at  one  point  may  not  be 
so  at  another. 

Associated  rocks.  —  Most  coal  beds  are  interbedded  with  shales,1 
clays  or  sandstones,  but  conglomerates  and  limestones  are  at  times 
found  not  far  from  the  coal  above  or  below  it,  and  sometimes  may 
form  either  the  floor  or  the  roof.  The  sedimentary  rocks  associated 
with  lignite  or  even  subbituminous  coal  are  not  as  often  consolidated, 

1  These  are  usually  but  incorrectly  called  slates,  while  the  coal  bed  is  frequently 
called  a  seam  or  vein,  although  both  names  are  incorrect. 


COAL  SERIES 


537 


or  at  least  as  much  so,  as  those  which  are  interbedded  with  bituminous 
and  anthracite  coal. 

Coal  beds  are  often  underlain  by  a  bed  of  clay,  which  in  some  re- 
gions is  of  refractory  character;  but  the  widespread  belief  that  all 
these  underclays  are  fire  clays  is  wholly  unwarranted. 

The  character  of  the  rock  overlying  a  coal  bed  is  of  some  impor- 
tance to  the  engineer.  If  firm  and  solid  it  forms  a  good  roof,  but  if 
soft  and  crumbly  it  requires  support. 

If  the  coal  measures  are  strongly  folded  the  associated  rocks  are 
sometimes  so  badly  fractured  as  to  give  considerable  trouble,  and  in 
some  regions  of  this  character  the  beds  appear  to  be  under  such  strain 
that  when  the  coal  is  mined  the  roof  or  floor  rock  being  no  longer  con- 
fined bulges  out  into  the  workings.  Sudden  movements  of  this  sort 
are  called  bumps. 

Variations  in  extent  and  thickness.  —  Few  coal  beds  are  traceable 
over  large  areas;  on  the  contrary  they  are  lens-like  in  their  nature, 


12  13  14  15  16 


FIG.  202.  —  Sections  of  Clarion  coal,  Foxburg  quadrangle,  Pa.  The  coal  has  two 
beds  with  a  variable  interval  of  clay,  shale  or  sandstone  in  between.  The 
lower  bed  has  a  persistent  "binder"  one-quarter  to  six  inches  thick  near  the 
middle  and  in  places  additional  binders.  Nos.  1,  2,  3,  4,  5  represent  both  upper 
and  lower  Clarion  coal,  while  Nos.  6  to  16  inclusive  represent  the  lower  Clarion. 
(After  Shaw  and  Munn,  U.  S.  Geol.  Survey,  Bull.  454,  1911.) 

thinning  out  eventually  in  all  directions.     But  a  bed  which  thins  out 
completely  may  reappear  a  little  farther  on  at  the  same  or  a  slightly 


538 


ENGINEERING  GEOLOGY 


different  stratigraphic  level.  Again  a  bed  of  sufficient  thickness  to 
work  in  one  mine  may  be  so  thin  in  a  neighboring  one  as  to  be  scarcely 
noticeable.  This  thinning  and  thickening  is  commonly  called  pinch- 
ing and  swelling  (Fig.  203).  In  regions  of  strong  folding  the  coal 
beds  are  sometimes  found  in  separate  synclinal  basins,  the  interven- 
ing anticlinal  folds  having  been  removed  by  erosion. 

The  thickness  between  adjoining  beds  also  varies  from  place  to 
place  and  the  separating  beds  may  thin  out  so  that  two  coal  beds 
coalesce.  Structural  features  like  this  often  render  it  difficult  to 
identify  the  same  coal  beds  in  different  sections. 

The  Mammoth  bed,  so  prominent  in  the  anthracite  basins  of  Penn- 
sylvania, splits  into  three  separate  beds  in  the  Wilkesbarre  basin. 
This  splitting  is  caused  by  the  appearance  of  beds  of  shale  (called 
"  slate  "  by  coal  miners),  which  often  become  so  thick  as  to  split  up 
the  coal  seam  into  two  or  more  beds.  (See  Fig.  202.)  When  narrow, 


FIG.  203.  —  Section  showing  irregularities  in  a  coal  bed.     a,  split;    6,  parting  of 
shale;  c,  pinch;  d,  swell;  e,  cut  out.     (From  Ries'  Economic  Geology.) 

such  a  bed  of  shale  is  called  a  parting.  The  Pittsburg  seam  of  western 
Pennsylvania  shows  a  fire-clay  parting  or  "  horseback  "  from  six  to 
ten  inches  thick  over  many  square  miles. 

Other  partings  are  sometimes  found  cutting  across  the  beds  from 
top  to  bottom.  In  some  cases  they  represent  erosion  channels  formed 
in  the  coal  during  or  subsequent  to  its  formation,  and  later  filled  by 
deposition  of  sand  and  clay.  In  other  cases  they  are  due  to  the  filling 
of  fissures  formed  by  different  causes. 

Coal  beds  may  pass  into  shale,  the  latter  representing  possibly 
islands  of  mud  or  ridges  which  rose  above  the  level  of  the  marsh  in 
which  the  coal  plants  accumulated. 

Variation  in  quality.  —  Coal  beds  change  in  quality  in  a  variety 
of  ways.  A  given  bed  may  show  uniform  composition  throughout 
its  entire  extent,  or  it  may  vary,  being  of  excellent  grade  at  one  point 
and  poor  quality  at  another.  So,  too,  a  bed  may  vary  vertically,  the 
upper  part  perhaps  being  of  a  different  nature  from  the  lower  half. 


COAL  SERIES 


539 


Several  beds,  lying  one  above  the  other,  and  separated  by  an  interval 
of  barren  rock,  may  likewise  be  quite  dissimilar. 

Such  variations  in  quality  are  commonly  due  to  varying  conditions 
of  accumulation. 

Folding.  —  All  degrees  of  folding  may  be  seen  in  different  coal 
regions,  from  the  very  gentle  folds  of  the  Ohio  field  to  the  intense 


Hi  if. 

w   & 

<?§|1  H* 


u 


flection  (Oacroes  the  Panther  Cm*  Basin 


FIG.  204.  —  Section  in  coal  basins  of  Pennsylvania,  showing  several  beds  in  same 
section  and  also  intense  folding.     (From  2d.  Penn.  Geol.  Survey.) 

crumples  of  the  Pennsylvania  anthracite  region  (Fig.  204).  In  cases 
like  the  latter,  the  intense  folding  not  only  breaks  up  the  coal,  but 
also  the  enclosing  rocks.  Since 
variations  in  the  intensity  of  the 
folding  cause  differing  dips,  the 
amount  of  inclination  of  the  beds 
affects  to  some  extent  the  method 
of  mining  to  be  employed. 

Faulting.  —  Faulting  is  not  an 
uncommon  feature  of  some  coal 
beds,  and  the  coal  is  sometimes 

badly  crushed  on  either  side  of  the    Fm.  205.  -Section  of  faulted  coal  seam, 
fracture.     The    amount    of    slip,         (After  Keyes,  la.  Geol.  Survey,  II.) 
number,  and  kind  of  faults  is  vari- 
able, depending  on  character  and  force  of  compression  or  tension  to 
which  the  beds  have  been  exposed,  nature  of  the  rocks,  etc.     In  the 


540  ENGINEERING  GEOLOGY 

Appalachian  region,  for  example,  faults  are  rare  at  the  northern  end,  as 
the  rocks  though  strongly  folded  were  more  yielding,  but  at  the  south- 
ern end  in  Alabama  where  the  associated  formations  contained  more 
rigid  beds,  faults  are  numerous. 

Classification  of  Coals 

A  number  of  different  types  of  coal  are  recognized  by  the  trade  in 
both  the  United  States  and  Canada,  and  their  differentiation  is  based 
on  physical* and  chemical  characters.  However,  no  sharp  line  of  divi- 
sion exists  between  them,  and  moreover  the  terms  are  often  used  in 
a  loose  way. 

Numerous  attempts  have  been  made  to  construct  a  satisfactory 
classification,  but  none  of  those  suggested  have  met  with  widespread 
approval.  (Refs.  7-13.)  Some  of  the  classifications  are  complex,  sev- 
eral have  to  be  figured  on  a  pure  coal  basis,  and  others  require  an  ele- 
mentary analysis  of  the  coal. 

A  simple  and  early  attempt  was  that  of  P.  Fraser,  Jr.,  which  was 
based  on  the  fuel  ratio.  It  was: 

Fuel  Ratio. 

Anthracite 100-12 

Semi-anthracite 12-8 

Semi-bituminous . 8-5 

Bituminous 5-0 

This  was  good  for  the  higher  grades,  but  is  open  to  the  objection 
that  it  does  not  separate  good  and  poor  bituminous  coals  or  indeed 
any  of  the  grades  below  bituminous. 

A  somewhat  careful  and  detailed  analysis  of  the  situation  is  involved 
in  Parr's  classification.  He  points  out,  and  it  seems  correctly,  that  the 
term  volatile  combustible  as  often  used  is  incorrect,  as  it  consists  of 
combustible  hydrocarbons  and  non-combustible  hydrogen,  oxygen  and 
nitrogen.  Thus  in  the  case  of  a  Pocahontas  coal  with  18.70  per  cent 
volatile  combustible,  14.5  per  cent  is  hydrocarbon  and  4.2  per  cent 
hydrogen,  oxygen  and  nitrogen.  Again,  a  North  Dakota  lignite  had 
41.91  per  cent  volatile  combustibles,  made  up  of  20.28  per  cent  hydro- 
carbons and  21.63  per  cent  hydrogen,  oxygen  and  nitrogen.  In  a  logical 
classification,  therefore,  allowance  should  be  made  for  this  inert  vol- 
atile matter. 

Parr  in  his  classification  uses  the  terms:  vc,  or  volatile  carbon 
unassociated  with  hydrogen,  obtained  from  C  —  fc  (total  carbon 
minus  fixed  carbon);  C,  or  total  carbon  as  determined  by  analy- 


COAL  SERIES 


541 


sis;  and  .inert  volatile  matter,  obtained  by  subtracting  from  100  per 
cent  the  sum  of  total  carbon,  available  hydrogen,1  sulphur,  ash  and 
water. 

It  will  be  seen  that  Parr's  classification,  which  follows,  requires 
data  from  both  the  elementary  and  proximate  analysis  of  the  coal. 


PARR'S  CLASSIFICATION. 
Anthracites  Proper         j  Ratio  ^  below  4%. 

r    Anthra- 
citic 


Coals 


(  vc 

Semianthracite  j  Ratio  ^  between  4%  and  8%. 


Semibituminous 


Bitumi- 
nous 


r  Bituminous  Proper 


Black  Lignites 


Brown  Lignites 


Ratio  ^from  10%  to  15%. 


C  VC 

J  Ratio  ^  from  20%  to  32%. 

A] 

I  Inert  volatile  from  5%  to  10%. 

f  Ratio  ^  from  20%  to  27%. 
Bl  C 

[  Inert  volatile  from  10%  to  16%. 

f  Ratio  ^  from  32%  to  44%. 
C\  C 

[  Inert  volatile  from  5%  to  10%. 

f  Ratio  ^  from  27%  to  44%. 

D  \ 

I  Inert  volatile  from  10%  to  16%. 

\  Ratio  p  from  27%  up. 

I  Inert  volatile  from  16%  to  20%. 

Ratio  ^  from  27%  up. 


I 


Inert  volatile  from  20%  to  30%. 


Campbell  has  also  suggested  the  possibility  of  recognizing  the  two 
classes  of  coal  below  bituminous  by  means  of  their  physical  charac- 
ters. He  believes  that  the  manner  of  weathering  can  be  used  as 
a  criterion  for  separating  the  bituminous  from  the  subbituminous, 
the  former  cleaving  into  prisms,  while  the  latter  checks  irregularly  on 
drying  (Plate  XCI)  and  when  weathered  on  the  outcrop  cleaves  into 

1  That  part  of  hydrogen  content,  excluding  the  hydrogen  united  with  oxygen  to  form 
water,  which  is  free  to  enter  into  combustion  with  oxygen  for  the  production  of  heat. 


PLATE  XCI,  FIG.  1.  —  Subbituminous  coal,  showing  the  irregular  checking  devel- 
oped in  drying. 


FIG.  2.  —  Bituminous  coal,  showing  prismatic  structure.     (After  Campbell,  Econ. 

Geol,  III.) 

(542) 


COAL  SERIES  543 

plates  parallel  to  the  bedding.  The  subbituminous  coals  with  their 
black  color  he  claims  can  be  distinguished  from  lignites,  because  the 
latter  are  brown. 

Origin  of  Coal 

Reference  to  the  members  of  the  coal  series  already  described  will 
show  that  there  is  an  undoubted  gradation  between  plant  beds  and 
anthracite  coal  (see  Refs.  1-6).  This  theory  is  strengthened  by  the 
fact  that  coal  in  addition  to  containing  the  same  elements  as  plant  tis- 
sue often  shows  the  presence  of  plant  fibres,  leaves,  stems,  seeds,  etc. 
Furthermore,  we  sometimes  find  stumps  or  trunks  of  trees  standing 
upright  in  the  coal,  with  their  roots  penetrating  the  underlying  bed 
of  clay,  just  as  trees  at  present  stand  in  bogs. 

The  early  stages  in  coal  formation  are  not  hard  to  trace,  for  we 
know  that  if  dead  vegetable  matter  accumulates  under  water,  with 
little  access  of  air,  as  in  a  peat  bog,  that  it  undergoes  a  slow  process 
of  decay,  and  physical  change,  forming  the  material  known  as  peat. 
This  differs  from  the  living  vegetable  tissue  chemically  in  having  less 
hydrogen,  oxygen  and  nitrogen  and  more  carbon,  and  physically  in 
being  more  compact,  darker,  and  showing  fewer  distinct  plant  remains. 

That  pressure  alone  will  convert  the  peat  into  a  mass  resembling 
lignite  or  even  subbituminous  coal  is  shown  by  the  behavior  of  peat 
in  the  briquetting  machine.  It  is  therefore  reasonable  to  assume 
that  as  a  deposit  of  peat  became  buried  under  a  considerable  thick- 
ness of  sediment,  it  would  become  compacted  and  consolidated. 

It  is  therefore  assumed  that  prolonged  burial  of  a  peat  bed  under 
many  feet  of  stratified  rocks  gradually  changes  the  vegetable  accumu- 
lation into  lignite  and  still  further  into  subbituminous  coal. 

It  has  been  noted  in  many  cases,  however,  that  the  rocks  are  at 
least  slightly  folded  in  bituminous  coal  areas,  and  this  leads  to  the 
suggestion  that  the  folding  not  only  indicates  additional  pressure, 
but  that  the  same  force  generated  some  heat,  and  drove  off  more 
volatile  matter. 

The  fact  that  the  anthracite  coal  of  Pennsylvania  is  found  in  a 
region  of  strong  folding  lends  color  to  this  view. 

As  further  bearing  on  this  point  we  may  refer  to  some  of  the  coals 
of  Montana,  where  the  lignite  is  found  in  practically  flat  rocks  un- 
derlying the  Plains,  while  the  bituminous  coals  occur  in  the  moun- 
tains where  the  beds  are  tilted  due  to  folding. 

There  is  some  question  of  course  how  much  heat  was  involved  in 
the  process  of  coal  formation,  and  whether  long  pressure  with  mod- 


544 


ENGINEERING  GEOLOGY 


erate  temperature  could  not  have  brought  about  considerable  meta- 
morphism  in  the  coal. 

A  condition  also  stipulated  by  some  (Campbell)  is  that  the  rocks 
must  have  been  sufficiently  broken  by  joints  to  permit  the  escape  of 
the  more  volatile  matter  during  the  coal  metamorphosis,  otherwise 
marked  folding  of  coal  beds  might  result  without  changing  them  much. 

Heat  alone  is  no  doubt  a  powerful  factor  in  changing  coal,  for  where 
the  beds  have  been  cut  by  dikes  of  igneous  rock  we  find  the  coal  on 
either  side  changed  to  natural  coke  or  in  some  cases  graphite.  Or 
as  in  the  Crested  Butte  area  of  Colorado,  where  a  bed  of  bituminous 
coal  has  been  locally  changed  to  anthracite  by  the  intrusion  of  ba- 
saltic rock  into  the  underlying  beds. 

While  the  foregoing  explanation  assumes  that,  in  general,  the  suc- 
cession peat,  lignite,  etc.,  is  a  strictly  lineal  one,  this  theory  cannot 
be  said  to  be  universally  accepted.  J.  J.  Stevenson,  among  others, 
has  argued  that  anthracite  coal  has  not  been  developed  from  bitumi- 
nous coal  by  metamorphism,  but  that  the  volatile  constituents  were  in 
part  removed  by  longer  exposure  of  the  vegetable  matter  to  oxidation 
before  burial. 

Technology  of  Coal 

Calorific  power  of  coals  (Ref .  20) .  —  The  calorific  power  of  coal 
may  be  expressed:  (1)  in  calories,  or  the  number  or  units  (kilograms) 
of  water,  which  one  unit  (kilogram)  of  fuel  will  raise  1°  C.  or,  (2)  in 
British  thermal  units  (B.  T.  U.),  or  the  number  of  pounds  of  water 
which  one  pound  of  fuel  will  raise  1°  F. 

The  following  figures  are  taken  from  the  reports  of  the  United 
States  Geological  Survey.  Many  others  will  be  found  there. 

CALORIFIC  POWER  OF  COALS 


Kind. 

Calories. 

B.T.U. 

Peat,  high  ash,  York,  Me  

2019 

3  634 

Peat  

4559 

8,206 

Lignite,  Tesla,  Cal  

4503 

8,105 

Brown  lignite   Lehigh  N   Dak 

3421 

6  158 

Brown  lignite,  Williston,  N.  Dak.    .  .  . 

3603 

6  485 

Subbituminous,  Miles,  Mont.         

4432 

7977 

Bitum  nous,  Coffeen,  111  

6031 

10856 

Carterville,  111  

6666 

11,999 

West  Mineral  Kan 

7181 

12  926 

'        Straight  Creek  Ky 

7986 

14  375 

Westernport,  Md                                   ... 

7696 

13  853 

'        Roslyn,  Wash            

8352 

15034 

Semibituminous,  Bonanza,  Ark  

6067 

10,920 

Semianthracite    Blacksburg    Va 

7112 

12  801 

Anthracite,  Scranton,  Pa.  . 

6929 

12.472 

546 


ENGINEERING  GEOLOGY 


Coke  (Refs.  21,  22). —  Artificial  coke  is  made  by  subjecting  bitu- 
minous coals  to  a  high  temperature  either  with  the  air  entirely  ex- 
cluded or  by  permitting  the  access  of  only  enough  air  theoretically 
to  burn  the  volatile  matter  given  off  from  the  coal.  The  former 
process  is  distillation,  the  latter  partial  combustion. 

In  distillation  the  coal  is  usually  crushed  to  half  an  inch  or  smaller,  and  charged 
into  retorts  which  are  about  30  feet  long,  6  to  8  ft.  high,  and  17  to  22  inches  wide. 
The  heat  is  supplied  by  the  combustion  gases  of  the  coal  which  pass  through  flues 
in  the  walls  of  the  retort  oven  as  it  is  called. 

The  coking  by  partial  combustion  is  done  in  beehive  ovens  usually  12  to  13  feet 
diameter,  6  to  7  feet  high  in  the  center,  and  3  feet  at  the  circumference.  Each  oven 
holds  6  to  8  tons  of  coal,  and  the  coking  process  takes  48  to  72  hours.  A  modification 
of  this  is  now  much  used. 

In  the  retort  ovens  the  volatile  gases  are  saved  as  by-products,  yielding  gas,  tar, 
ammonium  sulphate,  etc.  The  coke  from  either  type  of  oven  is  suitable  for  blast 
furnace,  foundry  or  smelter  purposes. 

The  following  analyses  give  I,  the  analysis  of  coal  from  Ellsworth, 
Pa.;  II,  coke  from  same;  and  III,  range  of  composition  of  Penn- 
sylvania cokes. 

ANALYSES  OF  PENNSYLVANIA  COKES 


I. 

II. 

III. 

Moisture 

4  73 

0  23 

0  23-  0  91 

Volatile  matter. 

34  29 

1  19 

0  29-  2  26 

Fixed  carbon.  .  .  . 

56  27 

91  63 

80  84-92  53 

Ash  

4  71 

6  95 

6  95-15  99 

Sulphur  

0  94 

0  81 

0  81-  1  87 

The  upper  limits  in  III  for  ash,  sulphur  and  volatile  matter  are 
extreme  cases  either  of  imperfectly-made  coke  or  of  coke  made  from 
coal  that  is  not  generally  used  for  the  purpose. 

While  it  is  recognized  that  many  bituminous  coals  will  coke,  still 
the  cause  of  coking  is  not  clearly  understood,  and  the  chemical  analysis, 
so  far  as  we  are  able  to  interpret  it,  does  not  appear  to  throw  much 
light  on  the  matter.  It  may  have  some  connection  with  the  char- 
acter of  the  plants  which  formed  the  coal. 

The  safest  way  to  determine  the  coking  qualities  of  a  coal  is  by 
means  of  a  practical  test.  It  has  been  suggested,  however,  that  the 
coking  qualities  of  a  coal  can  be  inferred  with  fair  accuracy  by  its 
behavior  when  ground  in  an  agate  mortar.  Coals  of  good  coking 
character  stick  to  the  mortar,  while  those  of  opposite  quality  are 
easily  brushed  loose. 


COAL  SERIES 


547 


David  White  has  also  claimed  that  the  coking  value  of  a  coal  seems 
to  be  indicated  with  fair  accuracy  by  the  hydrogen-oxygen  ratio,  cal- 

TT 

culated  on  a  moisture-free  basis.     Practically  all  coals  with  -^  =  58 

IT 

possess  coking  qualities.    Most  coals  with-yr  down  to  55  make  coke 

of  some  kind,  a  few  with  ratios  as  low  as  50  will  coke,  though  the  prod- 
uct is  rarely  good.  This  test  may  fail  as  a  guide  in  those  coals  which 
are  undergoing  anthracitization.  However,  while  these  laboratory 
tests  may  indicate  whether  a  coal  will  coke,  they  do  not  give  us  definite 
evidence  regarding  the  physical  character  of  the  product,  which  is  a 
matter  of  considerable  importance. 

Natural  coke  or  carbonite.  —  Natural  coke  is  occasionally  found 
in  coal  deposits,  and  has  been  formed  by  igneous  rocks  cutting  across 
coal  seams.  Thus  in  Utah,  for  example,  "  dikes  of  igneous  rock  ten 
feet  in  width  have  cut  vertically  across  a  coal  bed  nine  to  sixteen  feet 
thick,  metamorphosing  the  coal  into  a  coke-like  substance  to  a  dis- 
tance of  three  feet  on  either  side.  The  coke  thus  formed  is  distinctly 
columnar,  the  columns  standing  perpendicular  to  the  face  of  the  dike; 
it  has  a  graphitic  lustre,  but  is  not  vesicular  like  artificial  coke." 
Natural  coke  is  also  found  in  the  Cerillos  field  of  New  Mexico,  the 
Crested  Butte  area  of  Colorado,  and  the  Richmond  coal  basin  of 
Virginia. 

In  the  following  analyses  I  and  II  give  the  composition  of  two 
natural  cokes  and  III  of  artificial  coke. 

ANALYSES  OF  COKE 


I. 

II. 

III. 

Moisture  

1.116 

1.66 

0.29 

Volatile  matter 

11  977 

18  35 

0  59 

Fixed  carbon 

75.881 

67.13 

93.84 

Ash  ... 

11.881 

12.86 

5.28 

Sulphur  .,  

4.70 

0.357 

Phosphorus 

0  018 

It  will  be  noticed  that  the  quantity  of  volatile  matter  is  higher  in 
carbonite  than  in  artificial  coke.  This  may  be  due  to  its  having  been 
formed  at  some  depth  below  the  surface,  thus  preventing  the  escape 
of  the  volatile  matter;  or  it  may  be  due  to  short  heating,  or  enrich- 
ment by  gases  from  the  neighboring  coal. 

At  all  events,  carbonite  is  of  no  commercial  value. 


550 


ENGINEERING  GEOLOGY 


Coke-oven  tar  (Ref.  19).  —  The  tar  which  is  saved  from  the  by-product  coke  ovens 
is  of  interest  to  engineers  as  the  use  of  refined  coal  tar  for  the  treatment  and  construc- 
tion of  roads  is  rapidly  increasing  in  this  country.1 

The  growing  demand  for  it  will  probably  lead  to  a  more  widespread  use  of  retort 
coke  ovens.  As  one  ton  of  coal  yields  on  the  average  about  10  gallons  of  tar,  it  has 
been  estimated  from  the  amount  of  coal  coked  in  non-recovery  ovens,  that  the  quan- 
tity of  tar  now  allowed  to  escape  is  sufficient  to  build  9000  miles  of  tar-macadam 
road  15  feet  wide. 

Hubbard  states  that  straight  coal-tar  roadbinders  or  refined  coal  tars  are  usually 
made  by  distilling  the  crude  material.  For  construction  work  a  soft  and  almost 
fluid  pitch  is  often  used.  If  it  runs  too  high  in  free  carbon,  crude  water-gas  tar  may 
be  mixed  with  it  before  distillation,  as  this  is  low  in  free  carbon.  A  high-carbon  tar 
is  difficult  to  distil  properly. 

Coke-oven  tars  are  considered  well  adapted  to  roadbinders. 

"In  an  ordinary  road-tar  for  use  in  construction  work  where  free  carbon  is  present 
to  the  extent  of  about  20  per  cent,  the  proportion  of  total  distillate,  below  315°  C., 
to  pitch  residue  is  approximately  1  to  4.  Where  this  relation  exists  the  pitch  residue 
is  hard  and  brittle.  A  residue  which  is  soft  or  plastic  is  to  be  preferred,  as  it  would 
indicate  longer  life  during  service,  and  where  such  a  residue  is  present  the  proportion 
of  distillate  would  naturally  be  lower  for  a  given  consistency,  as  the  distillate  may 
be  considered  as  fluxes  for  the  residues." 

Of  31  pitch  residues  from  coke-oven  tars,  14  were  soft  or  plastic  after  distillation, 
a  condition  rare  in  gas-house  coal  tars. 

The  following  are  some  analyses  of  coke-oven  tars  given  on  a  water-free  basis, 
but  in  most  of  these  this  was  under  3  per  cent. 

ANALYSES  OF  COKE-OVEN  TARS 


Locality. 

Fractions  by  weight. 

Per  cent 
free  carbon. 

Per  cent 
up  to 
110°  C. 

Per  cent 
110-170°  C. 

Per  cent 
170-270°  C. 

Per  cent 
270-315°  C. 

Per  cent 
of 
pitch. 

Syracuse,  N.  Y  

7.82 
4.73 
9.00 
14.22 
2.81 
17.17 

0.30 
1.30 
1.42 
2.34 
1.03 
0.30 

0.70 
0.60 
0.20 
0.51 
0.30 
1.73 

11.59 
15.57 
18.10 
20.81 
19.07 
10.12 

7.35 

8.44 
5.79 
14.69 
6.70 
10.42 

79.73 
74.07 
74.36 
60.91 
72.37 
76.68 

Lebanon,  Pa  

Dunbar,  Pa  
Everett,  Mass  

Gary   Ind 

Buffalo,  N.  Y.. 

Use  of  coals  in  gas  producers  (Refs.  15,  16).  —  Some  of  the  western 
states  have  but  little  good  coal.  The  low-grade  coals  which  occur  in 
large  quantities  cannot  be  used  in  boiler  furnaces,  and  many  will  not 
bear  long  transportation.  In  other  states  where  good  coals  occur, 


1  P.  Hubbard,  Dust  Preventives  and  Road  Binders,  p.  239;  also  U.  S.  Dept. 
Agric.,  Office  Public  Roads,  Circ.  97,  1912. 


COAL  SERIES 


551 


there  is  also  a  considerable  quantity  of  bone 1  coal,  and  also  slack  2 
coal,  which  goes  to  waste. 

The  proposition  of  saving  these  by  utilizing  them  in  gas  pro- 
ducers has  been  strongly  advocated  by  many,  including  the  Bureau 
of  Mines,3  and  tests  made  by  the  above-named  Bureau  have  shown 
that  the  method  is  not  only  practicable  but  economically  possible. 

It  has  been  estimated  that  on  an  average  each  coal  tested  in  the 
producer-gas  plant  developed  two  and  one-half  times  the  power  that 
it  would  develop  in  the  ordinary  steam-boiler  plant. 

Thus  a  low-grade  North  Dakota  lignite  when  converted  into  producer  gas  de- 
veloped as  much  power  as  the  best  West  Virginia  bituminous  coal,  burned  under  the 
steam  boiler. 

For  simply  steam-boiler  work,  it  is  questionable  whether  any  advantage  is  gained 
by  the  use  of  producers  for  high-grade  fuels,  except  the  reduction  or  elimination  of 
smoke,  but  for  low-grade  fuels  there  is  a  decided  gain. 

The  composition  of  producer  gas  varies  greatly,  depending  on  the  type  of  the  pro- 
ducer, method  and  skill  used  in  operating,  regulation  of  air  and  steam  supply,  fuel 
used,  etc. 

The  following  are  given  in  the  Bureau  of  Mines  Report  as  typical  analyses  of 
up-draft,  pressure-producer  gas. 

TYPICAL  ANALYSES  OF  UP-DRAFT  PRESSURE-PRODUCER  GAS 
(Percentages  by  volume) 


Constituents. 

From  bitumi- 
nous coal. 

From  lignite. 

From  peat. 

Carbon  dioxide  (CO2)  

9.84 

10.55 

12.40 

Oxygen  (O2)               

0.04 

0.16 

0.00 

Ethylene  (C2H4) 

0  18 

0.17 

0.40 

Carbon  monoxide  (CO) 

18.28 

18.72 

21.00 

Hydrogen  (H2)                                   

12.90 

13.74 

18.50 

Methane  (CH4)                      

3.12 

3.44 

2.20 

Nitrogen  (N2)      

55.64 

53.22 

45.50 

100.00 

100.00 

100.00 

Carbon  monoxide,  hydrogen,  ethylene  and  methane  are  regarded  as  desirable 
constituents,  but  the  suitability  of  the  gas  for  a  particular  industrial  application 
depends  on  the  relative  proportion  of  these  constituents. 

Coal  briquetting  (Refs.  27,  29).  --The  term  briquet  is  applied  to 
the  product  obtained  by  compressing  fine  coal  or  lignite  into  different 
shapes,  either  with  or  without  the  use  of  binding  material.  The  forms 
made  are  known  as  briquets,  eggettes,  boulets,  carbonets,  coalettes, 

etc. 

1  Coal  with  shaly  streaks. 

2  Fine  or  broken  coal. 

3  Bur.  of  Mines,  Tech.  Paper  9  and  Bull.  13. 


PLATE  XCV,  FIG.  1.  —  Lignite  briquets  at  beginning  of  weathering  test. 


FIG.  2.  —  Same  after  9  days.     (After  Wright,  U.  S.  Bur.  Mines,  Bull.  14,  1911.) 
(552) 


PLATE  XCVI,  FIG.  1.  — Same  briquets  as  in  Plate  XCV,.  after  226  days' 

weathering. 


FIG.  2.  —  The  same  after  286  days'  weathering.     (Wright,  U.  S.  Bur.  Mines, 

Bull.  14,  1911.) 

(553) 


554  ENGINEERING  GEOLOGY 

The  successful  development  of  the  industry  in  the  United  States 
depends  on  the  ability  to  use  low-grade  fuel  materials,  and  the  pro- 
duction of  an  article  that  will  compete  successfully  with  raw  coal  or 
coke. 

The  low-grade  fuels  that  could  be  used  are:  (1)  Anthracite  culm; 
(2)  slack  coal  from  semianthracite,  bituminous,  and  subbituminous 
coals  of  non-coking  character;  and  (3)  lignite,  which  disintegrates  in 
storage  or  long  transportation. 

The  briquetting  industry  has  not  developed  very  rapidly  in  the 
United  States  because  of  (1)  a  large  supply  of  cheap  fuel,  (2)  high 
labor  cost,  and  (3)  unsuccessful  attempts  to  exploit  secret  processes. 

The  industry  will  probably  be  most  valuable  in  those  regions  where 
there  are  large  fields  of  lignite  somewhat  remotely  located  from  areas 
of  better  coal.1 

Escape  of  gas  from  coal  (Ref.  26).  —  Inflammable  gas,  consisting 
chiefly  of  marsh-gas  (methane,  CH4),  called  also  fire  damp  by  coal 
miners,  escapes  from  coal  in  many  mines.  Little  is  said  to  be  known 
regarding  the  condition  of  this  gas  in  the  coal  or  its  quantity  and  rate 
of  escape.  An  additional  quantity  may  also  come  from  the  rocks 
above  and  below  the  coal. 

Coal  does  not  give  off  gas  alone  during  the  period  of  mining,  but  may  yield  it 
continuously  for  a  long  time  after  it  has  been  mined,  this  fact  having  been  demon- 
strated by  experiments  made  on  certain  American  coals.  The  escape  is  rapid  at 
first,  but  diminishes  in  rate  and  ceases  after  several  months.  The  loss  in  fuel  value 
due  to  the  escape  of  gas  is  small,  but  the  danger  of  accumulation  of  explosive  gas  from 
this  source  in  mines  and  coal  bunkers  is  sufficient  to  call  for  proper  ventilation  in 
mines  and  coal  storages. 

Coal  on  the  whole  seems  to  suffer  but  slight  calorific  loss  when 
stored  for  some  time.  Storage  under  water  preserves  strength,  but  it 
is  questionable  whether  this  gain  offsets  the  disadvantages  of  having 
to  fire  wet  coal.  It  of  course  prevents  spontaneous  combustion,  and 
so  might  be  justified  where  the  coal  is  particularly  dangerous  on  this 
account.2 

Distribution  of  Coal  in  the  United  States 

The  occurrences  of  coal  and  lignite  in  the  United  States  can  be 
grouped  in  the  following  regions: 

Area, 
sq.  mi. 

1.  Appalachian,  including  parts  of  Pennsylvania,  Ohio,  Mary- 
land, Virginia,  West  Virginia,  Eastern  Kentucky,  Ten- 
nessee, Georgia,  and  Alabama 69,812 

1  See  Bur.  of  Mines,  Bull.  14;  U.  S.  Geol.  Survey,  Bull.  316,  343,  366,  385,  403. 

2  See  Tech.  Paper  16,  Bur.  of  Mines. 


COAL  SERIES  555 

Area, 

2.  Atlantic  Coast  Triassic,   including  parts  of  Virginia  and        **• mi" 

North  Carolina 210 

3.  Eastern  Interior,  including  parts  of  Indiana,  Illinois,  and 

Western  Kentucky 48,500 

4.  Northern  Interior,  including  parts  of  Michigan 11,000 

5.  Western  Interior,  including  parts  of  Iowa,  Missouri,  Ne- 

braska, Kansas,  Oklahoma,  and  Arkansas 71,664 

6.  Southwestern,  including  parts  of  Texas 13,500 

7.  Gulf  Coast  Lignite  region,  including  parts  of  Alabama,  Mis- 

sissippi, Louisiana,  Arkansas,  and  Texas 84,300 

8.  Rocky  Mountain  region,  including  parts  of  Colorado,  Ari- 

zona, New  Mexico,  Utah,  Wyoming,  Idaho,  Montana, 

North  Dakota,  and  South  Dakota 195,960 

9.  Pacific  Coast  region,  including  parts  of  Washington,  Oregon, 

and  California 1,830 

10.   Alaska 

The  estimates  of  areas  given  above  are  from  calculations  made  by 
the  United  States  Geological  Survey,  and  are  to  be  considered  as 
fairly  accurate,  although  they  may  be  extended  by  future  develop- 
ment of  areas  now  regarded  as  unproductive.  Much  coal  now  lies 
too  deep  to  be  profitably  mined,  but  this  may  be  sought  in  the  future 
when  other  more  easily  accessible  supplies  become  exhausted,  or 
mining  methods  cheapened. 

Statistics  of  production  show  that  the  production  of  the  individual 
fields  is  by  no  means  proportional  to  their  area.  Proximity  to  mar- 
kets, value  of  the  coal  for  fuel,  and  relative  quantity  of  coal  per  square 
mile  of  productive  area  are  factors  of  importance  in  determining  the 
output  of  a  field. 

Each  field  may  be  surrounded  by  a  zone  whose  markets  are  domi- 
nated by  it,  while  between  it  and  the  neighboring  field  there  is  a  belt 
in  which  the  coals  from  both  fields  compete,  assuming  them  to  be  of 
the  same  character. 

Geologic  distribution.  —  There  is  not  necessarily  any  direct  re- 
lation between  the  kind  of  coal  and  its  geological  age,  especially  in 
formations  later  than  Carboniferous. 

Coals  belonging  to  the  Carboniferous  system  are  found  east  of  the 
100th  meridian  and  include  not  only  the  best  coals  of  the  country, 
but  also  most  of  those  east  of  the  line  mentioned.  Triassic  coals  are 
found  in  Virginia  and  North  Carolina. 

Cretaceous  coals  lie  between  the  100th  and  115th  meridian,  and 
Tertiary  coals  chiefly  west  of  the  120th;  an  exception  to  the  latter 
being  a  large  area  of  Tertiary  lignites  in  the  Gulf  States. 

The  character  and  structure  of  the  coals  may  be  briefly  referred  to. 


556 


ENGINEERING  GEOLOGY 


Appalachian  region.  —  This  is  the  most  important  coal  region  in  the 
United  States,  extending  a  distance  of  850  miles  from  northeastern 
Pennsylvania  to  Alabama,  and  it  is  estimated  that  about  75  per  cent 
of  its  area  contains  workable  coal. 

At  the  southern  end  the  coal  measures  pass  beneath  the  Coastal 
Plain  deposits,  and  they  may  possibly  connect  to  the  westward  with 
the  Arkansas  coals. 

The  coals  of  this  region  are  closely  associated  with  the  Appalachian 
Mountain  uplift,  and  hence  show  a  similar  structure.  Thus  on  the 
eastern  edge  of  the  field,  the  coal-bearing  formations  are  much  folded, 
while  at  its  southern  end  they  are  faulted  in  addition. 

To  the  westward  the  folds  become  gentle.  The  coal  beds  are  not 
continuous  over  the  entire  field,  for  extensive  erosion  has  left  them 
as  a  series  of  somewhat  disconnected  basins. 

This  variation  in  structural  conditions,  coupled  with  variations  in 
topography,  demands  therefore  a  somewhat  diversified  method  of 
mining. 

The  coal  measures  of  the  Appalachian  region  consist  of  a  great  thick- 
ness of  overlapping  lenses  of  conglomerate,  sandstone,  limestone, 
shale,  fire  clay,  and  coal.  This  means  that  the  coal  beds  are  not  as 


FIG.  206.  —  Coal  breaker  in  Pennsylvania  anthracite  region.     (From 
Hies'  Economic  Geology.) 

a  rule  continuous  over  long  distances,  and  while  a  fairly  uniform  suc- 
cession of  beds  is  identifiable  in  Pennsylvania,  Ohio,  Maryland,  and 
West  Virginia,  the  problem  is  less  clear  in  the  more  southern  states. 
The  coals  range  from  bituminous  to  anthracite. 

The  anthracite  is  confined  to  the  highly-folded  area  of  northeast- 


101°    Longitude 


BITUMINOUS  AND  ANTHRACITE  COAL 
A   Indicates  anthracite  coal    Q  coking  coal 


Areas  Area3  that  may  Ajgs(a  ^^^  containlng 

containing  workable  contain  workable  Workable  coal  beds  under 

coal  beds  such  heavy  cover  as  not 

to  be  available  at  present 


Areas 

contacting  workabh 
coal  beds 


PLATE  XCVII.  —  Map  of  coal  fields  of  tl 


bom          Greenwich      93° 
UBBTTUMINOUS  COAL 


77° 


LJGNITE 


Areas  that  may  Areas  probably  containing 

contain  workable  workable  coal  beds  under 

coal  beds  such  heavy  cover  aa  not 

to  be  available  at  present 


CH3 


Areas  Areas  that  may 

containing  workable  contain  workable 

lignite  beds  lignite  beds 


United  States.     (U.  S.  Geol.  Survey.) 


COAL  SERIES  557 

central  Pennsylvania,  and  is  utilized  for  fuel  purposes.  Owing  to  its 
peculiar  physical  character  it  can  be  crushed  to  any  size,  and  in  this 
operation  much  of  the  shale  and  shaly  coal,  called  bone,  is  separated. 
The  fine-grained  refuse  from  the  coal  breakers,  known  as  culm,  has, 
however,  during  the  period  of  mining  accumulated  in  enormous  quan- 
tities, and  its  utilization  has  presented  an  interesting  engineering 
problem.  A  great  deal  of  it  is  now  washed  and  screened  to  save  the 
fine  particles  of  clean  coal;  and  much  is  also  washed  into  the  mines 
to  support  the  roof,  so  that  the  pillars  of  coal  originally  left  for  that 
purpose  can  be  removed. 

The  finer  sizes  such  as  buckwheat,  rice,  and  barley,  which  are  ob- 
tained by  washing  the  culm,  are  important  as  steam-raising  fuels,  and 
are  much  used  for  this  purpose  in  large  buildings,  especially  where 
smoke-prohibiting  ordinances  exist.  Their  use  requires  special  grates 
and  furnaces,  but  they  represent  the  only  grade  of  anthracite  that  can 
compete  with  bituminous  coal  for  steam  raising  in  the  eastern  markets. 

Outside  of  the  anthracite  fields,  the  coal  with  very  few  exceptions 
is  bituminous  or  semibituminous.  Coking  coal  is  found  throughout 
the  entire  extent  of  the  field;  but  most  of  the  coke  is  made  from  coals 
along  the  eastern  border,  the  coking  qualities  seeming  to  disappear 
towards  the  western  margin.  The  Connelsville  district  in  Fayette 
and  Westmoreland  counties  of  southwestern  Pennsylvania  is  espe- 
cially prominent. 

Excellent  steaming  coals  are  mined  in  Clearfield,  Allegheny,  and 
Washington  counties  of  Pennsylvania;  in  the  Hocking  district  of 
Ohio;  in  northern  West  Virginia;  and  in  the  Pocahontas  district  of 
southwest  Virginia,  the  latter  being  known  as  smokeless,  as  well  as 
coking. 

Other  well  known  grades  are  the  Youghiogheny  gas  coals  of  south- 
western Pennsylvania;  the  Cumberland  smithing  coal  of  Maryland 
and  adjacent  counties  in  West  Virginia;  the  Kanawha  splint  and  gas 
coals,  the  Massillon  domestic  coal  of  Ohio,  and  high-grade  steaming 
and  domestic  coals  of  the  Jellico  basin. 

Eastern  Interior  field.  —  This  field  is  an  oval  elongated  basin 
extending  northeast  and  southwest,  with  the  marginal  beds  dipping 
gently  towards  the  lowest  portion,  which  lies  in  Illinois. 

The  coal  of  this  field  is  all  bituminous,  but  varies  in  quality.  That 
on  the  eastern  edge  of  the  field  is  called  block  or  semi-block,  because 
of  its  peculiar  jointing.  It  is  very  pure,  dry,  and  non-coking. 

The  rest  of  the  coal,  which  is  known  locally  as  bituminous  and  forms 
more  persistent  beds  than  the  block  coal,  is  classed  as  coking  and  gas 


!3 

e* 


§ 

HS 


COAL  SERIES 


559 


coal,  but  is  not  sufficiently  high-grade  to  compete,  for  these  purposes, 
with  the  high-grade  coking  coals  of  the  eastern  states.  For  steaming 
purposes  it  competes  with  the  Appalachian  coals.  Cannel  coal  is 
mined  at  one  or  two  points. 

Northern  Interior  or  Michigan  region.  —  This  region  forms  a  large 
basin,  with  the  beds  dipping  irregularly  from  the  margin  towards  the 
center.  Owing  to  the  heavy  covering  of  unconsolidated  deposits  such 
as  glacial  drift,  outcrops  are  scarce,  and  prospecting  has  to  be  done 
by  drilling.  The  coals,  which  are  all  bituminous,  are  used  chiefly  for 
fuel,  but  some  are  coking  and  others  may  prove  of  value  for  gas 
manufacture. 

Western  Interior  region.  —  The  coal  measures,  composed  of  lime- 
stones, shales,  and  coal  beds,  have  in  general  a  gentle  western  dip,  and 
are  divisible  into  two  parts,  of  which  the  lower  is  on  the  whole  the 
more  important. 

The  coals  are  all  essentially  bituminous.  Those  of  Iowa  are  mostly 
of  low-grade,  non-coking  character,  but  have  fairly  good  steaming 


Vertical  scale 


FIG.  207.  —  Generalized  section  of  Michigan  coal  region.  Shows  irregularity  of  beds 
and  the  entire  absence  of  outcrops  due  to  heavy  surface  covering.  (After  Lane, 
U.  S.  Geol.  Survey,  22d  Ann.  Kept.,  III.) 

qualities.  On  account  of  the  high  sulphur  content  of  many,  they  do 
not  stock  well.  Much  bituminous  coal  is  mined  in  Kansas,  and  some 
coking  coal  is  found.  The  Missouri  coals  are  similar  to  the  Iowa  ones 
in  quality.  Arkansas  produces  both  bituminous  and  semibituminous 
coal;  indeed  the  latter  is  sometimes  termed  semianthracite.  The 
quality  increases  from  east  to  west,  and  the  beds  are  often  folded. 

Rocky  Mountain  region.  —  This  includes  a  number  of  separate  areas 
extending  from  the  Canadian  boundary  southward  into  New  Mexico 
and  Arizona.  The  coals  range  in  grade  from  lignite  to  anthracite. 
Portions  of  this  area  are  only  slightly  disturbed,  but  in  others  moun- 


560  ENGINEERING  GEOLOGY 

tain-building  forces  and  igneous  intrusions  have  affected  a  large  pro- 
portion of  the  region,  often  materially  changing  the  character  of  the 
coal.  Thus  the  bituminous  coal  in  the  Crested  Butte  area,  Colo- 
rado, or  the  Cerrillos  field,  New  Mexico,  has  been  locally  changed  to 
anthracite  by  igneous  intrusions.  Some  of  the  coal  yields  a  high- 
grade  coke,  such  as  that  of  Trinidad  and  Glenwood  Springs,  Colorado. 
Some  of  the  lignite  is  also  profitably  used  because  of  its  nearness  to 
market. 

Pacific  Coast  region.  —  Coals  ranging  from  lignitic  to  bituminous 
occur  scattered  over  a  wide  area  in  the  states  of  California,  Wash- 
ington, and  Oregon,  but  the  individual  fields  are  not  large.  Those 
of  Washington,  which  are  mainly  bituminous,  are  the  most  important. 


References  on  Coal 

Origin. —  1.  Campbell,  Econ.  Geol.,  I,  p.  26,  1905.  2.  Clarke, 
U.  S.  Geol.  Surv.,  Bull.  491,  p.  705,  1911.  3.  Ries,  Economic  Geology, 
3rd.  ed.,  p.  7,  1910.  4.  Stevenson,  Amer.  Philosophical  Soc.,  L,  pp. 
1  and  519,  1911.  5.  Stevenson,  Geol.  Soc.  Amer.,  Bull.  V,  p.  39,  1894. 
6.  White,  Econ.  Geol.,  Ill,  p.  292,  1908. 

Classification.  —  7.  Campbell,  Econ.  Geol.,  Ill,  p.  134,  1908,  and 
Amer.  Inst.  Min.  Engrs.,  Trans.  XXXVI,  p.  324,  1906.  8.  Collier, 
U.  S.  Geol.  Surv.,  Bull.  218,  1903.  9.  Bowling,  Can.  Min.  Inst., 
Quart.  Bull.  No.  1,  p.  61,  1908.  10.  Frazer,  Amer.  Inst.  Min.  Engrs., 
Trans.  VI,  p.  430.  11.  Grout,  Econ.  Geol.,  II,  p.  225,  1907.  12. 
Parr,  111.  Geol.  Surv.,  Bull.  3,  1906.  13.  White,  U.  S.  Geol.  Surv., 
Bull.  382,  1909. 

Technologic.  —  14.  Clement,  Adams  and  Hoskins,  Factors  in  forma- 
tion of  producer  gas,  Bur.  Mines,  Bull.  7,  1911.  15.  Fernald  and 
Smith,  Producer  gas  investigations,  Bur.  Mines,  Bull.  13,  1911.  16. 
Fernald,  Producer  Gas  Power  Plants,  Bur.  Mines,  Bull.  9,  1910,  and 
Bull.  55,  1913.  17.  Frazer  and  Hoffman,  Coal  Constituents  Soluble 
in  Phenol,  Bur.  Mines,  Tech.  Paper  5,  1912.  18.  Holmes,  Sampling 
Coal  in  Mine,  Bur.  Mines,  Tech.  Paper  1,  1911.  19.  Hubbard,  Coke 
Oven  Tars,  U.  S.  Dept.  Agric.,  Office  Pub.  Roads,  Cir.  97,  1912.  20. 
Parker  and  others,  U.  S.  Geol.  Surv.,  Prof.  Paper  48  and  Bulls.  261 
and  290,  containing  analyses,  tests,  etc.  21.  Moldenke,  Coke  Indus- 
try of  U.  S.,  Bur.  Mines,  Bull.  3,  1910.  22.  Pishel,  Econ.  Geol.,  Ill, 
p.  265,  1908.  (Test  for  coking  coal.)  23.  Pope,  Sampling  Coal 
Deliveries,  and  Government  Specifications,  Bur.  Mines,  Bull.  63,  1913. 


COAL  SERIES  561 

24.  Porter  and  Ovitz,  Volatile  Matter  in  Coal,  Bur.  Mines,  Bull.  1, 
1910.  25.  Porter  and  Ovitz,  Deterioration  of  Coal  in  Storage,  Bur. 
Mines,  Tech.  Paper  16,  1912.  26.  Porter  and  Ovitz,  Escape  of  Gas 
from  Coal,  Bur.  Mines,  Tech.  Paper  2,  1911.  27.  Porter  and  others, 
Can.  Dept.  Mines,  Mines  Branch,  Vols.  I,  II,  III  (Investigation  of 
Canadian  coals).  28.  Randall  and  Kreisinger,  N.  Dak.  lignite  as  fuel 
for  power  plant  boilers,  Bur.  Mines,  Bull.  2,  1910.  29.  Wright,  Bri- 
quetting  tests  of  lignite,  Bur.  Mines,  Bull.  14,  1911. 

References  on  Peat.  —  30.  Bastin  and  Davis,  U.  S.  Geol.  Surv., 
Bull.  376,  1909.  (Maine,  and  many  analyses.)  31.  Dachnowski, 
Ohio  Geol.  Surv.,  4th  ser.,  Bull.  16,  1912.  (Uses  and  origin.)  32. 
Davis,  Mich.  Geol.  Surv.,  Ann.  Kept.,  1906,  p.  105,  1907.  (General 
and  Mich.)  33.  Haanel,  Dept.  Mines,  Can.,  Spec.  Bull.  1912.  (Peat 
as  fuel  for  power.)  34.  Nystrom,  Dept.  Mines,  Can.,  Spec.  Bull. 
1908.  (Manufacture  and  uses.)  35.  Parmelee  and  McCourt,  N.  J. 
Geol.  Surv.,  Kept,  1905,  p.  223,  1906.  (N.  J.  and  analyses.)  36. 
Ries,  N.  Y.  State  Museum,  54th  Ann.  Rept.,  1903.  (New  York,  origin 
and  uses.) 

Area!  Reports.  —  The  Contributions  to  Economic  Geology  issued 
annually  by  the  U.  S.  Geological  Survey  contain  a  number  of  short 
papers  on  different  coal  districts,  especially  western  ones.  Professional 
Paper  48  of  the  U.  S.  Geological  Survey,  and  Bulletins  261  and  290, 
contain  a  number  of  analyses  and  tests.  Bulletin  22,  U.  S.  Bureau  of 
Mines,  contains  many  analyses. 

Special  reports  have  also  been  issued  by  the  Geological  Surveys  of 
Alabama,  Georgia,  Illinois,  Indiana,  Iowa,  Kansas,  Kentucky,  Mary- 
land, Michigan,  Missouri,  Montana,  North  Dakota,  Ohio,  Pennsyl- 
vania, Texas,  Virginia,  Washington,  and  West  Virginia.  Alaska  coal 
is  treated  in  a  number  of  the  U.  S.  Geological  Survey  Bulletins,  deal- 
ing with  the  mineral  resources  of  that  territory. 

The  Department  of  Mines,  Canada,  has  issued  several  bulletins 
on  peat  and  others  dealing  with  tests  of  Canadian  coal.  Reports  on 
the  more  important  coal  fields  have  been  issued  by  the  Canada  Geo- 
logical Survey. 


CHAPTER  XV 

PETROLEUM,  NATURAL  GAS,  AND  OTHER  HYDROCARBONS 
Petroleum  and  Natural  Gas 

Introductory.  —  Under  this  heading  is  included  a  series  of  sub- 
stances, chiefly  compounds  of  carbon  and  hydrogen  (hydrocarbons), 
with  variable  amounts  of  oxygen,  sulphur  and  nitrogen.  These  sub- 
stances range  from  gases,  through  liquids  and  viscous  materials  to 
solids,  the  four  physical  conditions  being  represented  by  natural  gas 
petroleum,  mineral  tar  or  maltha,  and  asphalt. 

All  of  these  materials  are  of  economic  value,  and  some  of  them  are 
of  importance  to  the  engineer. 

Natural  gas  is  widely  used  for  heating  and  lighting.  Petroleum  is 
of  importance  as  a  fuel,  illuminant,  and  lubricant,  and  the  residue  of 
asphaltic  oils  as  an  ingredient  of  paving  mixtures. 

Asphalt  in  its  pure  form  is  employed  for  paints,  varnishes  and 
insulation,  while  the  larger  deposits  of  impure  nature  are  utilized  for 
paving  purposes. 

Properties  of  petroleum.  —  Crude  petroleum  is  a  liquid  of  complex 
composition,  variable  color  and  density.  It  consists  of  a  mixture  of 
liquid,  gaseous,  and  solid  hydrocarbons,  the  last  being  in  solution, 
and  the  second  predominating.  The  variation  in  density  is  due  to 
varying  amounts  of  the  three  kinds  of  hydrocarbons  mentioned  above. 

American  petroleum  may  have  either  a  paraffin  base  (most  Pennsyl- 
vania oils),  an  asphaltic  base  (Texas  and  many  California  oils),  or 
a  mixed  asphaltic  and  paraffin  base  (some  Illinois  petroleums). 

The  paraffin  oils  predominate  east  of  the  Mississippi,  while  the 
asphaltic  oils  are  abundant  west  of  it.  Most  petroleum  contains  some 
nitrogen>  but  the  quantity  present  rarely  exceeds  2  per  cent. 

Sulphur,  though  usually  present,  is  abundant  only  in  exceptional 
cases,  and  then  the  oil  requires  special  treatment  to  eliminate  it. 

Petroleums  commonly  vary  in  specific  gravity  between  about  0.8 
and  0.98,  but  the  gravity  is  usually  expressed  in  terms  of  the  Beaume 
scale,  on  which  10°  is  equivalent  to  a  specific  gravity  of  1  as  compared 
with  water.  Thus  a  heavy  oil  would  be  12°  or  14°  Beaume,  while  a 
light  one  would  be  about  46°  Beaume. 

562 


PETROLEUM,   NATURAL  GAS,   ETC. 


563 


Petroleum  also  varies:  (1)  In  the  temperature  at  which  it  solidifies; 
(2)  in  the  minimum  temperature  at  which  it  gives  off  inflammable 
vapors  (flashing  point) ;  and  (3)  in  the  boiling  point. 

When  petroleum  is  subjected  to  a  rising  temperature,  the  lighter 
oils  pass  off  first,  and  then  the  heavier  ones,  the  more  important  oils 
which  can  be  separated  being  gasoline,  benzine,  and  heavy  napthas, 
while  there  is  left  behind  a  residue  of  paraffin  or  asphalt-like  char- 
acter. 

The  oils,  found  in  different  fields,  or  even  in  the  several  sands  of  the 
same  field,  will  consequently  yield  different  percentages  of  the  same 
kind  of  distillate. 

Illuminating  oil  is  of  low  gravity,  lubricating  oil  of  medium,  and 
fuel  oil  of  high  gravity  (comparatively  speaking).  Fuel  oils  are  com- 
monly used  in  their  crude  form. 

Properties  of  natural  gas.  — Natural  gas  consists  chiefly  of  marsh 
gas  —  fire  damp  —  CH4.  It  is  colorless,  odorless,  burns  readily  with 
a  luminous  flame,  and  when  mixed  with  air  is  highly  explosive. 

Other  hydrocarbons,  such  as  ethane  (C2H6),  ethylene  (C2H4),  car- 
bon monoxide,  and  carbon  dioxide  may  be  present.  The  nitrogen 
content  is  variable,  and  rarely  large. 

The  following  analyses  of  natural  gas  will  serve  to  show  how  it  may 
vary  in  composition: 

ANALYSES  OF  NATURAL  GAS1 


Constituents. 

1 

2 

3 

4 

5 

Methane  (CHj) 

94.40 
0.00 
0.00 
0.00 
0.00 
0.23 
5.08 
0.00 
0.183 

82.25 
0.00 
0.12 
0.61 
0.00 
tr. 
16.40 
0.00 
0.616 

14.85 
0.41 

73.81 

Ethane  (C2H6) 

98.90 

Olefine  (C2H4) 

Carbon  dioxide  (CO2)   .... 

0.00 
0.00 
0.20 
82.70 
tr. 
1.84 

0.81 

0.40 

Carbon  monoxide  (CO)  

Oxygen.  . 

3.46 
21.92 

"6'76" 

xS  itrogen 

Hydrogen 

Helium    .  . 

undete 

rmined 

Hydrogen  sulphide  (HaS) 

1.  Tola,  Kas. 

2.  Fredonia,  Kas. 


5.  Pittsburgh,  Pa. 


3.  Dexter,  Kas. 

4.  Pittsfield,  111. 


The  one  analysis  from  Dexter,  Kas.,  is  exceptional,  because  of  its 
high  nitrogen  content. 

1  Many  additional  analyses  can  be  found  in  the  U.  S.  Geol.  Survey,  Min.  Res., 
1911,  II,  p.  324,  1912. 


564 


ENGINEERING  GEOLOGY 


Natural  gas  is  used  chiefly  for  heating  and  illumination,  but  the 
wasteful  use  of  this  product  in  some  states  has  nearly  exhausted  the 
supply  in  those  regions. 

The  table  given  below  brings  out  the  essential  differences  between 
natural  gas  and  other  fuel  or  illuminating  gases. 

ANALYSES  OF  NATURAL  AND  MANUFACTURED  GASES 


Constituents. 

Average 
Pa.  and 
W.  Va. 

Average 
Ohio  and 
Indiana. 

Average 
Kansas. 

Average 
Coal 
Gas. 

Average 
Water 
Gas. 

Average 
Producer 
Gas  Bit. 
Coal. 

Marsh  gas,  CH4  

80.85 

93.60 

93.65 

40.00 

2.00 

2.05 

Other  hydrocarbons  
N  

14.00 
4.60 

0.30 
3.60 

0.25 
4.80 

4.00 
2.05 

0.00 
2.00 

0.04 
56.26 

CO, 

0  05 

0.20 

0.30 

0.45 

4.00 

2.60 

CO 

0  40 

0  50 

1  00 

6  00 

45  00 

27  00 

H  
H2S... 

0.10 
0.00 

1.50 
0.15 

0.00 
0.00 

46.00 
0.00 

45.00 
0.00 

12.00 
0.00 

0.  

tr. 

0.15 

0.00 

1.50 

1  50 

0.05 

Lbs.  in  1000  cu.  ft  

47  50 

48  50 

49  00 

33  00 

45  60 

75  00 

Sp.  gr. 

0  624 

0  637 

0  645 

0  453 

0  600 

0  985 

B.T.U.  per  1000  cu.  ft  

1,145,000 

1,095,000 

1,100,000 

755,000 

350,000 

155,000 

During  the  past  three  years  the  separation  of  the  more  volatile 
grades  of  gasoline  from  natural  gas  derived  from  gas  wells  has  become 
an  industry  of  some  importance.1  Tests  made  show  that  the  natural 
gas  from  different  regions  yields  from  zero  to  8  or  10  gallons  of  gaso- 
line per  1000  cubic  feet,  the  average  being  about  2  gallons. 

In  recent  years  considerable  attention  has  been  given  to  the  waste 
of  natural  gas,  and  methods  for  preventing  it,  for  the  early  exhaustion 
of  some  large  fields  has  been  due  to  the  reckless  manner  in  which  it 
has  been  used. 

The  following  are  the  chief  causes  of  waste  according  to  Day:2  (1)  Free  escape 
of  gas  from  natural  gas  wells  that  have  not  been  closed  in.  (2)  Free  escape  of  gas 
from  oil  wells.  (3)  Abuse  of  gas  by  the  use  of  its  pressure  to  drive  steam  engines  in 
oil  fields.  (4)  Abuse  by  jetting  the  gas  into  oil  wells,  for  the  purpose  of  a  gas  lift 
instead  of  an  air  lift  in  oil  production.  (5)  Wasteful  installation  of  gas  burners  and 
lights  in  oil-well  drilling.  (6)  Waste  by  selling  at  flat  rate.  (7)  Waste  from  open 
grates,  inefficient  furnaces,  improperly-adjusted  mixers,  and  other  causes,  by  con- 
sumers. (8)  Overheated  buildings. 

Occurrence  of  oil  and  gas.  —  Oil  and  gas  are  with  few  exceptions 
always  found  in  sedimentary  rocks.  At  least  a  little  gas  usually  oc- 


1  Bur.  Mines,  Tech.  Paper  10. 

2  U.  S.  Geol.  Survey,  Min.  Res.,  1911,  II,  p.  280,  1912. 


PETROLEUM,   NATURAL  GAS,  ETC. 


565 


curs  with  the  oil,  but  the  gas  is  at  times  alone.  A  well  may  yield 
either  one  or  the  other. 

The  two  are  sometimes  found  in  separate  beds,  or  in  different  parts 
of  the  same  bed.  In  most  cases  the  oil  or  gas  has  collected  in  the  pores 
of  the  rock,  but  occasionally  they  are  found  in  joint  planes  or  other 
kinds  of  cavities. 

The  rock  containing  the  oil  or  gas  is  known  as  the  oil  or  gas  rock, 
or  sand.  It  is  usually  a  sandstone  of  varying  coarseness  and  porosity, 
and  less  often  a  limestone  or  even  shale.  Even  an  apparently  dense 
rock  can  hold  a  surprisingly  large  amount  of  oil  or  gas.  White  esti- 
mated that  fairly  productive  sands  may  hold  from  six  to  twelve  pints 
of  oil  per  cubic  foot,  but  that  probably  not  more  than  three-fourths 
of  the  quantity  stored  in  the  rock  is  obtainable. 

That  portion  of  a  formation  containing  the  oil  or  gas  is  known 
as  a  pool.  A  district  may  contain  several  pools,  and  in  each  one 
there  may  be  one  or  more  sands  lying  at  different  levels  (Fig.  209). 
Indeed,  in  some  districts  as  many  as  10  or  12  sands  may  be  struck 
in  drilling,  but  all  are  not  necessarily  productive  in  all  parts  of 
the  area. 

The  thickness  of  the  producing  rock  ("  pay  sand  ")  will  vary  in 
the  different  fields.  In  some,  the  sand  is  as  thin  as  2  feet,  in  others 


a  GAS 


OIL 


C    WATER 


d     CAPROCK 


FIG.  208.  —  Section  showing  association  of  oil  and  gas  with  anticline. 
(After  Hayes.) 

as  much  as  75  or  100  feet.     Its  depth  below  the  surface  may  range 
from  500  or  600  to  3000  or  4000  feet. 

Well  pressure.  —  Both  oil  and  gas  are  usually  under  pressure,  so 
that  if  any  line  of  escape  is  opened  up  they  rise  towards  the  surface, 
sometimes  with  sufficient  force  to  eject  the  string  of  drilling  tools. 
In  fact,  it  is  a  natural  avenue  of  escape  that  sometimes  leads  to  the 
discovery  of  oil  or  gas.  The  natural  pressure  of  the  oil  or  gas  is  often 


566 


ENGINEERING   GEOLOGY 


high,  several  hundred  pounds  per  square  inch  being  not  uncommon, 
but  it  does  not  bear  any  direct  relation  to  the  depth  of  the  sand  below 
the  surface,  and  it  not  unusually  decreases  with  time. 

Yield  of  wells.  —  Quite  variable  also  is  the  yield  per  well.  In  the 
Appalachian  field  not  a  few  wells  of  a  few  barrels  daily  capacity 
have  been  pumped  for  years,  while  in  California  and  Texas,  some 
wells  have  been  drilled  that  had  an  enormous  flow  for  a  short 
while. 

Structure  of  sands.  —  In  many  fields  the  oil  and  gas  seem  to  be 
associated  with  archlike  structures,  not  always  distinct  anticlines,  but 
in  other  areas  other  types  of  structure  sometimes  hold  them  (Ref.  3). 

If  the  first  structural  case  exists,  and  the  bed  is  porous  throughout, 
the  oil,  gas,  and  saline  water,  which  is  often  present,  are  arranged 


FIG.  209.  —  Diagrammatic   section   of   sands   in   the   central   Appalachian  region. 
(After  Griswold  and  Munn,  U.  S.  Geol.  Survey,  Bull.  318.) 

according  to  their  gravities,  the  gas  at  the  top,  the  oil  next,  and  the 
water  at  the  botton^.  This  is  known  as  the  anticlinal  theory  of  ac- 
cumulation. 

Origin  of  Oil,  Gas  and  Asphalt 

There  is  evidently  a  close  genetic  relationship  between  the  different 
hydrocarbons,  as  is  seen  from  the  fact  that:  (1)  The  gases  given  off 
by  petroleum  are  similar  to  those  predominating  in  natural  gas;  (2) 
the  exposure  of  many  petroleums  to  the  air  results  in  a  change  to  a 


PETROLEUM,   NATURAL  GAS,  ETC.  567 

viscous  mass  and  finally  to  a  solid  asphalt  or  paraffin-like  substance; 
(3)  oil  and  gas  often  occur  together  in  the  same  rock;  and  (4)  asphalt 
deposits  are  frequently  found  in  close  association  with  oil. 

The  most  generally-accepted  theory  of  the  origin  of  oil  and  gas  is 
that  they  have  been  derived  from  animal  or  plant  remains  which  have 
become  buried  in  sedimentary  rocks.  By  a  process  of  decay,  the 
hydrocarbon  compounds  have  been  evolved  and  accumulated  in  the 
pores  of  the  rocks.  They  are  held  there  either  because  the  containing 
rock  is  capped  by  impervious  ones,  or  as  some  suppose  the  hydro- 
carbons are  held  in  by  hydrostatic  pressure. 

There  are  several  less  widely-accepted  theories  all  based  on  the 
hypothesis  that  the  hydrocarbons  are  of  inorganic  origin  (Refs.  7, 1,  2). 

Since  the  solid  bitumens  are  often  found  in  veins,  it  is  supposed  that 
the  oil  has  seeped  into  these,  and  subsequently  hardened  by  the  loss 
of  its  more  volatile  constituents. 

Cases  are  known  where  oil  has  oozed  from  fissures,  spread  over  the 
surface,  and  gradually  changed  to  asphalt  or  paraffin. 

Distribution  of  Petroleum  in  the  United  States 

The  important  oil  fields  of  the  United  States  are  the  Appalachian, 
Ohio-Indiana,  Illinois,  Mid-Continental,  Gulf  coast,  California,  Colo- 
rado and  Wyoming.  Outside  of  these,  small  areas  have  been  devel- 
oped or  prospected  in  Michigan,  Utah,  Missouri,  Arizona,  New  Mexico, 
Alaska,  etc. 

It  is  almost  impossible  to  have  a  map  showing  accurate  distribution 
without  revising  it  from  year  to  year. 

Appalachian  field.  —  This  field  covers  the  largest  area  of  those  in 
the  United  States,  but  is  no  longer  the  most  important,  as  it  supplies 
but  little  over  10  per  cent  of  the  country's  production. 

The  oil-bearing  rocks,  which  range  from  Ordovician  to  Carboni- 
ferous in  age,  are  chiefly  sandstones,  with  a  few  limestones,  em- 
bedded in  and  underlain  by  a  great  thickness  of  shales,  while  below 
these  are  probably  limestone  beds.  The  oil-bearing  rocks  occupy 
the  bottom  and  west  side  of  a  great  structural  trough,  within  which 
are  a  number  of  subordinate  folds.  The  sands  range  in  depth  from 
100  to  4000  feet.  In  recent  years  much  drilling  has  been  done  in 
Kentucky  and  Tennessee,  and  more  recently  in  Ohio,  resulting  in 
finding  oil  at  lower  horizons  than  in  other  parts  of  the  field,  but 
the  output  from  these  has  not  been  sufficient  to  overcome  the  gen- 
eral decline. 


568 


ENGINEERING  GEOLOGY 


PETROLEUM,   NATURAL  GAS,   ETC.  569 

The  oil  obtained  from  the  Appalachian  field  has  been  of  high  grade, 
practically  free  from  sulphur,  and  usually  from  asphalt,  but  rich  in 
paraffin  wax.  The  Kentucky  and  Tennessee  oils  are  inferior  to  those 
of  Pennsylvania. 

Ohio-Indiana  field.  —  The  oil  in  this  field  was  found  in  Trenton 
limestone.  It  is  high  in  sulphur  and  requires  special  treatment. 
Though  formerly  a  large  producer,  the  output  has  dropped  off  con- 
siderably, but  in  1911  oil  was  found  in  the  Trenton  limestone  at  a 
depth  of  about  1000  feet  below  the  previous  pools. 

Illinois  field.  —  This  field  has  increased  remarkably  and  almost 
steadily  since  1900.  The  main  portion  of  the  field  is  associated  with 
a  structural  feature  known  as  the  La  Salle  anticline,  extending  from 
the  northeastern  part  of  the  state  into  southwestern  Indiana.  The 
oil  is  thick,  asphaltic  and  contains  sulphur  in  the  northern  portion, 
but  in  the  southern  part  of  the  field  it  is  found  at  a  greater  depth 
(2200  +  ft.);  is  thinner,  and  contains  little  or  no  sulphur.  The  oil 
sands  are  of  Carboniferous  age. 

Mid-Continental  field.  —  This  field  underlies  a  portion  of  south- 
eastern Kansas  and  northeastern  Oklahoma,  and  extends  roughly 
from  Paola,  Kansas,  to  Muskogee,  Oklahoma.  The  north  Texas  and 
north  Louisiana  (Caddo  field)  might  also  be  included  as  the  products 
are  similar,  although  they  are  not  found  at  the  same  geologic  horizon. 
The  oil  of  Kansas  and  Oklahoma  is  in  general  found  in  Carboniferous 
sandstones.  Those  of  north  Texas  and  Louisiana  in  Cretaceous. 
Most  of  the  Kansas  oils  are  asphaltic,  but  in  Oklahoma  oils  of  both 
paraffin  and  asphaltic  types  are  found. 

In  northern  Louisiana  and  Texas,  paraffin  oils  free  from  sulphur 
predominate,  but  heavier  oils  are  known. 

Gulf  field.  —  Within  this  field  are  included  a  number  of  scattered 
areas  lying  in  the  Coastal  Plain  region.  The  oils  have  been  found  in 
association  with  certain  salt  domes,  which  also  carry  limestone  and 
gypsum.  The  oils  are  usually  heavy,  asphaltic  and  sulphurous,  but 
exceptionally  lighter,  non-asphaltic  ones  occur. 

California  field.  —  This  state  is  now  the  leading  producer,  the 
output  coming  from  a  number  of  fields,  which  differ  so  that  it  is  diffi- 
cult to  generalize  regarding  them.  The  California  oils  have  been 
usually  characterized  by  much  asphalt,  although  in  recent  years  not 
a  few  lighter  ones  have  been  found.  They  are  often  in  rocks  that 
have  been  much  disturbed. 

The  following  table  gives  a  summary  of  the  occurrence  of  oil  in  the 
principal  fields: 


570  ENGINEERING  GEOLOGY 

SUMMARIZED  TABLE  OF  OIL  OCCURRENCES  IN  THE  UNITED 


STATES 


Field. 

Structure. 

Geologic  Age. 

Kind  of  Rock. 

Kind  of  Oil. 

Appalachian. 

Geosyncline      with 
subordinate  anti- 
clines. 

Ordovician  to  Car- 
boniferous. 

Mostly  sandstone. 

Paraffin  base. 

Lima-Indiana. 

Anticlines. 

Ordovician. 

Mostly  limestone. 

Paraffin  base. 
Sulphur. 

Illinois. 

Low  anticlines  (?) 

Carboniferous. 

Sandstones. 

Paraffin  and  mixed 
oils. 

Michigan. 

Probably  anticlines. 

Silurian. 

Sandstones. 

Paraffin  base. 

Mid-Continental  . 

Westerly    dip    with 
some  anticlines. 

Carboniferous. 

Shales,    sandstones, 
mostly. 

Both  paraffinic  and 
asphaltic. 

Wyoming. 

Usually  folded. 

Carboniferous    to 
Tertiary. 

Mostly  sandstone. 

Paraffinic  and  as- 
phaltic. 

Colorado. 

Folded. 

Cretaceous. 

Sandstone  and  shale. 

Paraffinic. 

Gulf  Coast. 

Domes. 

Tertiary  and  Oe- 
taceous. 

Dolomite  and  sand- 
stone. 

Mainly     asphaltic, 
sometimes    high 
sulphur. 

California. 

Folded  and  faulted. 

Tertiary. 

Sandstones,    shales, 
conglomerates. 

Mainly  asphaltic. 

Alaska. 

Folded  and  faulted. 

Jurassic  to  Terti- 
ary. 

Sandstones  and 
shales. 

Paraffin. 

Distribution  of  Natural  Gas  in  the  United  States 

The  distribution  of  natural  gas  is  practically  co-extensive  with 
petroleum,  and  most  oil  wells  yield  some  gas,  but  the  gas  regions  are 
fewer  in  number  than  the  oil  regions.  In  the  Appalachian  field  gas 
has  been  found  from  New  York  to  Alabama,  but  West  Virginia  is  at 
present  the  chief  producer,  although  considerable  quantity  is  ob- 
tained in  Pennsylvania.  The  gas  from  the  Trenton  limestone  of  the 
Ohio-Indiana  field  is  practically  exhausted,  but  much  gas  is  now  being 
obtained  from  the  Clinton  sand  of  Ohio. 

While  the  gas  field  of  southeastern  Kansas  is  practically  exhausted, 
its  lack  is  being  supplied  by  gas  from  Oklahoma,  and  in  this  state 
the  production  of  oil  is  complicated  by  high  gas  pressure  to  a  greater 
extent  than  in  most  other  oil  fields. 

Northern  Louisiana  still  remains  an  important  producer  and  re- 
cently some  strong  gas  fields  have  been  developed  in  California. 

Solid  and  Semi-solid  Bitumens 

Under  this  heading  are  included:  (1)  Bitumens  of  a  more  or  less 
solid  character,  which  occupy  fissures  in  rocks  or  in  rarer  cases,  basin- 
shaped  depressions  on  the  surface,  and  (2)  bitumen  of  viscous  char- 
acter, or  maltha,  which  oozes  from  fissures  or  pores  of  the  rocks,  and 
sometimes  collects  in  pools  on  the  surface. 


PETROLEUM,   NATURAL  GAS,   ETC. 


571 


Since  the  first  class  is  found  filling  fissures  or  associated  with  them, 
they  may  be  called  vein  bitumens. 

Vein  bitumens.  —  There  are  several  varieties  of  vein  bitumens,  all 
of  which  are  black  or  dark-brown  in  color,  usually  have  a  pitchy 
odor,  and  burn  easily  with  a  smoky  flame.  They  are  insoluble  in 
water,  but  soluble  to  a  varying  degree  in  ether,  oil  of  turpentine  and 
naptha.  They  are  closely  related  chemically,  and  in  their  mode  of 
occurrence,  but  they  differ  somewhat  in  their  behavior  towards  sol- 
vents, as  well  as  in  their  fusibility.  Their  specific  gravity  ranges  from 
1  to  1.1. 

It  may  be  added  that  all  authorities  do  not  regard  all  of  the  sub- 
stances included  under  flhis  head,  as  bitumens.  Some  of  them,  as 
albertite  and  wurtzilite,  are  not  so  considered  by  Richardson,1  because 
as  he  states  they  are  "  not  soluble  to  any  extent  in  the  usual  solvents 
for  bitumen,  nor  do  they  melt  at  comparatively  low  temperatures, 
nor  dissolve  in  heavy  asphaltic  oils." 

ELEMENTARY  ANALYSES  OF  BITUMENS  AND  MALTHA 


1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

Ozokerite, 
Utah. 

& 

eJ.S 

1! 

si 

H 

Mo 

Grahamite, 
W.  Va. 

Grahamite, 
W.Va. 

Albertite, 
Nova  Scotia. 

Gilsonite, 
Utah. 

Gilsonite, 
Utah. 

Wurtzilite, 
Utah. 

Lake  Pitch. 
Trinidad, 

c.  . 

85  25 

85  72 

86  57 

76  45 

59  20 

86  04 

88  30 

89  28 

80  00 

83  68 

H 

15  09 

11  83 

7  26 

7  83 

5  77 

8  96 

9  96 

8  66 

12  23 

10  84 

N... 

1  21 

1  48 

tr. 

1  01 

2  93 

o 

2  00 

13  46 

14  68 

1  97 

(A  00 
0.32 

0.79 

1.78 

0.45 

s  

Ash 

•» 

1.32 

1.38 
1  31 

tr. 
0  10 

19  34 

tr. 
0  10 

1.32 

0  10 

1.79 

5.83 

5.10 

Moisture 



1,8,9,10.  Richardson,  "Nature  and  Origin  of  Asphalt,"  1898.  2.  Munic.Eng.Mag.,  June-August,  1897. 
3.  Amer.  Jour.  ScL,  Sept.,  1899,  p.  221.  4.  Wurtz,  analyst,  Amer.  Jour.  Sci.,  iii,  VI;  415,  1873.  5.  Hite, 
analyst,  Geol.  Soc.  Amer.,  Bull.  X;  283,  1899.  A  proximate  analysis  made  on  another  sample  gave  1.13 
sulphur.  6.  Trans.  Amer.  Philos.  Soc.,  Phila.,  853, 1852.  7.  Jour.  Frankl.  last.,  CXL,  No.  837,  Sept.,  1895. 

On  the  other  hand,  Peckham2  includes  under  asphaltic  coals,  the 
vein  bitumens  grahamite,  albertite,  gilsonite,  etc.,  which  he  states 
"yield  paraffin  on  distillation." 

The  terms  asphalt  and  asphaltum  are  considered  as  synonymous 
by  many.  Peckham  3  gives  it  as  the  solid  form  of  natural  bitumen, 
while  Richardson,  in  practical  agreement  with  him,  includes  under 

1  Modern  Asphalt  Pavement,  p.  107. 

2  Solid  Bitumens,  p.  77,  1909. 

3  1.  c.,  p.  79. 


572  ENGINEERING  GEOLOGY 

asphalts,  "all  solid  native  bitumens  which  are  in  use  in  the  paving 
and  other  industries."  Specifically,  he  says,  "  true  asphalt  is  sharply 
differentiated  from  several  of  the  bitumens  which  are  used  industrially 
under  this  designation,  such  as  gilsonite  and  grahamite." 

The  Committee  on  Standard  Tests  for  Road  Materials,  in  its  report 
to  the  American  Society  for  Testing  Materials,  defines  asphalts  as 
"  solid  or  semi-solid  native  bitumens,  solid  or  semi-solid  bitumens 
obtained  by  refining  petroleum,  or  solid  or  semi-solid  bitumens  which 
are  combinations  of  the  bitumens  mentioned  with  petroleums  or  der- 
ivations thereof,  which  melt  upon  the  application  of  heat  and  which 
consist  of  a  mixture  of  hydrocarbons  and  their  derivations  of  complex 
structure,  largely  cyclic  and  bridge  compounds." 

Asphaltenes.  —  The  committee  define  these  as  the  components  of 
the  bitumen  in  petroleums,  petroleum  products,  malthas,  asphalt 
cements  and  solid  native  bitumens,  which  are  soluble  in  carbon  disul- 
phide  but  insoluble  in  paraffin  naphthas. 

The  following  are  some  of  the  more  important  types  of  the  purer 
bitumens,  which  occur  mostly  in  vein  form. 

Albertite.  —  A  black  bitumen  with  a  brilliant  lustre  and  conchoidal 
fracture,  a  hardness  of  1  to  2,  and  specific  gravity  of  1.097.  It  is 
barely  soluble  in  alcohol,  and  dissolves  to  the  extent  of  4  per  cent  in 
ether  and  30  per  cent  in  oil  of  turpentine. 

The  material  was  worked  in  New  Brunswick,  but  was  too  valuable 
to  use  in  pavements,  and  the  deposit  appears  to  be  exhausted. 

Grahamite.  —  This  is  a  brittle,  black  bitumen  with  a  hardness  of  2 
and  specific  gravity  of  1.145.  It  is  slightly  soluble  in  alcohol,  partly 
so  in  ether,  petroleum  and  benzole,  but  almost  completely  in  tur- 
pentine. Carbon  disulphide  and  chloroform  dissolve  it  completely.  It 
occurs  in  veins  but  never  in  large  amounts. 

According  to  Richardson1  it  is  differentiated  from  the  asphalts  and 
gilsonites  by  the  fact  that  it  yields  from  30  to  50  per  cent  fixed  carbon 
on  ignition. 

Grahamite  has  been  found  in  West  Virginia,  southeastern  Okla- 
homa, and  Colorado,  but  the  deposits  are  hardly  large  enough  to  be 
of  much  use  in  the  paving  industry. 

Gilsonite  or  Uintaite  is  a  black,  brilliant  bitumen,  with  conchoidal 
fracture,  hardness  of  2  to  2.5  and  specific  gravity  of  1.065  to  1.067. 
It  is  equally  soluble  in  cold  carbon  tetrachloride  and  carbon  disul- 
phide, thus  differentiating  it  from  grahamite  and  some  of  the  residual 
pitches.  According  to  Richardson  it  is  "  readily  soluble  in  the  heavy 

1  1.  c.,  p.  205. 


PETROLEUM,   NATURAL  GAS,  ETC.  573 

asphaltic  residues  from  California  and  Texas  petroleums,  and  when 
mixed  with  these  in  the  proper  proportion,  makes  a  material  which 
is  extremely  rubbery  and  more  or  less  elastic.  It  possesses  little 
ductility,  however,  and  in  this  respect  differs  from  similar  preparations 
made  with  asphalt.'*' 

The  material  is  used  chiefly  in  the  manufacture  of  Japan  varnishes, 
in  water-proof  cement  for  coating  reservoirs,  and  in  insulating 
materials. 

Glance  Pitch.  —  The  name  is  applied  to  a  somewhat  widely-dis- 
tributed bitumen  of  which  the  best  supplies  come  from  East  Syria  and 
the  Dead  Sea.  It  is  not  used  in  the  paving  industry. 

Manjak.  —  This  is  a  bitumen  found  only  on  the  Island  of  Barbadoes. 
It  is  of  high  purity,  black  color,  and  brilliant  lustre,  related  probably 
to  grahamite.  It  is  said  to  be  of  no  value  in  the  paving  industry. 

Maltha.  —  Under  this  term  are  included  viscous,  liquid,  natural  bit- 
umens, which  correspond  in  consistency  to  the  artificial  residuums, 
but  are  usually  denser. 

Richardson1  claims  that  they  are  rarely  of  a  suitable  character  for 
use  as  a  flux  because  on  heating  they  are  generally  rapidly  converted 
into  a  harder  material  by  the  loss  of  volatile  hydrocarbons. 

Maltha  is  not  known  to  occur  in  large  deposits  in  the  United  States, 
although  it  is  somewhat  widely  distributed  in  the  California  oil  fields. 

Trinidad  lake  asphalt.  —  This  represents  a  type  of  deposit  not  found 
in  the  United  States,  but  occurs  in  the  famous  pitch  lake  on  the  island 
of  Trinidad,  off  the  coast  of  Venezuela. 

The  deposit  appears  to  occupy  a  basin-shaped  depression  of  about 
100  acres,  and  the  pitch  has  evidently  oozed  up  from  below,  for  borings 
show  that  it  occupies  a  crater-like  depression  in  sandstones  which  are 
more  or  less  impregnated  with  bitumen.  An  analysis  is  given  in  the 
table  on  p.  571. 

Trinidad  asphalt  has  to  be  dried  and  agitated  with  steam  before 
use.  The  refined  material  has  a  specific  gravity  of  1.4,  dull  lustre, 
conchoidal  fracture,  and  hardness  of  2.  It  contains  about  56  per  cent 
of  bitumen  soluble  in  carbon  disulphide. 

Bituminous  Rocks 

Under  this  heading  are  included  consolidated  and  unconsolidated 
rocks,  whose  pores  are  more  or  less  completely  filled  with  bituminous 
matter.  In  some  cases  the  material  is  petroleum,  and  then  it  is 

1  1.  c.,  p.  122. 


574 


ENGINEERING   GEOLOGY 


possible,  though  not  always  commercially  practicable,  to  distil  the  oil 
from  the  rock.  In  other  cases  the  pore  filling  is  either  maltha  or 
asphalt,  and  the  material  is  sometimes  used  for  paving  purposes. 

Bituminous  rocks  may  be  classified  according  to  the  character  of  the 
rock  as  bituminous  sands  or  sandstones,  bituminous  limestones,  shales 
and  schists.  The  amount  of  bituminous  matter  in  the  rock  varies, 
and  as  a  rule  is  not  large,  as  the  table  of  analyses  given  below  shows. 

Some  difference  of  opinion  exists  as  to  the  value  of  bituminous  rocks 
for  paving  purposes.  The  advocates  of  this  material  claim  that  the 


FIG.  210.  —  Map  of  asphalt  and  bituminous  rock  deposits  of  the  United  States. 
(After  Eldridge,  U.  S.  Geol.  Survey,  22d  Ann.  Kept.,  IX.) 

bitumen  and  rock  occur  practically  mixed  by  nature,  requiring  only 
crushing  (if  the  rock  is  hard),  heating  and  spreading.  Cases  are 
quoted  also  in  print  of  bituminous  rock  pavements  which  have  given 
excellent  satisfaction. 

As  against  this  We  hear  that  the  character  of  bituminous  rocks  is 
variable,  that  the  texture  of  the  mineral  grains  is  not  always  such  as 
to  compact  to  a  tight  mass,  and  that  the  bituminous  matter  is  some- 
times maltha  and  not  asphalt,  which  becomes  brittle  with  time. 

A  mixture  of  bituminous  limestone  and  bituminous  sandstone  has 
sometimes  given  better  results  than  the  sandstone  alone.  In  France 
bituminous  limestone  has  been  successfully  used  for  paving  pur- 
poses. Bituminous  rocks  are  widely  distributed  in  the  United  States 
(Fig.  210). 


PETROLEUM,   NATURAL  GAS,   ETC. 


575 


In  Kentucky  asphaltic  sandstones  occur  in  Carter  and  Boyd  counties 
in  the  northeastern  part  of  the  state,  and  in  Breckenridge,  Grayson, 
Edmonson,  Warren  and  Logan  in  the  western  part. 

In  Oklahoma  a  number  of  quarries  of  bituminous  sands  and  lime- 
stones have  been  opened  in  the  Buckhorn  district  east  of  the  Washita 
River  and  in  the  vicinity  of  Rock  Creek. 

In  California  bituminous  sands  occur  near  Santa  Cruz,  Santa 
Barbara  and  San  Luis  Obispo. 

The  following  data  are  given  by  Richardson,  showing  the  percentage 
of  bituminous  matter  and  texture  of  a  number  of  rocks. 

BITUMINOUS  ROCKS  FROM  UNITED  STATES 


1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

Bitumen  soluble  in  CS2   
Passing  200  mesh-sieve 

9.1 
3  9 

7.7 
7  2 

11.1 
13  0 

8.2 
18.8 

11.8 
1  2 

13.2 
8.6 

11.4 
1  5 

11.4 
4.4 

5.9 
44.1 

7.5 

18  5 

100 

35.0 

26.6 

48.0 

9.0 

5.0 

5.2 

4.1 

6.1 

10.0 

14.0 

80 

36  0 

26  0 

23  0 

18  0 

16  0 

12  0 

12  0 

16  1 

5  0 

21  0 

50 
40 

15.0 
1  0 

23.4 
2  5 

5.0 
0  0 

16.0 
4  0 

59.0 
6  0 

40.0 
13  0 

35.0 
20  0 

44.0 
9  6 

9.0 

7  0 

25.0 
7  0 

30 

0  0 

0  4 

3.0 

1  0 

5  0 

11  0 

5  0 

7  0 

2  0 

20 

0  0 

0  2 

8  0 

0  0 

2  0 

4  0 

3  0 

6  0 

3  0 

10                          
Retained  on  10  mesh-sieve  

6.0 
9  0 

0.0 

1.0 

0.0 

1.0 

6.0 

2.0 

1.  Bituminous  sand,  Soldier  Creek,  Carter  County,  Ky.  2.  Same,  Breckenridge  County,  Ky.  3. 
Bituminous  sand,  Buckhorn  District,  Indian  Territory.  4.  Surface  mixture  made  from  bituminous  sand 
and  lime  rocks  from  Oklahoma.  5.  Bituminous  sandstone  near  Ardmore,  Oklahoma.  6.  Bituminous 
sand,  richest  rock,  Side  Hill  quarry,  Santa  Cruz,  Cal.  7.  Poorer  rock,  same  quarry.  8.  Bituminous  sand, 
San  Luis  Obispo,  Cal.  9.  Bituminous  limestone,  Seyssel,  France.  10.  Bituminous  limestone,  Vorwohle, 
Ger. 


References  on  Petroleum  and  Natural  Gas 

1.  Campbell,  Econ.  Geol.,  VI,  p.  363,  1911.  (Theories  of  Origin.) 
2.  Clapp,  Econ.  Geol.,  IV,  p.  603,  1909.  (Origin.)  3.  Clapp,  Econ. 
Geol.,  V,  p.  503,  1910.  (Classification  of  occurrence.)  4.  Clarke, 
U.  S.  Geol.  Surv.,  Bull.  491,  p.  681,  1911.  (Composition.)  5.  Hofer, 
Das  Erdol,  2nd  ed.,  1906.  (Brunswick,  Ger.)  6.  Redwood,  B., 
Treatise  on  Petroleum,  London.  7.  Ries,  Economic  Geology,  3rd 
ed.,  1910.  (P.  68  et  seq.  gives  resume  of  U.  S.  occurrence.) 

Many  of  the  bulletins  of  the  United  States  Geological  Survey 
contain  papers  on  special  districts. 

The  following  state  geological  surveys  have  issued  special  reports 
on  petroleum  or  natural  gas,  which  can  usually  be  obtained  free  of 
charge  by  application  to  the  State  Geologist:  Illinois,  Indiana, 
Kansas,  Michigan,  New  York,  Ohio,  Oklahoma,  Pennsylvania,  West 
Virginia. 


576  ENGINEERING  GEOLOGY 

References  on  Solid  and  Semi-solid  Bitumens 

1.  Eldridge,  U.  S.  Geol.  Surv.,  22nd  Ann.  Kept.,  Pt.  I,  1901.  (Dis- 
tribution in  U.  S.)  2.  Ells,  Dept.  Mines,  Canada,  Bulletins  55  and 
1107,  1910.  (Bituminous  shales,  Nova  Scotia  and  New  Brunswick.) 
3.  Gosling,  Sch.  of  M.  Quart.,  XVI,  p.  41.  (Ozokerite.)  4.  Hertle 
and  others,  Report  on  Asphalt  Paving  by  Commissioner  of  Accounts 
of  New  York  City,  1904.  5.  Peckham,  Solid  Bitumens,  New  York, 
1909.  (M.  C.  Clark  Pub.  Co.)  6.  Richardson,  The  Modern  Asphalt 
Pavement,  2nd  ed.,  N.  Y.  1908.  (Wiley  &  Sons.) 


CHAPTER  XVI 
ROAD  FOUNDATIONS  AND  ROAD  MATERIALS 

IN  the  construction  of  roads,  especially  in  the  country,  the  engineer 
should  consider  two  factors;  namely,  (1)  the  geological  conditions 
which  may  affect  the  permanence  and  stability  of  the  road  bed,  drain- 
age, etc.,  and  (2)  the  kind  and  character  of  rock  to  be  used  for  the 
road,  whether  sand,  gravel  or  crushed  stone.1  What  may  be  said 
under  the  first  head  applies  equally  well  to  rail  and  wagon  roads,  as 
both  are  affected  by  the  same  set  of  geological  conditions,  and  unless 
specifically  stated  to  the  contrary  this  can  be  understood  to  be  so. 

ROAD  FOUNDATIONS 

Kind  of  rock.  —  In  the  construction  of  cuts  either  on  or  through 
a  hillside,  it  is  necessary  to  consider  the  character  of  the  rock  and  its 
structure.  Some  rocks  are  hard,  massive  and  therefore  expensive  to 
blast,  while  others  are  soft,  or  contain  joint  or  stratification  planes, 
and  hence  are  easy  to  remove.  Gravels,  sands,  clays  and  even  shales 
(if  not  too  hard)  can  be  attacked  with  the  steam  shovel.  In  many  cases 
the  material  removed  from  the  cut  can  be  utilized  for  fills. 

Depressions  filled  with  peat  often  give  trouble,  more  with  railroads 
than  with  wagon  roads,  because  of  the  yielding  character  of  the  ma- 
terial, which  has  not  always  sufficient  strength  to  hold  up  the  road 
bed,  and  may  require  much  and  continual  filling  to  keep  the  top  of  the 
sinking  material  at  proper  grade.  Bogs  on  hillsides  give  similar  trouble 
and  moreover  are  usually  springy. 

Clay  sometimes  causes  much  trouble,  especially  on  railway  lines,  for 
two  reasons:  (1)  If  wet  and  on  a  slope,  it  shows  a  tendency  to  slide, 
even  though  very  slowly;  and  (2)  since  clay  expands  when  wet,  and 
shrinks  when  dry,  the  heaving  of  soil  is  likely  to  affect  the  road,  and  in 
the  case  of  tracks  to  throw  them  out  of  alignment. 

In  the  province  of  Alberta,  Canada,  for  example,  there  are  certain 
clay  formations  which  have  given  the  railway  engineers  considerable 

1  The  use  of  asphalt  is  referred  to  in  Chapter  XV,  and  the  use  of  cement  and 
concrete  hardly  lies  within  the  field  of  this  book. 

577 


578  ENGINEERING  GEOLOGY 

trouble,  because  the  material  absorbs  a  large  amount  of  water,  and  in- 
creases in  volume. 

Rock  structure.  —  In  igneous  rocks,  joint  planes  are  usually  present; 
in  metamorphic  rocks,  joints  and  sometimes  stratification  and  foliation 
planes;  while  in  the  sedimentary  ones  both  joints  and  stratification 
planes  occur.  A  rock  mass  which  is  unsupported  may  slide  along 
either  type  of  plane,  and  this  fact  should  not  be  overlooked  in  the 
construction  of  rock  cuts  (Chapter  VII). 

Take,  for  example,  the  case  of  a  slate  whose  cleavage  planes  are  in- 
clined at  right  angles  to  the  line  of  the  road.  On  the  side  of  downward 
dip  the  face  of  the  cut  can  be  quite  steep,  but  on  the  other  side  it  should 
be  sloping  if  possible  and  parallel  with  the  dip,  otherwise  slips  of  rock 
are  liable  to  be  frequent. 

If  much  water  seeps  along  these  planes,  and  the  rock  is  located  in  a 
region  of  frost,  the  tendency  to  loosen  pieces  of  it  will  be  great... 

Valley  crossings.  —  The  character  of  the  underground  structure 
has  to  be  considered  here  in  connection  with  bridge  foundations.  Piers 
for  railroad  bridges  are  often  of  large  size,  those  for  wagon  bridges  not 
usually  as  great,  but  in  either  case  it  is  essential  that  they  rest  on  firm 
ground.  The  material  on  the  sides  of  a  valley  is  sometimes  of  the 
character  of  slide  material,  which  does  not  remain  firm  under  great 
weight. 

In  other  cases  the  beds  may  dip  towards  the  stream,  and  contain 
slippery  layers  here  and  there  in  the  section.  If  now  a  large  bridge 
pier  is  placed  on  such  a  mass,  the  weight  of  it  may  cause  movement 
along  some  of  the  slip  planes,  unless  the  precaution  has  been  taken  to 
prevent  it.1 

Filled  valleys  are  not  uncommon  (p.  424).  In  some  cases  these 
contain  tightly  packed  sand  and  gravel  which  give  no  trouble.  In 
other  cases  the  material  is  peaty  (see  above),  or  in  still  other  instances 
a  comparatively  firm  surface  bed  of  sand  and  gravel  may  be  underlain 
by  wet  clay  or  quicksand. 

Embankments  constructed  across  a  valley  filling  sometimes  load  it  up 
to  a  greater  degree  than  it  can  stand,  so  that  the  fill  settles  down. 
One  cannot  tell,  without  boring  or  test-pitting,  how  thick  the  filling 
is;  and,  moreover,  the  deepest  part  of  the  original  rock  bottom  of  the 
valley  is  not  necessarily  under  its  central  portion. 

Slope  of  cuts.  —  This  should  be  carefully  considered  in  order  to 
insure  stability  of  the  sides  of  a  cut  and  prevent  constant  slides.  Firm 

1  Engineering  News,  XXXIX,  p.  278,  1898,  and  Railroad  Gazette,  XL,  p.  197, 
1906. 


ROAD  FOUNDATIONS  AND  ROAD  MATERIALS  579 

rock  can  usually  be  left  standing  with  a  steep  face,  but  unconsolidated 
materials,  like  sand  and  gravel,  must  be  given  their  proper  angle  of 
repose,  remembering  that  moisture  in  material  like  clay  increases  its 
tendency  to  assume  a  lower  angle.  The  same  material  in  a  dry  climate 
will  often  stand  up  better  than  in  a  moist  one.  On  p.  355  will  be  found 
the  allowable  slopes  suggested  for  different  kinds  of  rock. 

Along  many  lines  of  railway,  there  are  often  clay  cuts  which  have  to 
be  constantly  watched,  because  of  their  tendency  to  slide  down  on 
the  track. 

Another  cause  of  sliding  in  a  cut  is  the  presence  of  alternating  hard 
and  soft  beds.  Take  the  case  of  sandstone  interbedded  with  soft 
shale.  As  the  latter  weathers  back,  the  sandstone  beds  are  robbed  of 
their  support  and  fall  down. 

The  subject  of  sand  dunes  in  their  relation  to  road  work  need  simply 
be  mentioned  here  by  way  of  reminder,  since  it  has  been  discussed  in 
Chapter  II. 

Drainage.  —  If  the  foundation  of  a  road  is  not  of  such  character  as 
to  be  self-draining,  some  means  must  be  provided  to  accomplish  this 
artificially.  Before  constructing  a  road,  therefore,  the  character  of  the 
foundations  with  respect  to  its  moisture-holding  qualities  should  be  in- 
vestigated. Clay,  for  example,  has  strong  capillary  power;  it  absorbs 
moisture  and  holds  it,  and  therefore  requires  artificial  drainage. 

Sand  on  the  contrary,  unless  very  fine-grained,  will  permit  the  water 
to  drain  off  if  it  can  do  so.  The  permeability  and  capillarity  of  these 
materials  can  be  tested  by  bringing  a  sample  of  known  volume  in  con- 
tact with  a  known  volume  of  water,  and  noting  the  time  that  the  latter 
takes  to  pass  through  the  former.  For  the  permeability  test  the  water 
should  be  placed  above  the  material,  and  for  the  capillarity  test,  below, 
but  in  contact  with  it. 

ROAD  MATERIALS 

Raw  materials  used  for  highway  construction.  —  These  include 
clay,  sand,  gravel,  crushed  stone,  asphalt  and  bituminous  rock.  The 
last  two  have  been  referred  to  in  Chapter  XV.  The  properties  of  the 
others  are  discussed  in  the  following  pages  of  this  chapter,  but  the 
mode  of  occurrence  is  treated  in  Chapter  II. 

The  different  kinds  of  unconsolidated  and  consolidated  rock  em- 
ployed in  highway  construction  are  rarely  transported  for  long  dis- 
tances, local  sources  of  supply  on  the  contrary  being  usually  drawn 
upon.  It  therefore  frequently  devolves  upon  the  engineer  to  care- 
fully examine  these  local  sources  with  reference  to  the  quantity 


580  ENGINEERING  GEOLOGY 

and   quality  of   the  best  material,  its  accessibility  and  thickness  of 
overburden. 

The  engineer  engaged  in  road  construction,  or  the  preparation  of 
specifications  should  be  familiar  with  at  least  the  common  kinds  of 
rocks.  The  authors  have  in  mind  one  case  of  an  engineer  who  specified 
syenite  (a  comparatively  rare  rock)  for  use  on  roads  in  a  certain  district 
where  it  could  not  be  found,  and  if  used  would  have  to  be  hauled  a 
long  distance.  Why  he  did  not  call  for  granite,  which  would  have 
served  the  purpose  just  as  well,  and  could  have  been  obtained  nearer 
by,  is  not  known.  The  district  in  which  it  was  to  have  been  used  con- 
tained plenty  of  sandstone,  and  even  some  limestone. 

Clay 

Clay  is  sometimes  used  for  roads,  but  the  different  deposits  available 
vary  widely  in  their  characters.  Some  are  exceedingly  sticky  when 
wet,  and  dry  to  a  hard,  caked,  cracked  mass,  like  the  gumbo  of  the 
western  and  southwestern  states.  The  black  waxy  soil  of  Texas  is  of 
the  same  character.  When  very  wet  it  is  almost  impassable.  Other 
clays  are  sandy,  and  do  not  get  quite  so  sticky.  Under  continued  traffic 
clay  roads,  when  dry,  wear  down  to  a  dust  that  is  equally  disagreeable. 

Much  better  results  are  obtained  by  using  a  sand-clay  mixture,  in 
which  case  the  clay  fills  the  voids  between  the  sand  grains.  Roads  of 
this  type  (Refs.  12  and  13)  are  common  in  many  parts  of  the  South, 
especially  in  the  Coastal  Plain  region,  and  give  excellent  results,  pro- 
vided the  sand  and  clay  are  well  mixed  and  the  road  is  properly  drained. 
The  sand  forms  about  70  per  cent  of  the  whole.  These  materials,  un- 
less mixed  wet,  do  not  reach  their  best  condition  in  a  road  until  they 
have  been  made  thoroughly  wet  by  rain  several  times. 

Gravel 

Under  the  term  gravel  is  included  all  unconsolidated  material  that 
will  not  pass  a  4-mesh  sieve.  It  may  occur  in  nature  under  a  vari- 
ety of  conditions:  (1)  As  a  constituent  of  modified  glacial  drift; 
(2)  as  a  stream  deposit;  (3)  as  extensive  deposits  laid  down  by  water 
not  necessarily  confined  to  valleys;  and  (4)  as  delta  deposits.  It  will 
be  seen  from  this  that  grav«els  are  usually  of  the  transported  type,  so 
that  the  pebbles  are  more  or  less  .rounded.  Gravelly  deposits  of  a 
residual  character  are  known.,  and  are  used  especially  in  the  South, 
where  the  chert,  much  used  in  Alabama,  is  commonly  referred  to  un- 
der this  name  (Ref.  13).  As  found  in  nature  it  is  seldom  clean,  but  is 


ROAD  FOUNDATIONS  AND  ROAD  MATERIALS  581 

mixed  with  more  or  less  sand  and  clay,  which  if  abundant  is  removed 
by  washing  and  screening. 

The  rocks  which  make  up  the  pebbles  are  of  different  kinds,  but 
chiefly  those  which  show  more  or  less  resistance  to  weathering  and 
abrasion. 

Gravel  to  be  of  value  for  roads  should  not  disintegrate  under  traffic, 
and  the  pebbles  should  be  of  variable  size,  so  as  to  have  the  minimum 
quantity  of  voids.  A  certain  amount  of  fine  material  sufficient  to  fill 
these  spaces  is  desirable.  If  the  gravel  is  too  coarse,  some  finer  ma- 
terial should  be  added.  It  is  claimed  that  if  a  gravel  occurs  in  a  some- 
what cemented  condition  in  the  bank  it  is  likely  to  make  a  good  road 
material,  and  that  gravels  containing  many  pebbles  of  rocks  which 
have  good  roadmaking  qualities  are  desirable,  although  the  rounded 
pebbles  of  a  rock  have  generally  less  cementing  power  than  angular 
fragments  of  the  same  kind  of  stone. 

The  fact  that  a  gravel  packs  quickly  does  not  necessarily  indicate 
that  it  will  make  a  good  road,  for  clayey  gravels  do  this,  and  those 
containing  over  20  per  cent  of  clay  are  said  to  make  muddy  roads. 

Iron  oxide  is  a  good  cement,  and  many  gravels  with  it  pack  well 
under  traffic.  Examples  of  such  are  the  yellow  gravels  of  New  Jersey, 
and  the  Lafayette  formation  of  the  southern  states. 

Requirements  of  gravel.  —  These  as  stated  by  different  highway 
engineers  and  road  commissions  vary  somewhat.  As  an  example  we 
may  take  those  issued  by  the  Borough  of  Brooklyn  in  1912.  "The 
Hudson  River  road  gravel  required  shall  be  what  is  known  as  'double 
screened'  and  'fine'  gravel.  It  shall  be  free  from  all  foreign  substances 
and  meet  the  following  requirements.  Double  screened:  Per  cent  wear 
not  to  exceed  5  per  cent.  Percentage  voids  not  to  exceed  45  per  cent. 
The  U.  S.  Dept.  of  Agriculture  cementation  test  must  not  be  under  25. 
The  percentage  retained  on  a  IJ-inch  screen  not  to  be  greater  than  10 
per  cent,  nor  less  than  5  per  cent.  The  percentage  retained  on  a  f-inch 
screen  must  not  be  less  than  75  per  cent.  Fine  gravel:  Percentage  of 
substances  soluble  in  water  not  to  exceed  5  per  cent.  Percentage  re- 
tained on  a  f-inch  screen  not  to  exceed  5  per  cent.  Percentage  in  pow- 
der form  not  to  exceed  5  per  cent. 

Tests  of  gravel.  —  The  quality  of  road  gravel  can  be  determined 
by  several  tests  as  follows :  Mechanical  analysis :  This  is  to  determine 
the  percentage  of  pebbles  of  different  sizes,  and  is  accomplished  by 
passing  the  material  through  different  size  screens  and  noting  the  quan- 
tity caught  on  each.  That  which  passes  200  mesh  is  called  powder. 
Voids:  The  determination  is  made  as  on  crushed  stone  (p.  585). 


582 


ENGINEERING  GEOLOGY 


Quality:  This  is  determined  by  the  abrasion  and  cementation  test  as 
on  crushed  stone  (p.  584).  Solubility  in  water:  This  test,  which  is 
often  desired,  gives  the  amount  of  soluble  matter  obtained  by  boiling 
a  small  sample  in  water  for  one  hour. 

Tests  of  gravel  from  different  localities.  —  The  following  table 
from  Baker  (Ref.  1)  gives  the  tests  of  gravels  from  a  number  of  differ- 
ent localities.  All  the  samples  are  said  to  make  a  good  road  material. 

TESTS  OF  ROAD  GRAVEL 


5 

5 

>" 

[§»-; 

d 

fc.s 

A 

OS 

I 

1 

£>H 

B 

B 

Size  of  mesh. 

1 
p 

I 

s 

i 

1 

|s 

£ 

1 

M 

\±4  ^ 

& 
1 

1 

II 

1 
PH 

1 

Percent  caught  on    2-in.  mesh  

0.0 
0.3 

0.0 

1.1 

0.0 
4.6 

6.0 
35.6 

0.0 
2.1 

0.0 
0.0 

0.0 
0.0 

0.0 
0.0 

0.0 
1.5 

0.0 
9.1 

0.0 
20.5 

0.0 
1.9 

i       '      '.'.'.'. 

9.6 

12.2 

10.0 

23.5 

41.3 

11.7 

3.3 

0.8 

12.2 

12.6 

20.5 

17.8 

I       '      .... 

13.0 

10.5 

8.7 

7.4 

25.5 

11.0 

13.4 

12.7 

18.6 

9.1 

8.8 

11.0 

IB 

41.1 

14.8 

20  2 

9.1 

17.8 

8.4 

25.1 

33.2 

47.5 

21.3 

13.8 

22.3 

1                 ' 

12.1 

3.9 

15.3 

3.2 

2.2 

20.4 

14.0 

7.4 

8.9 

9.3 

5.2 

20.0 

&               '             ...' 

3.9 

7.8 

15.9 

2.7 

1.8 

8.2 

7.2 

5.5 

3.5 

9.3 

1.9 

10.5 

passing  5Vin.  mesh  
in  suspension 

16.2 
3  8 

40.0 
9  6 

21.0 
4  2 

8.7 
3  2 

8.4 
0.9 

20.2 
20.0 

16.3 
20.7 

20.0 
20.2 

6.1 
1.8 

18.8 
9.8 

21.8 

7.8 

10.4 
5.9 

Total  

100.0 

100.0 

99.9 

99.4 

100.0 

99.9 

100.0 

99.6 

100.1 

99.3 

99.7 

99.8 

Per  cent  voids  in  washed  gravel.  .  . 

25.5 

30.5 

27.3 

29.6 

30.5 

26.3 

34.0 

28.2 

25.6 

24.3 

24.5 

25.3 

Chert  gravel.  —  This  material  is  available  at  several  localities  in 
the  Gulf  states,  and  has  given  excellent  results  in  Alabama,  so  that  we 
may  quote  briefly  from  the  State  Geological  Survey  report  (Ref.  13). 

"  Chert,  when  of  the  best  varieties,  such  as  Fort  Payne  chert,  is  one  of  the  very 
best  of  road  materials.  If  it  is  crushed  or  broken  by  hand  so  that  no  piece  is  over 
two  inches  in  diameter,  and  the  majority  even  less,  it  will  make  a  very  lasting  road. 
If  any  fragments  of  larger  dimensions  are  used,  even  under  the  bottom,  it  will  soon 
wear  enough  to  expose  the  big  lumps,  and  then  you  will  have  a  road  that  is  very 
rough,  hard  on  horses,  and  especially  hard  on  rubber  tires.  Chert  binds  together 
better  than  any  of  the  pebble  gravels  or  even  limestone  macadam,  and  the  best 
chert  road  has  little  or  no  dust,  and  consequently  little  or  no  mud." 

There  are,  however,  several  kinds  of  chert,  sometimes  found  in  one 
county,  one  very  hard  and  durable,  and  another  soft  and  chalky. 
Chert  is  sometimes  found  in  large  solid  masses,  but  this,  if  blasted, 
crushed  and  sized,  makes  a  good  macadam  road. 

Broken  Stone 

The  broken  stone  used  for  roads  may  be  of  almost  any  kind  of  rock, 
included  under  the  three  groups,  igneous,  sedimentary  and  metamor- 
phic.  The  mineralogic  composition  and  textural  properties  of  the  dif- 


ROAD  FOUNDATIONS  AND  ROAD  MATERIALS  583 

ferent  kinds  have  already  been  given  in  Chapter  II  and  need  not 
be  repeated  here. 

Attention  should,  however,  be  called  to  the  fact  that  the  minerals 
found  in  rocks  may  be  divided  into  two  classes,  viz.,  primary  and  second- 
ary. The  former  includes  such  minerals  as  quartz,  feldspar,  pyroxene, 
amphibole,  biotite,  muscovite,  calcite,  dolomite,  garnet,  olivine,  etc.; 
the  latter,  minerals  like  chlorite,  kaolinite,  sericite,  limonite,  serpentine, 
epidote  and  sometimes  calcite  and  quartz.  A  small  amount  of  some 
of  these  secondary  minerals  may  increase  the  binding  power  of  the  rock, 
but  an  excess  is  likely  to  have  the  opposite  effect. 

The  weathering  qualities  are  important  and  depend  primarily  on  the 
mineral  composition,  rather  than  on  the  hardness  and  toughness. 

Rocks  whose  grains  are  loosely  held  together  lack  coherence,  and 
may  have  high  porosity,  as  well  as  low  abrasive  and  crushing  resistance. 
Hard  rocks,  whose  grains  are  tightly  interlocked  are  stronger  and  better 
than  the  preceding  class,  even  though  they  may  be  of  low  cementing 
value.  Easily  soluble  rocks,  such  as  limestones,  are  also  bad.  Many 
of  the  strongly  foliated  metamorphic  rocks,  such  as  chlorite  and  mica 
schists,  are  undesirable,  because  owing  to  their  softness  and  structure 
they  wear  easily. 

Properties  of  Crushed  Stone 

The  properties  that  are  commonly  considered  in  the  selection  of 
stone  for  roads  are:  (1)  Abrasive  resistance;  (2)  hardness;  (3)  tough- 
ness; (4)  cementing  value;  (5)  absorption;  and  (6)  specific  gravity. 

"  Resistance  to  wear.  —  Resistance  to  wear  is  a  special  property  in  a  rock,  and 
although  it  depends  to  a  large  extent  upon  both  the  hardness  and  the  toughness  of 
the  rock  it  is  not  an  absolute  function  of  these  qualities. 

The  per  cent  of  wear  in  the  table  refers  to  the  dust  and  detritus  below  one-six- 
teenth of  an  inch  in  size  worn  off  in  the  abrasion  test.  The  test  is  made  in  the  fol- 
lowing manner:  Eleven  pounds  (5  kg.)  of  broken  rock  between  1|  and  2|  inches  in 
size,  50  pieces  if  possible,  are  placed  in  a  cast-iron  cylinder  mounted  diagonally  on 
a  shaft  and  slowly  revolved  10,000  times. 

The  French  coefficient  of  wear  is  obtained  by  dividing  40  by  the  per  cent  of 
wear.  Thus  a  rock  showing  4  per  cent  of  wear  has  a  French  coefficient  of  wear  of 
10.  The  French  engineers,  who  were  the  first  to  undertake  road-material  tests, 
adopted  this  method  of  recording  results.  They  found  that  their  best  wearing 
rocks  gave  a  coefficient  equal  to  about  20.  The  number  20  was  therefore  adopted 
as  a  standard  of  excellence.  In  interpreting  the  results  of  this  test  a  coefficient  of 
wear  below  8  is  called  low;  from  8  to  13,  medium;  from  14  to  20,  high;  and  above 
20,  very  high.  Rocks  of  very  high  resistance  to  wear  are  only  suited  for  heavy 
traffic. 


584  ENGINEERING  GEOLOGY 

Hardness.  —  By  hardness  is  meant  the  resistance  of  a  rock  to  the  grinding  action 
of  an  abrasive  agent  like  sand,  and  it  is  tested  as  follows: 

A  core  1  inch  in  diameter,  cut  from  the  solid  rock,  is  faced  off  and  subjected  to 
the  grinding  action  of  sand  fed  upon  a  revolving  steel  disk  against  which  the  test 
piece  is  held  with  a  standard  pressure.  When  the  disk  has  made  1000  revolutions 
the  loss  in  weight  of  the  sample  is  determined.  In  order  to  report  these  results  on 
a  definite  scale  which  will  be  convenient  the  method  has  been  adopted  of  subtract- 
ing one-  third  of  the  resulting  loss  in  weight  in  grams  from  20.  Thus  a  rock  losing  6 
grams  has  a  hardness  of  20  —  6/3  or  18.  Experience  has  shown  this  to  be  the  most 
convenient  scale  for  reporting  results.  The  results  of  this  test  are  interpreted  as 
follows:  Below  14,  rocks  are  called  soft;  from  14  to  17,  medium;  above  17,  hard. 

Toughness.  —  By  toughness  is  meant  the  resistance  a  rock  offers  to  fracture 
under  impact;  such,  for  instance,  as  the  striking  blow  given  by  a  shod  horse.  This 
property  is  tested  in  a  specially  designed  machine  built  on  the  pile  driver  principle, 
by  which  a  standard  weight  is  dropped  upon  a  specially  prepared  test  piece  until  it 
breaks.  The  height  in  centimeters  of  the  blow  which  causes  the  rupture  of  the 
test  piece  is  used  to  represent  the  toughness  of  the  specimen.  Results  of  this  test 
are  interpreted  so  that  those  rocks  which  run  below  13  are  called  low;  from  13  to 
19,  medium;  and  above  19,  high. 

Cementing  value.  —  By  cementing  value  is  meant  the  binding  power  of  the  road 
material.  Some  rock  dusts  possess  the  quality  of  packing  to  a  smooth,  impervious 
mass  of  considerable  tenacity,  while  others  entirely  lack  this  quality.  Cementing 
value  should  not  be  confused  with  the  property  possessed  by  Portland  cement, 
which  causes  it  to  set  into  a  hard,  stone-like  mass  when  mixed  with  water.  The 
cementation  test  is  made  as  follows: 

The  rock  sample  is  ground  in  an  iron  ball  mill  with  sufficient  water  to  form  a 
stiff,  fine-grained  paste.  From  this  paste  small  briquettes  1  inch  (25  mm.)  in  diam- 
eter and  1  inch  high  are  molded  under  pressure.  After  thorough  drying  the  bri- 
quettes are  tested  under  the  impact  of  a  small  hammer  which  strikes  a  series  of 
standard  blows.  The  number  of  blows  required  to  destroy  the  briquette  is  taken 
as  a  measure  of  the  cementing  value  of  the  dust.  Some  rock  dusts,  when  thoroughly 
dried  into  compact  masses,  immediately  slake  or  disintegrate  when  immersed  in 
water.  It  is  considered  that  the  tendency  to  act  in  this  way  is  not  a  desirable 
characteristic  of  a  road  material,  as  it  would  lead  to  muddy  conditions  on  the  road 
surface  after  rains.  The  test  is  interpreted  so  that  cementing  values  below  10  are 
called  low;  from  10  to  25,  fair;  from  26  to  75,  good;  from  76  to  100,  very  good; 
and  above  100,  excellent. 

Weight  per  cubic  foot.  —  The  weight  per  cubic  foot  refers  to  the  weight  of  the 
material  in  the  form  of  a  solid  and  not  as  broken  stone."1 

Absorption.  —  The  absorption  is  expressed  hi  pounds  of  water  absorbed  per 
cubic  foot,  according  to  the  formula 


y 


w-wz 

1  Quoted  from  U.  S.  Office  of  Public  Roads  report. 


ROAD  FOUNDATIONS  AND  ROAD  MATERIALS  585 


in  which 


Wi  is  weight  of  sample  in  water  after  96  hours  immersion,  in  grams. 

W2  is  weight  of  sample  in  water,  just  after  immersion,  in  grams. 

W  is  weight  in  air,  in  grams. 

62.37  is  weight  of  cubic  foot  of  water.. 

Specific  gravity.  —  This  is  determined  in  the  usual  manner. 

Results  of  tests.  —  In  the  accompanying  table  are  given  the  aver- 
age, maximum  and  minimum  figures  obtained  for  the  several  tests  on 
different  rocks,  as  published  by  the  U.  S.  Office  of  Public  Roads. 

Significance  of  tests.1  —  The  attrition  loss  seems  to  be  conditioned 
by  texture,  mineral  composition  and  degree  of  freshness  of  the  miner- 
als. The  hardest  and  toughest  stones  seem  to  be  those  containing  an 
abundance  of  quartz  and  having  a  dense  fine-grained  texture. 

The  abundant  development  of  secondary  minerals  produced  by 
weathering  is  undesirable,  but  the  presence  of  secondary  minerals  pro- 
duced by  deep-seated  processes,  such  as  uralitic  hornblende  (p.  19), 
seems  to  strengthen  the  rock. 

A  study  of  the  tests  given  hi  the  table  below  leads  to  two  impor- 
tant conclusions:  (1)  The  number  of  different  kinds  of  rocks  used  for 
road  material  is  very  great,  and  (2)  the  tests  of  each  kind  consider- 
ing the  maximum  and  minimum  figures  shows  considerable  range. 
One  may,  therefore,  raise  the  point,  whether  in  engineering  specifications 
it  would  not  be  better  to  demand  that  the  material  meet  certain  tests, 
rather  than  to  simply  call  for  rock  of  a  certain  kind  or  its  "equivalent." 

Tests  made  by  the  U.  S.  Bureau  of  Roads  indicate  that  the  percent- 
age wear  is  less  in  fresh  igneous  and  metamorphic  rocks,  as  well  as  those 
rich  in  secondary  hornblende,  than  it  is  in  the  weathered  varieties. 
But  even  the  slightly  weathered  igneous  rocks  may  yield  better  results 
than  limestones,  dolomites,  calcareous  sandstones  and  cherts. 

Plutonic  rocks  with  granular  texture  are  usually  of  inferior  toughness 
to  their  volcanic  equivalents  (rhyolite,  basalt  and  diabase).  The  sedi- 
mentary rocks  show  a  relation  between  mineral  composition  and 
physical  properties.  Soft  non-resistant  calcareous  rocks,  such  as  lime- 
stones, dolomites  and  calcareous  sandstones,  are  composed  largely  of 
calcite  and  dolomite;  they  are  consequently  of  inferior  hardness, 
toughness  and  wearing  qualities  than  the  more  siliceous  sandstones 
and  cherts.  The  metamorphic  rocks  in  general  resemble  the  igneous 
ones. 

Rocks  in  which  one  or  more  of  the  primary  constituents  have  un- 

1  Bull.  31,  U.  S.  Bureau  Public  Roads,  has  been  largely  drawn  upon  for  these  data. 


586 


ENGINEERING  GEOLOGY 


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ROAD  FOUNDATIONS  AND  ROAD  MATERIALS  587 

dergone  alteration  mainly  through  the  action  of  atmospheric  agencies 
yield  powders  with  proportionately  higher  cementing  values,  than 
those  obtained  from  their  unaltered  prototypes. 

Qualities  of  Individual  Rock  Types 

Trap  and  fine-grained  basic  rocks.  —  The  term  is  a  very  compre- 
hensive one,  and  is  convenient  for  field  use.  It  includes  diabase, 
basalt,  andesite  and  even  fine-grained  gabbro.  Fresh  trap  rocks  are 
hard,  of  high  abrasive  resistance  and  good  cementing  value  if  the 
traffic  is  heavy  enough  to  wear  the  stone.  In  laboratory  tests  they 
give  a  rather  low  cementing  value. 

Fine-grained  volcanics.  —  These  show  a  hardness  similar  to  trap, 
but  are  of  inferior  toughness,  probably  due  to  the  fact  that  the  mineral 
grains  are  not  as  tightly  interlocked.  The  cementing  value  is  about 
the  same  as  trap,  and  they  are  excellent  for  light  traffic. 

Gabbros  and  other  coarse-grained  basic  igneous  rocks.  —  The 
wearing  qualities  of  these  are  not  so  good  as  those  of  the  two  preceding 
groups.  Their  cementing  value  and  hardness  about  equal  those  of 
trap,  but  they  are  of  inferior  toughness.  The  presence  of  small  amounts 
of  secondary  minerals  increases  the  cementing  value.  These  rocks  in 
their  general  properties  stand  intermediate  between  trap  and  granite. 

Granites  and  other  coarse-grained  acidic  igneous  rocks.  —  These 
are  usually  of  low  toughness  and  poor  cementing  value.  The  percent- 
age of  wear  is  about  the  same  as  the  coarse-grained  basic  rocks.  Their 
low  toughness  appears  to  be  largely  due  to  their  texture  which  is 
granular  instead  of  interlocking,  and  to  the  abundance  of  platy  mica. 
The  finer-grained  granites  show  greater  toughness.  The  low  cement- 
ing value  of  granites  may  be  due  perhaps  to  the  lack  of  secondary 
minerals  which  develop  in  basic  rocks.  Coarse  granites  should  if  pos- 
sible be  avoided  for  roads.  If  used  for  road  making,  they  should  be 
placed  in  the  foundation. 

Slates  and  argillaceous  schists.  —  These  in  general  show  a  moder- 
ately high  percentage  of  wear,  low  hardness  and  toughness,  and  only 
fair  cementing  value.  Their  foliated  character  causes  them  to  split 
readily  into  chips,  which  is  objectionable.  The  clayey  varieties  grind 
under  traffic. 

Quartzite  and  quartzitic  conglomerate.  —  These  have  good  wearing 
qualities  and  toughness,  but  are  of  low  cementing  value.  The  latter 
is  such  an  important  property,  that  quartzite  alone  is  not  recommended 
for  roads.  It  can  be  used  if  a  top  dressing  of  stone  with  good  cementing 
qualities  is  employed. 


588  ENGINEERING  GEOLOGY 

Limestone.  —  This  rock  is  generally  of  low  toughness,  low  hard- 
ness, high  wearing  qualities,  but  good  cementing  value.  If  used  alone 
it  sometimes  tends  to  crumble  and  form  dusty  and  muddy  roads,  but 
often  yields  excellent  results  as  a  top  dressing  for  rocks  of  greater 
hardness  and  better  wearing  qualities.  The  presence  of  clay  increases 
its  cementing  value.  It  is  not  adapted  to  heavy  traffic. 

Shales.  —  These  vary  considerably  in  their  nature.  Some  are  soft 
and  clayey,  and  grind  down  easily  to  a  mass  which  is  powdery  in  dry, 
and  muddy  in  wet  weather.  Others  are  hard  and  siliceous,  and  give 
better  results;  indeed  they  make  a  good  road  if  the  traffic  is  not  too 
heavy. 

Economic  considerations.  —  As  said  on  an  earlier  page,  crushed 
stone  for  roads  is  not  usually  hauled  long  distances.  Consequently, 
the  best  of  the  local  material  is  commonly  selected.  It  is  of  importance 
to  remember  in  this  connection,  however,  that  stratified  rocks  es- 
pecially vary  from  point  to  point.  If  a  shale  formation  contains  here 
and  there  heavy  beds  of  sandstone,  suitable  for  road  work,  that  one 
should  be  selected  (other  things  being  equal)  which  contains  the  least 
overburden.  Or,  if  none  are  free  from  it,  select  if  possible  one  con- 
taining the  thinnest  top  material,  or  where  the  slope  is  gentle,  so  that 
the  stripping  does  not  increase  too  rapidly  in  thickness. 

Limestones  may  also  vary  in  their  nature,  some  being  more  clayey 
and  of  better  cementing  value  than  others.  This  difference  may  not 
show  on  inspection,  so  that  it  is  well  to  test  samples  from  different 
outcrops. 

Where  igneous  rocks  are  to  be  employed,  and  considerable  tonnage 
of  stone  is  required,  the  precaution  should  be  taken  to  ascertain  that 
the  rock  selected  is  an  intrusive  rock,  or  flow  of  sufficient  size,  and 
not  merely  a  dike. 

Stone  Blocks 

Blocks  for  roadways  are  usually  made  of  granite,  although  sandstone, 
quartzite,  and  trap  are  sometimes  used.  Their  essential  properties  are 
resistance  to  weather,  and  sufficient  abrasive  resistance  to  prevent  their 
wearing  round  and  smooth  under  traffic. 

Granite  is  preferred  for  blocks  because  it  splits  easily.  Trap  is 
harder  and  tougher  and  hence  does  not  cut  so  readily,  neither  does  it 
wear  round  as  granite  does,  but  more  uniformly,  even  though  at  times 
somewhat  readily.  Sandstone  cuts  easily,  and  in  New  York  State  the 
Medina  sandstone  as  well  as  the  Potsdam  quartzite  are  said  to  have 
been  used  for  pavements  (Ref.  2).  Much  quartzite  has  also  been 
employed  in  Chicago. 


ROAD  FOUNDATIONS  AND  ROAD  MATERIALS  589 

The  size  of  stone  paving  blocks  is  variable.  Blanchard  (Ref .  2)  gives 
the  United  States  large  size  standard  for  a  first-class  pavement  as 
from  7  to  8  inches  deep,  3  to  4J  inches  wide,  and  8  to  12  inches  long. 
"  A  light  block  which  is  from  4  to  4J  inches  deep,  3J  to  4  inches  wide, 
and  8  to  12  inches  long,  is  also  used  under  certain  conditions." 

There  are  no  special  tests  for  paving  blocks. 


References  on  Road  Materials 

1.  Baker,  Roads  and  Pavements,  Wiley  &  Sons,  New  York.  2.  Blan- 
chard and  Drowne,  Textbook  on  Highway  Engineering,  Wiley  &  Sons, 
New  York,  1913. 

State  References 

3.  Ashley,  Term.  Geol.  Surv.,  I,  No.  2,  1911  (Chert).  4.  Blatchley, 
Ind.  Dept.  Geol.  and  Nat.  Res.,  30th  Ann.  Rept.,  1906  (Ind.).  5.  Buck- 
ley, Wis.  Geol.  and  Nat.  Hist.  Surv.,  Bull.  10,  1903  (Highway  Con- 
struction, Wis.).  6.  Clark,  Johnson  and  Reid,  Md.  Geol.  Surv.,  Vols. 
Ill,  1899;  IV,  1902;  V,  1905  (Md.).  7.  Landes,  Wash.  Geol.  Surv., 
Bull.  2,  1911  (Wash.).  8.  Leighton  and  Bastin,  U.  S.  Dept.  Agric., 
Office  Pub.  Roads,  Bull.  33,  1908  (s.  e.  Me.).  9.  Lewis,  N.  J.  Geol. 
Surv.,  Ann.  Rept.  1906,  1907  (Trap  in  N.  J.).  10.  Lord,  U.  S.  Dept. 
Agric.,  Office  Pub.  Roads,  Bull.  37,  1911.  (Examination  and  Classifica- 
tion of  Rocks  for  Roads.)  11.  Merrill,  N.  Y.  State  Museum,  Bull.  17, 
1897  (N.  Y.).  12.  Pratt,  N.  C.  Geol.  Surv.,  Economic  Paper  27,  1912. 
13.  Prouty  and  others,  Ala.  Geol.  Surv.,  Bull.  11, 1911  (Ala.).  14.  Sel- 
lards,  Fla.  Geol.  Surv.,  1st  Ann.  Rept.,  1908  (Fla.).  15.  Sloan,  S.  C. 
Geol.  Surv.,  ser.  4,  Bull.  2,  1908  (S.  C.).  16.  Snider,  Okla.  Geol.  Surv., 
Bull.  7,  1911  (Okla.). 

See  also  the  publications  issued  by  Office  of  Public  Roads,  Wash- 
ington, D.  C. 


CHAPTER  XVII 

ORE  DEPOSITS 

Nature  and  Occurrence 

THIS  chapter  gives  an  outline  of  the  general  principles  of  ore  deposits, 
including  the  origin,  character  and  more  important  changes  which 
take  place  in  them,  but  does  not  attempt  a  detailed  discussion  of  their 
distribution. 

Definition  of  ore  deposits.  —  The  term  ore  deposits  is  applied  to 
concentrations  of  economically  valuable  metalliferous  minerals  found 
in  the  earth's  crust. 

The  ore  may  be  said  to  include  those  portions  of  the  ore  deposit 
which  contain  the  metallic  mineral  in  sufficient  quantity  and  in  the 
proper  combination  to  make  its  extraction  both  possible  and  profitable. 

Ore  minerals  are  those  minerals  carrying  the  desired  metallic  con- 
tents which  occur  within  the  deposit.  Thus  galena  and  cerussite  are 
ore  minerals  of  lead;  chalcocite,  chalcopyrite,  and  azurite  are  ore  miner- 
als of  copper;  and  magnetite  and  siderite  are  ore  minerals  of  iron. 

An  ore  deposit  may  contain  ore  minerals  of  one  or  several  metals 
or  several  ore  minerals  of  the  same  metal. 

Compounds  serving  as  ore  minerals.  —  Only  a  few  elements,  such  as 
gold,  copper,  platinum,  and  mercury,  occur  in  ores  in  the  native  form. 

In  most  cases  the  metal  is  combined  with  other  elements,  forming 
sulphides,  hydrous  oxides,  carbonates,  sulphates,  silicates,  chlorides, 
and  phosphates. 

Gangue  minerals.  —  Associated  with  the  ore  minerals  there  are 
usually  certain  common  ones,  chiefly  of  non-metallic  character,  which 
carry  no  values  worth  extracting.  These  are  the  gangue  minerals,  and 
of  these  quartz  is  the  commonest,  but  calcite,  barite,  fluorite,  and  sid- 
erite are  also  common,  while  dolomite,  hornblende,  pyroxene,  feldspar, 
rhodochrosite,  etc.,  are  found  in  some  ore  bodies. 

The  gangue  minerals  may  be  more  or  less  intimately  mixed  with  the 
ore  minerals,  or  segregated  in  masses.  In  the  former  case,  if  there  is 
sufficient  difference  in  specific  gravity  between  the  ore  and  gangue 
minerals,  the  ore  can  be  crushed,  and  the  two  often  separated  by  me- 
chanical concentration.  In  the  latter,  the  masses  of  gangue  can  be 

590 


ORE  DEPOSITS  591 

avoided  or  thrown  out  in  mining.  If  the  ore  is  low  grade,  and  both  ore 
and  gangue  minerals  in  a  finely  divided  condition,  leaching  may  be  re- 
sorted to  as  the  first  step  in  separating  the  metal.  Or  if  the  metalliferous 
mineral  is  magnetic,  a  process  of  magnetic  separation  can  be  employed. 

Origin  of  Ore  Bodies 

In  an  early  paragraph,  ore  deposits  have  been  referred  to  as  natural 
concentrations.  This  being  so,  they  must  have  been  concentrated 
either  at  the  same  time  as  the  enclosing  rock  (contemporaneous  de- 
posits) or  else  they  have  been  formed  by  a  process  of  concentration  at 
a  later  date  (subsequent  deposits).  Most  ore  deposits  belong  to  the 
second  group,  not  a  few  to  the  first,  but  the  origin  of  many  is  still  in 
doubt. 

Contemporaneous  ore  deposits.  —  These  (known  also  as  syngenetic 
deposits)  may  occur  hi  igneous  or  sedimentary  rocks.  Those  found  in 
igneous  rocks  are  said  to  be  of  magmatic  origin,  and  the  field  evidence 
goes  to  show  that  they  have  been  derived  from  the  igneous  magma  by 
a  process  of  segregation  (see  also  Chapter  on  Rocks).  In  other  words, 
as  the  ore  minerals  crystallized  out  they  gathered  together.  In  many 
cases  the  ore  grades  into  the  surrounding  rock;  in  others  it  is  sharply 
separated  from  the  igneous  mass,  reminding  one  of  a  dike.  Indeed,  the 
supposition  is  that  it  represents  a  very  basic  segregation,  which  has 
been  forced  up  from  below,  subsequent  to  the  intrusion  of  the  igneous 
rock  itself,  but  not  necessarily  in  all  cases  before  the  enclosing  igneous 
mass  had  entirely  cooled. 

Most  magmatic  ores  are  usually  associated  with  basic  igneous  rocks. 
The  best-known  examples  in  the  United  States  are  the  titaniferous 
iron  ores  of  the  Adirondack  Mountains,  New  York;  Iron  Mountain, 
Wyo.,  etc.  The  nickel-copper  ores  of  Sudbury,  Ont.,  and  the  gigantic 
Scandinavian  iron-ore  deposits  of  Kirunavaara  and  Luossavaara  are 
other  well-known  cases.  Chromic  iron  ores  are  no  doubt  formed  hi 
this  manner. 

When  the  contemporaneous  deposits  are  of  sedimentary  origin  they 
may  be  either  interstratified  or  surface  deposits. 

The  former  have  originated  from  processes  similar  to  those  which 
have  formed  the  enclosing  rocks.  Some  have  accumulated  by  pre- 
cipitation from  sea  water  or  fresh  water,  while  others  have  had  a  me- 
chanical origin,  having  been  set  free  by  the  disintegration  of  rocks  on 
the  land,  the  grains  being  washed  into  the  sea  or  valleys. 

The  best  example  that  we  have  of  an  interstratified  deposit  is  the 
Clinton  iron  ore  (hematite)  found  from  New  York  to  Alabama  (Fig. 


592 


ENGINEERING  GEOLOGY 


FIG.  211.  —  Section  of  Red  Mountain,  Birming- 
ham, Ala.,  containing  a  bedded  ore  deposit  of 
contemporaneous  origin.  (After  Burchard, 
Amer.  Inst.  Min.  Engrs.,  XL,  1910.) 


211),  as  well  as  in  Ohio  and  Wisconsin.  It  is  of  medium  grade,  and 
though  of  great  areal  extent  is  not  much  worked,  except  in  the  Bir- 
mingham, Ala.,  region,  which  is  second  hi  importance  only  to  the 
Lake  Superior  iron  district.  The  Torbrook  ares  of  Nova  Scotia,  and 

the  Wabana  Island  hema- 
tite of  Newfoundland,  are 
probably  also  of  this  type. 
Surface  deposits  of  con- 
temporaneous origin  include 
the  placer  or  gravel  deposits 
so  well  known  to  the  gold 
miner.  They  represent  the 
heavier  products  of  rock 
decay  which  have  settled 
down  usually  in  stream 
channels,  and  in  other  cases 
have  accumulated  along  sea 
beaches.  If  the  formations  from  which  they  are  derived  contain 
metallic  minerals  of  durable  nature,  such  as  gold,  tin,  platinum,  etc., 
they  become  concentrated  in  the  lower  part  of  the  gravel  deposit. 
The  gold  gravels  of  California  and  Alaska  are  of  this  type. 

Tin  ore  and  platinum  are  also  obtained  chiefly  from  placers,  but 
neither  is  of  much  importance  in  the  United  States. 

Subsequent  ore  deposits.  —  In  the  formation  of  this  type  of  ores 
(known  also  as  epigenetic),  the  metallic  compounds  have  been  gathered 
from  the  different  rocks,  mainly  through  the  agency  of  water,  and  de- 
posited under  favorable  conditions.  These  facts  are  susceptible  of 
reasonably  strong  proof,  on  the  following  grounds: 

It  is  a  well-known  fact  that  metallic  minerals  in  small  quantities  are 
widely  distributed  through  both  igneous  and  sedimentary  rocks.  In 
the  former  they  are  not  impartially  distributed,  for  certain  metals  seem 
to  favor  certain  rocks.  Thus  iron,  manganese,  nickel,  cobalt,  chro- 
mium, platinum,  and  titanium  seem  to  favor  basic  rocks;  while  tin, 
tungsten,  and  some  rarer  metals  favor  the  acid  ones.  Although  the 
occurrence  of  metallic  minerals  in  the  rocks  of  the  earth's  crust  is 
widely  recognized,  few  probably  realize  the  .small  percentage  existing 
outside  of  the  concentrated  portions  of  ore  deposits,  and  the  follow- 
ing table,  which  shows  the  average  composition  of  the  earth's  crust, 
will  bring  out  this  point,  the  figures  being  those  given  by  F.  W. 
Clarke.1 

1  U.  S.  Geol.  Survey,  BuU.  491,  p.  27,  1910. 


ORE  DEPOSITS 


593 


AVERAGE  COMPOSITION  OF  EARTH'S  CRUST 
Oxygen 47 . 05        Manganese 077 


Silicon 28.26 

Aluminum '.     7 . 98 

Iron 4.47 

Calcium 3 . 43 

Magnesium 2 . 34 

Potassium 2.50 

Sodium 2.54 

Titanium 45 

Hydrogen 16 

Carbon .  .  .13 


Sulphur 11 

Barium 097 

Chromium 033 

Nickel 023- 

Lithium 004 

Chlorine 06 

Fluorine 10 

Zirconium 025 

Vanadium 018 

Strontium..  .033 


Phosphorus 11 

The  above  figures  make  clear  the  interesting  fact  that,  of  some  twenty 
metals  which  are  of  importance  to  us  for  daily  use,  only  five,  viz., 
aluminum,  iron,  manganese,  chromium,  and  nickel,  are  included  hi 
the  above  list,  and  that  the  others  must  be  present  in  amounts  of  less 

than  .01  per  cent. 

ANALYSES  OF  MINE  WATERS 
(Parts  per  million) 


j 

II. 

III. 

IV. 

SO4 

406.5 

2672. 

43.2 

2039.51 

Cl  

6.8 

13. 

7.9 

8.16 

CO3 

13  2 

110.5 

XO3 

PO4 

tr. 

tr. 

tr. 

B4O7 

tr. 

Br 

tr. 

F 

tr. 

SiO2 

23  2 

47  7 

25.9 

43.80 

K 

7  1 

13.1 

10.6 

70.0 

Na 

16  2 

39.6 

36.4 

106.27 

Li 

tr 

tr. 

Ca 

151.2 

132.5 

37.4 

187.15 

Mg 

28.2 

61.6 

12.25 

93.50 

Al  . 

83.5 

0.4 

3.12 

Mn 

0  5 

12  0 

0.8 

155.58 

Ni 

) 

J-  . 

0.5 

Co 

\ 

Cu     . 

tr. 

59.1 

tr. 

77.05 

Zn  

0.3 

852. 

0.2 

49.66 

Fe'".. 

I                 1     Q 

Fe".. 

}         1.8 

159.8 

.7 

164.82 

Cd 

41.1 

Pb 

tr. 

3.44 

C02  

• 

37.2 

I.  Green  Mountain  Mine,  Butte,  Mont.,  2200-foot  level  fissure  in  granite,  remote  from  known  veins; 
II.  St.  Lawrence  Mine,  Butte,  Mont;  III.  Geyser  mine,  Custer  Co.,  Col.;  IV.  Stanley  mine,  Idaho 
Springs,  Col.  All  quoted  by  Emmons,  U.  S.  Geol.  Survey,  Bull.  529,  pp.  60,  62,  and  63,  1913. 

Mode  of  concentration.  —  There  seems  to  be  little  doubt  that 
water  has  served  as  the  chief  concentrating  agent  of  subsequent  ores, 
for  the  following  reasons: 


594  ENGINEERING  GEOLOGY 

1.  Water  is  known  to  be  widely  distributed  through  the  rocks  of 
the  earth's  crust,  much  of  it  being  in  slow  but  constant  circulation. 
Some  of  it  is  surface  water  that  has  penetrated  to  a  moderate  depth, 
and  some  of  it  is  magmatic  water  that  has  been  given  off  by  igneous 
rocks  while  cooling  and  solidifying. 

2.  Water  if  pure  has  very  little  solvent  power,  but  if  it  contains 
acids  or  alkalies,  or  if  it  is  heated  or  under  pressure,  its  solvent  power 
is  increased. 

3.  Many  mine  waters  contain  metallic  compounds  in  solution,  and 
hotsprings  are  even  now  found,  which  are  depositing  such  metals  as 
gold,  tin,  copper  or  mercury  as  they  reach  the  surface.     The  analyses 
of  mine  waters  given  on  page  593  are  of  interest. 

Source  of  concentrating  waters.  —  Most  geologists  admit  that  cir- 
culating water  in  the  rocks  has  been  an  important  ore  carrier,  but  all 
are  not  in  agreement  as  to  its  source;  one  class  maintaining  that  it  is 
largely  of  meteoric  origin,  the  other  that  it  is  mostly  from  magmatic 
sources. 

Concentration  by  meteoric  waters.  —  According  to  Van  Hise,  who 
has  been  the  most  ardent  modern  exponent  of  this  theory,  water  filtering 
down  from  the  surface  into  the  rocks  flows  through  various  openings, 
such  as  fissures  due  to  jointing,  faulting,  stratification,  or  cleavage, 
through  the  pores  between  the  grains  or  through  irregular  openings. 

The  entering  waters  then  first  percolate  downward  by  gravity,  and 
then  in  general  laterally,  finally  emerging  again  at  the  surface.  The 
movement  is  supposed  to  be  from  areas  of  high  pressure  to  areas  of  low 
pressure,  and  the  path  traversed  may  be  very  irregular. 

Gravity  is  supposed  to  be  the  chief  cause  of  the  circulation,  but 
other  factors  are  not  to  be  overlooked. 

Thus,  as  the  waters  reach  increasing  depths,  their  temperature  rises, 
and  this  is  accompanied  by  a  decrease  in  the  viscosity  of  the  water, 
accelerating  its  circulation  at  those  depths. 

Concentration  by  magmatic  waters.  —  It  is  probably  safe  to  say 
that  the  majority  of  geologists  hold  the  view  that  most  ores  have  been 
primarily  deposited  by  magmatic  waters. 

These  waters  often  emanate  from  the  magma  in  vaporous  form 
because  of  high  temperature  and  pressure  conditions,  but  as  the  vapors 
travel  farther  away  from  the  eruptive  where  temperature  and  pressure 
are  less,  they  are  condensed  to  a  liquid  condition. 

This  water,  called  juvenile  water,  is  evidently  present  in  many  molten 
rock  masses  or  magmas,  although  some  have  disputed  it.  However, 
volcanic  gases  which  have  been  tested  show  its  presence,,  and  as  the 


ORE  DEPOSITS  595 

rock  solidifies,  and  minerals  which  have  little  or  no  water  of  combina- 
tion crystallize  out,  the  water  is  gradually  forced  from  the  cooling  and 
solidifying  mass.  With  the  water  there  are  usually  other  gaseous  sub- 
stances. As  these  solutions  leave  the  magma  they  carry  some  metallif- 
erous compounds  with  them  in  solution,  and  as  they  pass  through  the 
rocks  on  their  way  toward  the  surface  they  may  add  to  their  burden  of 
dissolved  substances. 

The  facts  in  general  rather  seem  to  favor  magmatic  waters  for  the 
following  reasons: 

1.  Meteoric  waters  do  not  reach  great  depths,  in  fact  probably  not 
more  than  2000  feet  or  even  less  from  the  surface,  unless  they  penetrate 
along  some  fissure. 

2.  The  lower  levels  of  many  deep  mines  are  so  dry  as  to  be  dusty. 

3.  Ore  deposits  reach  a  much  greater  depth,  than  that  penetrated  by 
surface  waters. 

4.  Igneous  rocks  are  known  to  expel  water  during  cooling. 

5.  Most  metalliferous  veins  and  ore  bodies  are  found  in  regions  of 
igneous  rocks,  and  many  have  been  formed  at  the  same  period  as  the 
associated  intrusives. 

It  should  be  said,  however,  that  a  few  ore  bodies  are  undoubtedly 
primarily  concentrated  by  surface  waters,  and  that  secondary  concen- 
tration is  in  the  vast  majority  of  cases  performed  by  these. 

Deposition  of  ores.  —  The  deposition  of  ores  from  solution  may 
occur  in  two  ways,  viz.,  (1)  hi  cavities,  and  (2)  by  replacement. 

Cavity  deposition.  —  The  deposition  of  ores  in  the  rocks  is  often 
due  to  the  presence  of  cavities  through  which  the  ore-bearing  solutions 
pass,  at  times  somewhat  freely,  and  many  ore  deposits  occupy  such 
spaces.  These  cavities  may  be  formed  in  different  ways,  and  may 
occur  in  all  kinds  of  rocks.  Thus  they  may  represent  solution  cavities 
in  limestones,  joint  or  fault  fissures,  and  interspaces  of  a  breccia,  gas 
and  shrinkage  cavities  in  igneous  rocks,  the  pores  between  the  grains 
of  a  sedimentary  rock,  etc. 

Precipitation  of  metals  from  solution.  If  the  metalliferous  and  other 
minerals  were  taken  into  solution  at  considerable  depths  where  tem- 
perature and  pressure  were  high,  then  as  the  waters  rose  towards  the 
surface,  where  both  of  these  were  less,  the  decreasing  solvent  power 
of  the  solution  would  cause  it  to  deposit  some  of  the  dissolved  material. 
In  other  cases  the  deposition  of  the  metals  may  have  been  due  to  the 
mingling  of  different  solutions,  resulting  in  chemical  reactions  which 
yielded  insoluble  compounds.  The  contact  of  solutions  carrying 
sulphates,  with  carbon,  organic  matter  or  other  reducing  agents,  would 


596 


ENGINEERING  GEOLOGY 


reduce  these  to  insoluble  sulphides.     Or,  in  other  cases  the  approach 

of  a  solution  to  the  surface,  where  it  is 
exposed  to  oxidizing  conditions,  could 
also  cause  precipitation,  as  the  change 
of  ferrous  sulphate  to  hydrous  ferric 
oxide. 

Where  precipitation  takes  place  on 
the  walls  of  a  cavity,  the  ore  and 
gangue  minerals  are  sometimes  built 
up  layer  upon  layer  (crustified).  There 
is  also  a  sharp  boundary  between  ore 
body  and  wall  rock. 

Replacement.  —  It  is  now  widely 
recognized  that  under  favorable  con- 
ditions mineral- bearing  solutions  may 
attack  the  rock  through  which  they 
move,  dissolving  them  wholly  or  in 
part,  and  depositing  other  mineral 
compounds  in  the  place  of  the  mineral 
matter  removed.  This  is  known  as 
replacement  or  metasomatism.  In  some 
cases  the  substitution  is  complete,  as 
when  calcite  is  removed  and  quartz  is 
deposited;  in  others  it  is  only  partial 
as  when  iron-bearing  silicates  are  decomposed  by  sulphur-bearing  solu- 
tions, and  pyrite  is  formed,  or  when  lime 
silicate  replaces  lime  carbonate. 

The  ore-bearing  solutions  enter  the 
rock  along  channels  of  access,  and  at- 
tack the  minerals,  penetrating  first  along 
cleavage  planes  or  fracture  lines,  and 
then  attacking  the  solid  portion  of  the 
grains.  The  change  then  is  a  progres- 
sive one,  which  seems  to  be  independent 
of  the  specific  gravity,  or  volume  of  the 
minerals  involved.  The  simplest  and 
most  common  type  of  replacement  is 
that  of  the  calcium  carbonate  of  fossils 
by  silica,  or  by  pyrite. 

Replacement  is  an  important  process 
in  the  formation  of  ore  deposits.  Certain  rocks  such  as  limestone  are 


FIG.  212.  —  Vein  filling  a  fault  fis- 
sure. Enterprise  mine,  Rico,  Col. 
(After  Rickard,  Amer.  Inst.  Min. 
Engrs.,  XXVI,  1897.)  Shows  ir- 
regular banding,  also  vugs  in 
center  of  vein.  White  vein  mate- 
rial is  quartz ;  dark,  is  blende  and 
rhodochrosite. 


FIG.  213.  —  Photomicrograph  of  a 
section  of  quartz  conglomerate, 
snowing  replacement  of  quartz 
(white)  by  pyrite  (black)  X  25 
diam.  (After  Smyth,  Amer. 
Jour.  Sci.,  XIX,  1905.) 


ORE  DEPOSITS 


597 


FIG.  214.  —  Photomicrographs  of  thin  sections  of  sulphide  ore  from  Austinville,  Va., 
mines  X  20  diameters,  crossed  nicols.  Show  crystalline  granular  dolomitic 
limestone,  and  the  filling  of  fine  cracks  accompanied  by  replacement  of  lime- 
stone grains  along  crystallographic  directions  by  the  sulphides.  Very  dark 
irregular  areas  in  center  represent  sulphides.  Re-entrant  angles  along  margins 
of  the  sulphides  and  the  spider-like  arrangement  of  the  sulphide  areas  as.  a 
whole  are  well  shown.  (After  Watson,  Va.  Geol.  Survey,  Bull.  1.) 


FIG.  215.  —  Section  through  the  Tuscon  shaft,  Leadville,  Col.,  showing  replace- 
ment ore  bodies.     (After  Argall,  Eng.  and  Min.  Jour.,  LXXXIX,  1910.) 


598  ENGINEERING  GEOLOGY 

more  easily  replaced  than  shales  or  quartzites,  but  few  rocks  under 
proper  conditions  entirely  resist  the  process.  Ferromagnesian  minerals 
like  hornblende  are  replaced  more  readily  than  the  more  acid  silicates, 
such  as  feldspar. 

The  process,  moreover,  is  sometimes  repeated  in  the  same  rock,  as  in 
the  lead-silver  mines  of  the  Coeur  d'Alene  district  of  Idaho,  where 
quartz  is  replaced  by  siderite,  and  both  in  turn  by  galena. 

The  boundaries  of  replacement  deposits  are  usually  indefinite,  but 
not  necessarily  so. 

Physical  conditions  of  ore  deposition.  —  It  has  been  pointed  out 
that  ore-bearing  solutions  are  given  off  by  igneous  rocks,  and  that  they 
move  towards  the  surface,  passing  through  zones  of  decreasing  pres- 
sure, and  gradually  becoming  cooler.  Thus  we  see  that  there  is  a 
gradual  change  of  physical  conditions  as  we  go  towards  the  surface. 

Starting  with  this  reasonable  hypothesis  as  a  basis,  and  carefully 
studying  all  available  evidence,  we  find  that  many  different  minerals 
appear  to  have  a  critical  level.  In  other  words,  certain  minerals  can 
exist  or  form  under  certain  conditions  of  temperature  and  pressure,  but 
not  under  others.  Some  minerals,  on  the  other  hand,  persist  through 
a  wide  range  of  conditions. 

In  addition,  the  wall  rocks  traversed  by  the  ore  solutions  may  be 
more  or  less  profoundly  and  .characteristically  altered.  It  must  not 
be  supposed  that  the  magmatic  solutions  arrived  undiluted  at  the 
surface,  for  as  they  approach  the  latter  they  no  doubt  mingle  with  sur- 
face waters. 

Close  to  the  igneous  rock  where  pressure  and  temperature  are  suffi- 
ciently high  to  heat  the  water  above  its  critical  point  (365°  C.)  it  must 
be  in  a  vaporous  form,  and  the  process  of  deposition  under  these  con- 
ditions is  termined  pneumatolysis  (gaseous).  If  deposition  occurs 
when  the  water  is  in  a  liquid  form,  it  is  termined  hydatogenesis  (aqueous). 

We  may  now  refer  to  several  types  of  deposits  which  are  more  or  less 
characteristic  of  certain  conditions. 

Pneumatolytic  deposits.  Tin  and  apatite  veins  belong  to  this  type. 
Around  the  borders  of  some  granitic  masses  there  are  found  pegmatite 
veins,  carrying  cassiterite,  wolframite,  etc.,  as  well  as  fluorspar,  topaz, 
and  tourmaline. 

The  wall  rocks  of  such  veins  have  been  strongly  altered,  the  feldspar 
and  mica  especially  being  attacked  by  the  water  vapors  carrying  fluorine, 
and  replaced  by  a  mass  of  quartz,  topaz,  tourmaline,  and  lepidolite 
giving  a  rock  type  termed  greisen.  Cassiterite  may  be  present  in  the 
wall  rock  as  well  as  in  the  vein. 


PLATE  C. —Photomicrographs 
of  polished  surfaces  showing: 
FIG.  1.  —  Intergrowth  of  bor- 
nite  and  chalcocite  indicating 
contemporaneous  deposition 
of  the  two  minerals.  Dark 
areas  bornite,  light  areas  chal- 
cocite X  40  diameters. 


FIG.  2.  —  Intergrowth  of  bornite  and  chalco- 
cite. Dark  areas  bornite,  light  areas  chal- 
cocite. 


FIG.  3.  —  Secondary  chalcocite  in  fractures  in  bornite,  X  Iff  diameters.  The  mass 
of  bornite  is  penetrated  hi  all  directions  by  a  network  of  chalcocite  veinlets. 
All  from  Virgilina  copper  district.  (After  Laney.) 

(599) 


600  ENGINEERING  GEOLOGY 

Such  tin  veins  are  believed  to  have  been  formed  from  a  mixture  of 
magma,  watery  vapor,  and  gases,  given  off  during  the  cooling  of  the 
igneous  mass. 

The  apatite  veins  form  an  analogous  group,  which  is  related  to 
basic  rocks  such  as  gabbro,  and  contain  chlorine,  in  place  of  fluorine, 
as  the  prominent  mineralizing  agent. 

They  may  carry  specularite  and  pyrrhotite  as  ore  minerals,  and 
scapolite,  diopside,  hornblende,  and  biotite  as  silicates. 

Contact-metamorphic  deposits.  —  These  include  certain  deposits  found 
in  some  sedimentary  rocks,  chiefly  calcareous  ones,  near  their  contact 
with  igneous  intrusions,  especially  those  of  a  more  or  less  acid  character. 

The  ore  deposits  are  a  mixture  of  silicates  and  ore  minerals.  The 
former  when  occurring  in  limestone  include  garnet,  wollastonite,"epidote, 
diopside,  amphibole,  etc.,  while  in  shale  or  slate  we  find  andalusite, 
sillimanite,  biotite,  etc. 

The  common  ore  minerals  are  magnetite  and  specularite,  mixed 
with  sulphides  such  as  bornite,  chalcopyrite,  pyrite,  pyrrhotite,  and 
more  rarely  galena  and  sphalerite.  Gold  and  silver  may  be  present. 

Since  these  contact-metamorphic  deposits  are  formed  sometimes  in 
limestones  which  in  their  unaltered  condition  are  practically  pure 
calcium  carbonate,  it  is  quite  evident  that  the  foreign  substances  came 
from  the  igneous  rock. 

They  were  given  off  in  solution  in  watery  vapor,  possibly  under 
gaseous  or  partly  gaseous  conditions.  These  were  forced  out  into  the 
fissures  and  pores  of  the  limestone,  and  replaced  the  latter  wholly  or 
in  part. 

The  deposits  are  somewhat  bunchy  in  character  and  of  irregular 
shape,  and  as  a  whole  do  not  extend  very  far  from  the  contact.  Where 
the  beds  of  sedimentary  rock  vary  in  their  character,  the  ore  is  con- 
fined to  or  more  abundant  in  those  which  are  more  easily  replace- 
able, and  this  fact  should  be  borne  in  mind  when  exploiting  such  ore 
bodies. 

Among  the  important  occurrences  of  this  type  may  be  mentioned 
the  Morenci,  Ariz.,  copper  deposits,  and  the  Iron  Springs,  Utah,  iron 
deposits.  Another  important  locality  is  that  of  Bingham  Canyon, 
Utah,  although  here  the  main  production  of  the  camp  now  comes  from 
the  disseminated  ore,  found  in  the  porphyry  near  its  contact  with  the 
limestone. 

Deep  seated  gold  and  silver  veins.  —  These  represent  a  class  of  veins 
which  have  probably  formed  at  considerable  depths,  where  temperature 
and  pressure  were  relatively  high.  They  are  usually  associated  with 


ORE  DEPOSITS  601 

granitic  intrusions  in  schists,  and  show  a  strong  replacement  of  the 
country  rock. 

The  characteristic  minerals  of  this  type  are  gold,  pyrite,  pyrrhotite, 
galena,  zinc  blende,  magnetite,  specularite,  ilmenite,  quartz,  biotite, 
tourmaline,  garnet,  hornblende,  chlorite,  apatite,  spinel,  and  epidote. 
The  amphibolites  and  micaceous  schists  show  replacement  by  tour- 
maline, garnet,  green  biotite,  and  epidote.  The  soda-lime  feldspars 
are  unstable  under  the  influence  of  the  vein-forming  solutions,  and 
alkali  feldspars  do  not  usually  form. 

Ore  deposits  at  shallow  depths.  —  The  veins  of  this  type  are  formed 
near  the  surface,  that  is  from  a  few  hundred  to  four  or  five  thousand 
feet,  this  being  shown  by  their  occurrence  in  beds  of  relatively  recent 
volcanic  rocks.  Additional  structural  features  indicative  of  their 
shallow  depths  are:  (1)  The  greater  number  and  width  of  fissures  near 
the  surface;  (2)  branching  of  the  upper  parts  of  the  fissures;  and  (3) 
changing  dip  of  fissures,  the  deeper  portions  of  which  are  likely  to  have 
a  flatter  dip. 

The  wall  rock  also  shows  strong  alteration,  which  may  extend  to  .a 
greater  distance  from  the  vein  than  it  does  in  deeper  ones.  If  the  wall 
rock  is  of  medium  acidity,  it  is  often  strongly  sericitized  (changed  to 
sericite,  p.  603)  along  the  veins,  and  it  is  often  pyritized  as  well. 

Another  important  type  of  alteration  is  propylitization  (p.  602),  which 
in  basic  rocks  extends  close  up  to  the  veins,  where  sericitization  takes 
its  place.  Such  propylitized  rocks  are  often  greenish-gray  hi  color, 
and  show  bright-green  epidote.  The  pyrite  usually  shows  well  de- 
veloped crystals,  but  oxidizes  easily  on  the  surface,  giving  the  rock 
a  red,  brown,  or  yellowish  color. 

But  while  propylitization  often  accompanies  ore  deposits  formed  at 
shallow  depths,  it  is  sometimes  very  extensively  developed  otherwise, 
and  does  not  necessarily  indicate  the  presence  of  ore  bodies. 

In  shallow-formed  veins,  gold  and  silver  are  the  prevailing  ores,  but 
the  silver  is  usually  relatively  more  abundant  than  it  is  in  the  deeper 
veins  with  quartz  gangue,  and  the  gold  is  commonly  more  finely  divided. 
Like  the  deeper  veins  they  may  carry  pyrite,  galena,  and  sphalerite, 
but  hi  addition  chalcopyrite,  arsenopyrite,  argentite,  and  stibnite  are 
characteristic  ore  minerals.  Magnetite  and  specularite  are  absent. 

Quartz  is  a  common  gangue  mineral,  and  chalcedony  or  opal  are 
sometimes  associated  with  it.  Calcite  and  dolomite  are  rather  abun- 
dant while  siderite  is  rare,  and  both  barite  and  fluorite  may  be  abundant 
locally.  Filling  of  open  spaces  is  an  important  process.  The  Cripple 
Creek,  Col.,  region  is  an  example  of  this  type  of  occurrence.  Here 


602  ENGINEERING  GEOLOGY 

the  ore  occurs  chiefly  as  veins,  in  Tertiary  volcanic  rocks,  which  fill 
the  throat  of  a  volcano  hi  older  granites.  The  veins  are  narrow,  and 
carry  mainly  tellurides  of  gold,  with  pyrite,  quartz,  and  fluorite  as 
common  associates.  Galena,  sphalerite,  tetrahedrite,  stibnite,  and 
molybdenite  occur  sparingly.  Propylitization  (p.  602)  of  the  wall 
rock  is  also  shown. 

Other  districts  of  this  type  are  Tonopah  and  Goldfield,  Nev.;  the 
San  Juan  district  of  Colorado,  etc. 

Cinnabar  deposits  also  belong  to  this  group. 

Deposits  formed  at  the  surface.  —  At  or  near  the  surface,  mineral  de- 
posits may  be  formed  by  hot  springs,  but  they  are  not  usually  of  eco- 
nomic importance.  Such  springs  may  deposit  earthy  carbonates  as 
sinter,  and  silica  as  opal  or  chalcedony.  Ore  minerals  developed  under 
these  conditions  in  crystallized  form  are  stibnite,  marcasite,  and  cinna- 
bar, but  other  sulphides  have  been  detected  by  chemical  means. 
Calcite,  fluorite,  barite,  and  celestite  may  also  develop. 

Distribution  of  magmatic  waters.  —  It  is  no  doubt  true  that  in 
many  cases  the  waters  which  came  from  the  igneous  magma  followed 
fissures,  and  either  deposited  the  ores  and  gangue  minerals  hi  them 
or  else  invaded  the  wall  rock  adjoining  the  fissure,  thus  giving  more 
or  less  tabular  deposits. 

In  some  cases,  however,  the  solutions  have  invaded  a  large  area  of  the 
country  rock,  giving  ore  bodies  of  irregular  shape  and  often  of  large  size, 
but  not  necessarily  great  richness. 

Hydrothermal  alteration.  —  The  hot  ascending  solutions  of  vary- 
ing composition  often  bring  about  a  most  profound  alteration  of  the 
rocks  which  they  traverse,  extracting,  it  may  be,  certain  elements  and 
adding  others.  Indeed  in  many  cases  the  alteration  is  so  extensive 
that  the  rock  bears  no  resemblance  to  its  former  self. 

Alteration  is  usually  most  intensive  along  the  fissures  which  con- 
ducted the  solution,  but  if  the  rock  is  extensively  fractured  it  is  affected 
over  a  large  area. 

The  types  of  hydrothermal  alteration  which  can  be  recognized  are 
propylitization,  sericitization,  silicification,  greisenization,  and  alunitiza- 
tion. 

Propylitization.  —  This  process  results  in  a  change  of  the  dark  silicates  to  chlo- 
rite, epidote,  and  pyrite,  and  of  the  feldspars  to  calcite,  epidote,  and  quartz.  The 
alteration  is  most  often  seen  in  rocks  of  intermediate  or  basic  composition,  and 
the  rocks  so  changed  are  usually  of  a  greenish-gray  color  with  bright  green  stains 
of  epidote.  The  feldspars  are  commonly  dull,  but  the  rock  texture  remains.  Pro- 
pylitization is  probably  a  somewhat  shallow  process.  The  volcanic  rocks  associated 
with  some  western  gold  and  silver  veins  often  show  strong  propylitization. 


ORE  DEPOSITS  603 

Serialization.  —  This  change  involves  a  loss  of  soda  and  a  gain  of  potash,  silica, 
and  pyrite,  as  well  as  carbon  dioxide  and  fluorine.  The  resultant  product  is  a 
fine-grained  mixture  of  sericite,  calcite,  quartz,  and  pyrite. 

Sericitization  is  a  common  type  of  hydrothermal  alteration,  which  is  common 
near  veins,  but  may  pass  outward  into  propylitic  alteration.  The  rocks  so  altered 
are  white  or  light  yellow  in  color,  and  the  mass  often  appears  clay-like.  Indeed 
sericite  masses  are  sometimes  mistaken  for  kaolin. 

Stticification.  —  Silicification  is  a  common  form  of  alteration  associated  with  the 
primary  deposition  of  ores,  and  is  more  often  noticed  in  acid  than  in  basic  rocks, 
although  it  is  by  no  means  uncommon  in  limestones. 

The  quartz  thus  formed  is  cherty  in  character,  and  the  original  structure  of  the 
rock  may  sometimes  be  clearly  preserved.  The  schist  carrying  the  disseminated 
copper  ore  at  Miami,  Ariz.,  for  example,  is  strongly  silicified. 

Alunitization.  —  This  is  a  somewhat  rare  type,  produced  as  at  Goldfield,  Nev., 
by  the  action  of  sulphuric  acid  solutions  on  feldspars.  The  alunite  here  occurs 
not  only  as  a  massive  crystalline  constituent  of  the  altered  rocks,  but  also  inter- 
grown  with  pyrite,  gold,  tellurides,  and  other  minerals  in  the  ore.  The  fragments 
of  alunitized  rock  on  the  dumps  give  them  a  whitish  appearance. 

Greisenization.  —  The  granite  walls  of  many  tin  veins  show  a  strong  and  char- 
acteristic alteration,  the  feldspar  and  muscovite  being  attacked  by  water  vapors 
carrying  fluorine,  resulting  in  the  development  of  a  mass  of  quartz,  topaz,  tourma- 
line, and  lepidolite,  to  which  the  name  greisen  is  applied.  Cassiterite  may  also  be 
present  in  the  altered  wall  rock. 

Forms  of  Ore  Bodies 

Ore  bodies  vary  greatly  in  form,  and  this  character  has  sometimes 
been  used  as  a  basis  for  classification,  instead  of  genesis  which  is  more 
satisfactory.  The  following  are  the  more  important  types. 

Fissure  veins.  —  A  fissure  vein  can  be  defined  as  a  tabular  mineral 
mass  occupying  or  closely  associated  with  a  fracture  or  set  of  fractures  hi 
the  enclosing  rock,  and  formed  either  by  filling  of  the  fissures  as  well 
as  pores  in  the  wall  rock,  or  by  replacement  of  the  latter  or  both.  In 
some  cases  bands  of  the  same  minerals  may  be  repeated  on  both  sides 
of  the  fissure. 

If  the  vein  is  formed  simply  by  filling,  the  ore  and  gangue  minerals 
are  often  deposited  in  successive  layers  (Fig.  216)  on  the  fissure  walls, 
but  if  deposition  of  both  goes  on  simultaneously,  the  banded-structure 
(called  crustification)  is  absent.  The  boundaries  of  a  filled  fissure  are 
usually  sharp. 

Replacement  veins  show  great  irregularity  of  width  and  usually  lack 
well-defined  boundaries;  they  do  not,  moreover,  as  a  rule  show  sym- 
metrical banding,  or  breccias  cemented  by  vein  material. 

The  term  vein  material  applies  to  the  aggregate  of  materials  which 
make  up  the  ore  body.  A  layer  of  soft,  clayey  material  known  as 


604 


ENGINEERING  GEOLOGY 

Present         Surface 


FIG.  216.  —  Sketch  of  a  fissure  vein  indicating  how  deposition  may  take  place  on 
walls  of  fissure  B  or  by  replacement  of  wall  rock  A. 


ORE  DEPOSITS 


605 


gouge  or  selvage  sometimes  forms  between  the  vein  and  country  rock, 
and  may  originate  in  crushing  caused  by  movement  along  the  vein 
wall.  The  ore  sometimes  follows  certain  streaks  hi  the  vein  known  as 
shoots  (q.v.),  or  again  it  may  be  restricted  to  pockets  of  great  richness 
known  as  bonanzas. 

Fissure  veins  vary  in  width  and  persistence;  splitting  and  inter- 
secting veins  are  also  known.  If  a  vein  is  inclined,  the  lower  wall  is 
termed  the  footwall  and  the  upper  the  hanging  wall.  Lode  is  a  vein 
consisting  of  closely-spaced  parallel  fissures,  sometimes  accompanied 
by  mineralization  of  the  intervening  rock.  Vein  system  is  a  larger 
assemblage  of  vein  fissures  and  may  include  several  lodes.  Con- 
jugate veins  are  parallel  intersecting  veins  of  opposite  dip,  examples  of 


FIG.  217.  —  Section  across  veins  of  Pennsylvania,  Rarus,  Mountain  View,  and  West 
Colusa  mines,  Butte,  Mont.  A  series  of  steeply  dipping  veins,  broken  by 
faults.  (After  Weed.) 


which  are  in  the  Encampment  district  of  Wyoming.  Apex  is  the  term 
applied  to  the  top  of  a  vein.  It  does  not  necessarily  reach  the  surface, 
or  even*  the  top  of  the  bed  rock.  Bedded  vein  is  a  term  sometimes 
applied  to  a  deposit  conformable  with  the  bedding,  as  hi  the  Snowstorm 
mine,  Coeur  d'Alene  district,  Idaho. 

Chimney.  —  This  is1  a  term  applied  to  ore  bodies  which  are  rudely 
circular  or  elliptical  hi  horizontal  cross-section,  but  may  have  great 


606 


ENGINEERING  GEOLOGY 


vertical  extent;  the  Yankee  Girl  mine  at  Red  Mountain,  Col.,  is  of 
this  type. 

Stock.  —  An  ore  body  similar  to  a  chimney  but  of  greater  irregu- 
larity of  outline. 

Fahlband.  —  A  term  originally  used  by  German  miners  to  indicate 
certain  bands  of  schistose  rocks  impregnated  with  finely-divided  sul- 
phides, but  not  always  rich  enough  to  work.  The  Homestake  ore  body 
at  Lead,  S',  Dak.,  belongs  to  this  type. 

Disseminated  deposit.  —  A  type  of  ore  deposit  in  which  the  ore 
minerals  occur  as  small  particles  or  veinlets  scattered  through  the 
country  rock.  Though  not  very  abundant,  such  deposits  are  some- 
times of  great  size,  and  in  some  parts  of  the  west  form  important  sources 
of  copper  ore.  They  are  found  mostly  in  schists  and  intrusives,  espe- 
cially those  which  have  been  fissured  or  shattered.  This  type  of  ore 
is  worked  at  Bingham,  Utah;  Clifton,  Ariz.,  etc. 

Residual  deposits.  —  In  the  case  of  some  iron,  manganese,  lead, 
and  zinc  ores,  the  rock  containing  the  primary  ore  has  been  weathered 
to  a  mass  of  residual  clay.  During  this  process  the  metallic  compounds 
have  been  changed  to  oxidized  forms  (p.  607)  and  concentrated  in 
lumps  and  nodules,  stringers  or  crusts,  within  the  clayey  mass.  Many 
of  the  eastern  limonites  are  of  this  type.  So,  too,  are  some  of  the  lead 
and  zinc  ores  of  Virginia,  and  the  manganese  ores  of  the  southern  states. 


FIG.  218.  —  Vertical  section  showing  structure  of  a  residual  deposit  of  brown  ore, 
from  Reed  Island,  Va.     (After  Harder,  U.  S.  Geol.  Survey,  Bull.  380,  1909.) 

Ore  shoots.  —  Few  ore  deposits  are  of  uniform  character  through- 
out; indeed  the  occurrence  of  pay  ore  is  apt  to  be  more  or  less  irregular, 
the  richer  ore  being  sometimes  more  or  less  localized.  These  richer 


ORE  DEPOSITS  607 

pockets  are  commonly  called  ore  shoots,  and  they  usually  owe  their 
formation  to  some  structural  feature  that  has  guided  the  ore  solutions. 

Thus  more  abundant  fissuring  or  brecciation,  hi  certain  parts  of 
the  rock,  may  operate  to  promote  deposition  hi  those  portions  of  the 
mass;  clay  walls  may  be  influencing  factors  hi  guiding  the  ore  solutions 
towards  certain  spots;  or  intersecting  fissures  may  permit  the  mingling 
of  reacting  solutions,  thereby  bringing  about  more  abundant  precipi- 
tation of  the  ore  at  these  crossing  points. 

Several  classifications  of  ore  shoots  have  been  suggested.  Among 
them  is  that  of  Van  Hise,  who  groups  them  as  follows:  (1)  Those 
explained  largely  by  structural  features;  (2)  those  formed  by  the 
influence  of  wall  rocks;  and  (3)  those  formed  by  secondary  concen- 
tration by  descending  waters. 

Primary  and  Secondary  Ores 

Primary  ores  are  those  which  have  remained  practically  unchanged 
by  surface  agencies  since  their  deposition.  Secondary  ores  are  those 
which  have  been  altered  by  surface  agencies,  especially  descending 
meteoric  waters.  Unfortunately  the  two  terms  are  not  always  used 
hi  exactly  this  sense. 

Weathering  and  secondary  enrichment.  —  Weathering  has  often 
changed  an  ore  deposit  hi  its  upper  part,  and  sometimes  to  a  considerable 


No.3  Le.el  Czar  and  Holbrook 

No.4  Le.,1  Car  aod  Holbrook 

—  No.5  Le.el  Holbrook 


FIG.  219.  —  Section  through  Copper  Queen  mine,  Bisbee,  Ariz.,  showing  variable 
depth  of  weathering.     (After  Douglas,  Amer.  Inst.  Min.  Engrs.,  Trans.,  XXIX.) 

depth,  while  the  lower-lying  portion^  below  the  groundwater  level 
are  often  enriched  by  secondary  processes.  The  lower  limit  of  the 
zone  of  weathering  may,  however,  be  very  irregular  (Fig.  219). 

Zones  in  an  ore  body.  —  In  passing  downward  from  the  surface 
the  following  zones  may  sometimes  be  distinguished  (Fig.  220),  although 
they  are  not  always  separately  recognizable  hi  all  ore  bodies. 


608 


ENGINEERING  GEOLOGY 


I.  Zone  of  weathering      (a)  Surface  zone  of  complete  oxidation. 
(6)  Zone  of  complete  leaching, 
(c)  Zone  of  oxide  enrichment. 
II.  Zone  of  secondary  sulphides. 
III.  Zone  of  primary  sulphides. 

Zone  of  weathering.  —  Nearly  all  minerals  are  attacked  by 
weathering  agents,  but  the  metallic  minerals  are  more  easily  attacked 
and  more  profoundly  affected  than  the  non-metallic  ones. 

The  weathering  processes  involve  both  chemical  and  physical  changes, 
but  the  chemical  reactions  especially  are  more  intricate  in  ores  than 


IRON  CAP  DEVELOPS  HER! 


ZONE  OF  OXIDE  ENRICHMENT 


PRIMARY  ORE 


FIG.  220.  —  Section  of  an  ore  body  showing  the  several  zones  that  may  be  developed 
by  weathering  and  secondary  enrichment.  (After  Tolman,  Min.  and  Sci. 
Press,  Jan.  4,  1912.) 

they  are  in  the  country  rock.  As  a  result  of  weathering,  worthless 
minerals  may  be  removed,  leaving  the  weathered  part  more  porous,  so 
that  the  richness  may  be  increased,  because  we  have  a  greater  quantity 
of  metals  per  ton  of  rock.  On  the  other  hand,  weathering  through 
solution  may  remove  some  of  the  metallic  compounds,  leaving  the  upper 
part  of  the  ore  body  impoverished. 

The  first  process  in  weathering  is  the  breaking  down  of  insoluble 
sulphides,  which  takes  place  above  the  water  level,  where  moisture 
and  oxygen  can  attack  them,  changing  them  first  to  sulphates  and 
in  some  cases  finally  to  oxides  or  other  compounds. 

They  are  not  attacked  in  the  same  order,  and  different  authorities 


ORE  DEPOSITS  609 

do  not  agree  on  this  point.  Thus  Weed1  gives  the  order  of  decom- 
position as  arsenopyrite,  pyrite,  chalcopyrite,  sphalerite,  galena,  and 
chalcocite,  while  Beck2  states  the  order  as  marcasite,  pyrite,  pyrrhotite, 
chalcopyrite,  bornite,  millerite,  chalcocite,  galena,  and  sphalerite.  The 
variation  in  order  of  decomposition  may  be  due  to  varying  conditions. 
Moreover,  the  oxidation  of  any  one  sulphide  does  not  necessarily 
always  proceed  hi  the  same  manner,  as  the  following  equations  indi- 
cating the  change  of  pyrite  show. 

FeS2  +  4  O  =  FeSO4  +  S. 

FeSs  +  6  O  =  FeSO4  +  SO2. 

FeS2  +  7  O  +  H20  =  FeSO4  +  H2S04. 

The  FeSO4  in  presence  of  oxygen  will  be  further  changed  thus: 

6  FeSO4  +  3  O  +  3  H2O  =  2  Fe2  (SO4)3  +  2  Fe  (OH)3 
and 

Fe,  (SO4)3  +  6  H2O  =  2  Fe  (OH),  +  3  H2SO4. 

But  the  ferric  hydroxide  may  break  down  as  follows: 

Limonite 

4  Fe  (OH)3  =  2  Fe2O3  +  6  H2O  =  2  Fe2O3  •  3  H2O  +  3  H2O. 

There  is  a  tendency,  therefore,  for  much  of  the  pyrite  to  be  converted 
into  limonite. 

While  iron  sulphide  as  shown  above  may  oxidize  to  iron  sulphate 
and  sulphuric  acid,  other  sulphides  like  galena  and  sphalerite  may 
oxidize  to  sulphates  without  liberating  any  acid.  Thus: 

ZnS  +  4  O  =  ZnS04, 
or 

CuFeS,  +  80  =  FeSO4  +  CuSO4. 

In  addition  to  sulphates,  we  sometimes  have  carbonates  or  silicates 
formed,  somewhat  as  in  the  following  reactions. 

Malachite 

2  CuS04  +  2  CaCO3  +  5  H2O  =  CuCO3  -  Cu(OH)2 

Gypsum 

+  2  (CaSO4  -  2  H20)  +  2  CO2 
or 

Chrysocolla 

CuS04  +  H2Ca(CO3)2  +  H4SiO4  =  CuOH4Si04  +  CaSO4  +  H20  +  2  C02. 

We  see  from  the  above  that  weathering  may  develop  comparatively 
insoluble  compounds  like  hydrous  oxides  or  silicates,  and  hi  some  cases 
carbonates  as  smithsonite  (zinc  carbonate),  or  at  other  times  soluble 

1  Trans.  Amer.  Inst.  Min.  Engrs.,  XXX,  p.  429,  1901. 

2  Nature  of  ore  deposits,  p.  337. 


610 


ENGINEERING  GEOLOGY 


ones  like  sulphates.  In  the  upper  zone  of  the  belt  of  weathering,  oxi- 
dation has  been  carried  to  an  extreme,  and  at  the  surface  there  is 
frequently  an  iron  cap  or  gossan,  composed  of  limonite  and  hematite, 
often  with  much  residual  silica.  It  may  also  carry  residual  gold,  silver 
chloride  (in  arid  regions)  or  even  weathered  compounds  of  lead,  zinc,  and 
copper;  provided  of  course  these  metals  are  present  in  the  primary  ore. 

Below  this  zone  may  follow  one  which  is  more  or  less  thoroughly 
leached.  Then  in  the  lower  part  of  the  belt  of  weathering,  or  just 
above  the  sulphide  zone,  the  minerals  are  sometimes  only  partly  oxi- 
dized, forming  oxides,  carbonates,  silicates,  and  native  elements.  Some- 
times rich  oxidized  ores  are  found  in  this  zone,  especially  where  the 
wall  rock  is  limestone. 

Secondary  sulphide  zone.  —  In  many  ore  bodies,  rich  masses  of  ore 
occur  below  the  oxidized  zone,  which  are  of  secondary  character,  or 


FIG.  221.  —  Section  of  ore  showing  precipitation  of  secondary  chalcocite  on  pyrite. 
(After  Paige,  U.  S.  Geol.  Survey,  Bull.  470,  1911.) 

there  may  be  a  zone  of  ore  which,  if  not  rich,  is  at  all  events  richer  than 
the  primary  ore.  This  is  seen  most  often  in  copper,  gold,  and  silver, 
and  to  a  less  extent  in  lead  and  zinc  ores. 

It  is  due  to  the  soluble  products  of  weathering  being  carried  below 
the  water  level,  where  they  (sulphates)  react  with  sulphides  and  are 
again  reduced  to  sulphides. 


ORE  DEPOSITS  611 

This  is  known  as  secondary  enrichment,  and  many  important  ore 
bodies,  such  as  most  of  the  copper  deposits  of  the  West,  owe  their  work- 
able character  to  this  enriching  process. 

The  two  equations  given  by  Tolman1  may  be  taken  as  illustrating 
the  reactions  which  occur  hi  this  zone,  the  sulphate  in  both  cases  having 
been  derived  from  the  weathered  zone  above  in  solution. 

7  ZnS04  +  4  FeS2  +  4  H2O  =  7  ZnS  +  4  FeS04  +  4  H2S04 
or 

14  CuS04  +  5  FeS2  +  12  H20  =  7  Cu2S  +  5  FeSO4  +  12H2S04. 


Evidence  of  this  process  can  be  seen  to  advantage  in  some  copper 
deposits,  where  in  the  secondary-sulphide  zone  rims  of  chalcocite  sur- 
round grains  of  pyrite.2  (Fig.  221.) 

Since  the  position  of  the  secondary  sulphide  zone  is  thought  to  be  de- 
termined by  the  level  of  the  water  table,  it  may  vary  from  a  few  feet 
in  depth  to  several  hundred  feet  in  semi-arid  and  elevated  regions,  or  in 
exceptional  cases  even  deeper.  Moreover,  the  thickness  of  the  zone 
is  extremely  variable,  for  the  process  is  affected  by  various  conditions. 

If  the  ore  body  below  the  water  level  is  dense  (impervious)  and  un- 
fractured  the  downward  migration  of  the  metals  is  stopped.  Secondary 
enrichment  may  also  be  lacking  in  arctic  regions  where  the  frozen 
ground  prevents  downward  seepage. 

Change  of  ore  with  depth.  —  It  has  been  pointed  out  that  all 
metallic  minerals  do  not  weather  with  equal  rapidity,  consequently 
some  may  be  carried  downward  more  rapidly  than  others.  Thus  zinc 
sulphide  weathers  more  rapidly  than  lead  sulphide,  resulting  some- 
times eventually  in  an  ore  deposit  which  yields  chiefly  lead  above  and 
zinc  below.  By  the  operation  of  similar  processes,  we  may  have  de- 
veloped from  a  copper-gold  ore,  a  gold  deposit  above  and  an  auriferous 
copper  deposit  below. 

Gold  is  leached  under  favorable  conditions.  When  held  in  solution 
as  chloride,  it  is  precipitated  by  ferrous  sulphate  unless  an  oxidizing 
agent,  such  as  manganese  oxide,  is  present,  hi  which  case  it  remains  in 
solution.  Gold  may,  therefore,  be  carried  in  an  acid  solution  so  long 
as  the  higher  oxides  of  manganese  are  present.  The  precipitation  of 
the  gold  from  chlorine  solution  may  be  caused  by  native  metals,  sul- 
phides, organic  matter,  and  other  materials. 

1  Min.  &  Sci.  Press,  Jan.  4,  1913. 

2  The  recent  investigations  of  L.  C.  Graton  show  that  the  change  from  pyrite  to 
chalcocite  is  not  always  a  direct  one,  but  that  intermediate  sulphides  may  be  formed. 


612  ENGINEERING  GEOLOGY 

Zone  of  primary  sulphides.  —  The  boundary  between  the  secondary- 
sulphide  zone  and  that  of  primary  sulphides  next  below  is  very  irregular 
and  often  somewhat  indefinite.  The  primary  ore  is  often  too  low  grade 
to  work.  Sections  of  the  ore  when  examined  under  the  microscope 
sometimes  show  that  more  than  one  ore  mineral  has  been  deposited 
at  a  time  (Plate  C,  Figs.  1  and  2). 

Outcrops  of  Ore  Bodies 

Many  ore  bodies  outcrop  on  the  surface.  Where  the  ore  is  more 
resistant  than  the  wall  rock  it  may  stand  out  in  more  or  less  strong 
relief,  and  where  it  is  less  resistant  than  the  country  rock  it  weathers 
more  rapidly.  In  the  latter  case,  its  presence  might  be  indicated  by 
a  depression.  Veins  with  predominant  quartz  are  usually  resistant, 
while  those  with  predominant  sulphides  are  likely  to  be  the  reverse. 
Strong  persistent  fissure  veins  on  the  surface  are  not  unlikely  to  continue 
so  with  depth,  but  small,  narrow,  branching  veins  are  less  reliable. 

If  a  vein  or  other  ore  deposit  of  irregular  width  is  more  resistant 
than  the  wall  rock,  the  wearing  down  or  erosion  is  likely  to  stop  at  the 
widest  part,  hence  below  this  the  vein  may  narrow. 

If  the  vein  is  softer,  it  may  increase  in  width,  and  the  surface  close 
together  after  the  vein  material  is  weathered  out.  Indeed,  in  such 
cases  the  position  of  the  vein  may  be  indicated  by  a  gouge-filled 
fissure. 

If  a  vein  outcrops  on  a  steep  hillside,  the  creep  of  the  surface  material 
will  carry  fragments  of  the  outcropping  ledge  down  the  hillside.  These 
become  mixed  with  the  surface  material  and  are  termed  "float." 

Silicified  ledges  and  limonite  gossans  sometimes  form  prominent 
outcrops. 

Distribution  of  Ore  Deposits  in  the  United  States 

A  map  showing  the  occurrence  of  ore  deposits  in  the  United  States 
at  once  conveys  the  idea  that  the  useful  and  precious  metals  are  not 
uniformly  distributed;  indeed  one  is  impressed  with  the  predominant 
variety  of  metals  found  in  the  western  states  and  the  practical  absence 
of  them  in  the  region  of  the  Great  Plains.  Their  general  occurrence 
in  the  several  physiographic  provinces  (Fig.  222)  may  be  briefly 
referred  to. 

Coastal  Plain.  —  In  this  province  which  borders  the  Atlantic 
Ocean  and  Gulf  of  Mexico,  and  extends  from  Cape  Cod  to  Mexico,  we 
have  a  nearly  level  plain,  which  contains  practically  no  metalliferous 


ORE  DEPOSITS 


613 


614  ENGINEERING  GEOLOGY 

deposits  of  commercial  importance,  even  though  the  belt  is  rich  in 
non-metallic  substances,  such  as  clays,  sands,  phosphates,  and  marls. 

Piedmont  Plateau.  —  West  of  the  Atlantic  Coastal  Plain  is  a  strip 
of  ancient  crystalline  rocks,  which  extends  from  central  Alabama 
northeastward  through  New  England.  The  Piedmont  Plateau  proper 
is  that  portion  lying  south  of  New  York,  while  the  northern  continu- 
ation is  known  as  the  New  England  Plateau.  It  represents  an  ancient 
plain  of  erosion,  formed  at  sea  level,  but  since  uplifted  and  dissected 
by  later  weathering  and  stream  cutting. 

A  number  of  metalliferous  deposits  of  iron,  copper,  manganese,  and 
gold  with  some  silver,  lead,  and  zinc  are  found  in  this  belt,  but  since 
most  of  them  are  chiefly  of  historic  interest,  they  add  with  few  exceptions 
little  to  the  total  production  of  the  United  States.  Some  of  these  will 
undoubtedly  prove  more  productive  in  the  future. 

Most  prominent  among  these  are  the  magnetites  of  southeastern 
Pennsylvania  and  the  states  farther  south,  and  the  gold  and  copper 
ores  of  the  southern  states. 

Appalachian  Province.  —  On  the  western  side  of  the  Piedmont 
Plateau,  and  extending  from  about  Montgomery,  Ala.,  to  Albany,  N.  Y., 
is  a  belt  whose  parallel  ridges  and  valleys  are  cut  in  folded  stratified 
rocks. 

This  belt  is  of  importance  in  the  metal-mining  industry  as  it  carries 
deposits  of  bedded  (Clinton)  iron  ore,  residual  brown  iron  ores,  and 
manganese,  as  well  as  the  copper  deposits  of  Tennessee  and  the  lead 
and  zinc  ores  of  Virginia  and  Tennessee.  The  bauxite  deposits  of  the 
Georgia-Alabama-Tennessee  district  also  lie  in  this  province. 

Allegheny  Plateau.  —  This  consists  of  a  great  block  of  stratified 
rocks,  which  begins  as  a  steep  eastward-facing  slope  on  the  western 
edge  of  the  Appalachian  Province,  and  dips  gently  westward  to  the 
interior  plains,  its  altitude  ranging  from  three  or  four  thousand  feet 
on  the  east  to  the  level  of  the  Mississippi  Valley  on  the  west. 

With  the  exception  of  the  magnetite  deposits  of  the  Adirondack 
Mountains,  which  rise  above  the  plateau  at  its  northern  end  in  New 
York  state,  there  are  few  metalliferous  deposits  of  importance  in  this 
province. 

Prairie  Plains.  —  In  the  central  part  of  the  country  we  have  an 
irregular  lowland,  which  extends  from  the  Gulf  Costal  Plain  on  the 
south  to  the  Great  Lakes  on  the  north.  Two  areas  of  somewhat  strong 
relief,  lying  within  this  province,  are  the  Superior  Highlands  on  the 
north  and  the  Ozark  region  of  domed  rocks  in  Missouri  and  adjoining 
states  on  the  west  and  south.  This  is  an  exceedingly  important  province 


ORE  DEPOSITS  615 

for  it  contains  the  vast  iron  deposits  of  the  Lake  Superior  region,  the 
native  copper  deposits  of  Keweenaw  Point,  Mich.,  and  the  lead  and 
zinc  deposits  of  the  upper  and  lower  Mississippi  Valley  region. 

Outside  of  these  districts  few  metals  have  been  found. 

Great  Plains.  —  This  belt  lies  between  the  Prairie  Plains  and  the 
Rocky  Mountains,  and  has  a  maximum  width  of  500  miles.  Its  surface 
rises  from  1,000  to  2,000  feet  on  the  east  to  4,000  or  5,000  feet  on  the 
west.  With  the  exception  of  the  isolated  mass  of  rocks  forming  the 
Black  Hills  of  South  Dakota,  which  contain  gold  ores,  and  the  mercury 
area  of  Brewster  County,  Texas,  the  province  is  singularly  free  from 
metalliferous  deposits. 

Cordilleran  Region.  —  This  area  includes  that  portion  of  the 
country  lying  between  the  foothills  of  the  Rocky  Mountains  and 
the  Pacific  coast.  It  consists,  however,  of  a  number  of  provinces,  most 
of  which  are  important  producers  of  different  metals.  The  provinces 
are  known  as  the  Rocky  Mountains,  Colorado  Plateau,  Columbia 
Plateau,  Basin  Range  province,  and  Pacific  Mountain  province. 

In  the  Rocky  Mountains  province  which  consists  of  mountain  ranges 
and  high  peaks,  with  many  igneous  rocks,  a  number  of  valuable  ore 
deposits  are  found.  These  include  the  gold  deposits  of  Cripple  Creek, 
Col.,  the  lead  and  zinc  ores  of  Leadville,  Col.,  the  lead-silver  ores  of 
the  Coeur  d'Alene  district,  Idaho,  etc.  Copper  also  occurs  associated 
with  other  ores. 

Not  less  important  is  the  Basin  Range  province.  This  contains  im- 
portant gold  and  silver  ores,  associated  with  recent  volcanic  rocks,  as 
at  Goldfield,  Tonopah,  and  Virginia  City,  Nev.  In  this  same  province 
also  are  found  the  enormous  deposits  of  disseminated  copper  ores  ob- 
tained at  Bingham,  Utah,  Ely,  Nev.,  and  several  points  in  Arizona. 

The  Pacific  Mountain  province  is  chiefly  important  as  a  source  of 
gold  quartz  ores,  such  as  the  Mother  Lode  of  California,  and  gold- 
bearing  gravels.  Mercury  has  been  found  at  scattered  points  in  the 
south  of  the  province,  and  iron  ore  in  the  northern  portion. 

Occurrence  of  the  More  Important  Ore  Types 
Iron  Ores 

In  spite  of  the  abundance  of  iron  in  the  rocks  of  the  earth's  crust, 
there  are  few  ore  minerals  of  the  metal.  The  iron  ores  of  the  greatest 
commercial  value  are  those  which  occur  hi  great  quantity,  are  favorably 
located,  and  easily  mined. 


616 


ENGINEERING  GEOLOGY 


The  quantity  of  iron  ore  mined  annually  in  this  country  is  large,  and 
the  average  grade  is  higher  than  that  obtained  in  many  other  countries, 
so  that  if  we  include  our  deposits  of  medium  grade  the  country  contains 
large  ore  reserves. 

Iron-ore  minerals.  —  The  ore  minerals  of  iron,  together  with  their 
composition  and  theoretic  percentage  of  metallic  iron,  are: 


Name. 

Composition. 

Per  cent,  iron, 

Magnetite. 
Hematite. 

Magnetic  iron  ore  
(  Specular  iron  ore,  red  hematite,  fossil 
(      ore,  Clinton  ore 

Fe3O4  
Fe2O3 

72.4 
70  0 

Limonite.1 
Siderite. 

Brown  hematite,  bog  iron  ore,  ochre.  . 
(  Spathic  ore,  carbonate  ore,  black- 
(      band,  clay  iron  stone,  kidney  ore  .  .  . 

2  Fe2O3  -  3  H2O 
FeCO3  

59.80 

48.27 

1  The  group  name  brown  ore  is  perhaps  preferable  as  the  ore  may  contain  other  hydrous  oxides. 

Pyrite,  a  very  common  mineral,  is  not  used  as  an  ore,  except  in  rare 
cases,  and  then  only  after  the  sulphur  has  been  expelled  by  roasting. 
Its  chief  use  is  for  sulphuric-acid  manufacture,  although  the  "  blue- 
billy"  iron  residue  after  desulphurizing  is  used  to  some  extent  for  the 
manufacture  of  pig  iron. 

Few  ores  of  iron  approach  in  richness  the  theoretic  amount  shown 
above,  the  deficiency  in  iron  content  usually  shown  being  due  to  the 
presence  of  a  variable  amount  of  gangue  minerals.  The  impurities 
which  they  supply  are  alumina,  lime,  magnesia,  silica,  titanium,  arsenic, 
copper,  phosphorus,  and  sulphur,  of  which  the  last  six  produce  a  weaken- 
ing effect  on  the  iron. 

Silica  occurs  in  practically  all  ores,  but  in  variable  amounts.  It  is 
always  high  in  residual  limonites,  and  these  may  likewise  show  high 
alumina.  Pyrite  is  a  common  source  of  sulphur,  but  in  some  limonites 
it  may  come  from  gypsum  or  barite.  Manganese,  when  present,  is 
found  mostly  in  limonite  ores,  and  for  certain  purposes  is  desirable. 
It  is  also  prominent  in  some  Lake  Superior  ores.  Apatite  yields  the 
phosphorus.  Titanium  is  prominent  only  in  certain  magnetites. 

Types  of  iron-ore  bodies.  —  Iron-ore  bodies  are  of  varied  form, 
but  many  of  the  important  ones  known  in  this  country  are  lens-  or 
basin-shaped  in  outline.  They  may  be  classified  as  follows: 

1.  Magmatic   segregation  deposits,   usually  of  irregular  form,   but 
sometimes  dike-like  in  character. 

2.  Contact-metamorphic  deposits,  commonly  of  somewhat  pockety 
form,  although  the  pockets  may  be  large. 


618  ENGINEERING  GEOLOGY 

3.  Sedimentary  ores  of  bedded  character,  like  the  Clinton  ore,  and 
bog  ore  occurring  as  nodules  in  bog  deposits. 

4.  Ores  concentrated  by  meteoric  waters,  and  deposited  as  replace- 
ments in  different  kinds  of  rocks.     (Some  Lake  Superior  hematites  and 
Oriskany  limonites  of  Virginia.) 

5.  Residual  deposits,  as  nodules  or  crusts  in  residual  clays    (some 
Virginia  and  Pennsylvania  limonites). 

6.  Lenticular  masses  in  metamorphic  rocks,  of  variable  origin  (some 
magnetite  and  pyrite  deposits). 

7.  Gossan   ores,   as   the   limonite   capping   of   many   sulphide    ore 
bodies. 

Magnetite.  —  Magnetite  is  black,  often  granular,  and  occurs  commonly 
as  lenses  or  disseminations  in  metamorphic  rocks.  It  sometimes,  as  in 
the  Great  Basin  province,  may  be  mixed  with  hematite  or  even  copper 
in  contact-metamorphic  deposits.  Deposits  of  magnetite  are  also 
found  at  times  in  very  basic  igneous  rock  (Lake  Sanford,  Adirondacks; 
Iron  Mountain,  Wyo.),  and  these  are  usually  formed  by  magmatic 
segregation.  They  run  too  high  in  titanium  to  be  smelted  in  the  blast 
furnace,  but  could  serve  for  making  ferro-titanium  alloys. 

The  most  important  ore  bodies  are  the  non-titaniferous  magnetites 
found  in  acid  metamorphic  rocks  of  the  Adirondacks,  in  New  York 
state,  northern  New  Jersey,  and  some  of  the  Atlantic  states  farther 
south. 

Hematite. — Hematite  is  red  to  brownish  red,  steel  gray,  or  even 
black.  It  is  commonly  fine  grained,  but  the  specular  varieties  may  be 
quite  coarse.  At  present  it  forms  the  most  valuable  ore  of  iron  mined 
in  the  United  States. 

Bedded  deposits,  known  as  the  Clinton  ore,  are  found  in  the  Appa- 
lachian province,  as  well  as  in  New  York,  Ohio,  and  Wisconsin.  They 
may  show  considerable  silica  and  phosphorus,  and  outside  of  the 
Birmingham,  Ala.,  district  are  not  worked  very  extensively.  Here 
the  ore  beds  are  found  in  the  southeasterly-dipping  Clinton  formation 
in  Red  Mountain.  There  are  four  well-marked  ore  beds  which  are 
interstratified  with  shales  and  sandstones.  The  ores  range  from  37 
to  54  per  cent  in  iron,  and  form  the  basis  of  an  important  iron  and 
steel  industry,  both  the  limestone  for  flux  and  coal  being  obtained 
near-by. 

In  the  Lake  Superior  region  extensive  deposits  of  hematite  are  found, 
and  form  the  most  important  source  of  domestic  iron-ore  supply. 

The  formations  present  in  the  iron  ranges  or  districts  include  a 
complex  series  of  pre-Cambrian  igneous  and  sedimentary  rocks  which 


ORE  DEPOSITS 


619 


have  been  highly  metamorphosed  and  folded.  The  ores  occur  in  the 
so-called  iron  formations,  the  latter  representing  alternations  of  chemi- 
cally-deposited sediments,  consisting  of  varying  mixtures  of  iron  and 
silica.  Since  their  formation  the  rocks  have  been  folded  and  faulted, 


FIG.  223.  —  Map  showing  distribution  of  hematite  and  magnetite  in  the  United 
States.     (After  Harder,  U.  S.  Geol.  Survey,  Min.  Res.,  1907.) 

the  iron  has  been  concentrated  by  surface  waters,  and  in  some  cases 
these  concentrations  changed  by  metamorphism. 

The  ores  vary  from  hard  blue  to  soft  earthy  ones,  and  some  are  of  very 
high  grade. 

Owing  to  the  enormous  quantity  of  ore,  its  location  and  the  ease 
with  which  it  can  be  mined  and  shipped,  the  region  has  become  of  great 
importance,  and  contributes  most  of  the  domestic  production. 

Specular  hematite  is  also  found  in  southeastern  Wyoming  and  in 
Shelby  County,  Ala. 

In  the  Great  Basin  province  of  the  west  hematite  occurs  associated 
with  magnetite  (as  at  Iron  Springs,  Utah)  and  copper  (Bingham  Canyon, 
Utah,  etc.). 

Limonite.  —  Limonite  is  never  crystalline,  and  varies  widely  in  its 
appearance;  it  is  sometimes  powdery,  or  at  other  times  massive,  and 
the  latter  may  be  porous,  vesicular,  stalactitic,  or  even  solid.  The 
color  is  brown  to  brownish-yellow  on  the  fracture,  but  may  be  black 
and  shiny  on  the  surface. 


ORE  DEPOSITS 


621 


Limonite  and  the  other  hydrous  oxides  of  iron  may  form  deposits 
of  several  different  types.  The  commonest  of  these  is  the  residual 
type,  in  which  nodules  of  the  ore  are  scattered  through  residual  clay. 
The  ore  has  to  be  separated  from  the  clay  by  washing  and  screening, 


FIG.  224.  —  Map  showing  distribution  of  limonite  and  siderite  in  the  United  States. 

(After  Harder.) 


so  that  the  workability  of  the  deposit  depends  on  the  quantity  of  ore 
in  the  clay.  A  second  type  is  gossan  limonite,  formed  by  the  weathering 
of  sulphide  deposits.  In  the  Appalachian  belt  these  are  formed  from 
pyrite,  or  mixtures  of  the  latter  with  pyrrhotite  and  chalcopyrite. 
They  have  supplied  some  iron  ore  in  the  past. 

In  the  Cordilleran  region  gossan  limonite  is  found  over  not  a  few 
sulphide-ore  bodies,  but  here  the  ore  may  carry  some  of  the  precious 
metals  and  is  of  more  value  as  a  flux  than  as  an  iron  ore. 

Sedimentary  limonite  deposited  as  a  chemical  precipitate  hi  ponds 
or  swamps  is  widely  distributed,  but  of  no  commercial  value  in  the 
United  States. 

Siderite.  —  This  ore  of  iron  is  not  of  much  commercial  value,  because 
of  the  small  extent  of  the  deposits  and  its  low  iron  content.  It  occurs 
most  commonly  as  bands  or  concretions  in  shales,  especially  of  the 
coal  measures.  Such  concretions  are  not  uncommon  in  some  clay  beds. 

Production.  —  The  production  of  iron  ores  in  the  United  States  in 
1912  amounted  to  57,017,614  long  tons,  valued  at  $107,050,153.  This 


622 


ENGINEERING   GEOLOGY 


was  an  increase  of  25.69  per  cent  in  quantity  from  1911.  Of  the  above 
amount,  hematite  formed  nine-tenths,  magnetite  and  brown  ore  each 
less  than  one-twentieth,  and  siderite  about  .02  per  cent. 


Copper  Ores 

Ore  minerals  of  copper.  —  While  the  total  number  of  ore  minerals 
of  copper  is  considerably  larger  than  those  of  iron,  not  many  of  them 
are  of  widespread  importance.  Unlike  iron,  the  ore  minerals  of  copper 
are  found  associated  with  many  different  metals  under  a  variety  of 
conditions.  The  number  of  important  copper-producing  districts  is 
small,  and  the  ores  mined  are  usually  of  low  grade.  Indeed,  such 
low-grade  ore  bodies  as  are  mined  can  only  be  worked  economically 
on  a  large  scale. 

The  following  are  the  ore  minerals  of  copper,  the  more  important 
ones  being  italicized. 


Ore  minerals. 

Composition. 

Per  cent,  Cu. 

f  Chalcopyrite 

CuFeSo 

34  5 

Chalcocite  •  

Cu2S.  .  . 

79  8 

Bornite  

Cu3FeS3.  . 

55  58 

Unweathered 

Enargite  

Cu3AsS4  

48  00 

zone 

Covellite  

CuS  

66  5 

Telrahedrite  

Cu8Sb2S7  

52.10 

Tennantite  

CUgASuSr  

57.00 

..  Native  copper 

Cu 

100  00 

r  Azurite 

2  CuCO3  Cu(OH)2 

55  10 

Malachite 

CuCO3  Cu(OH)2 

57  27 

Chrysocolla 

CuSiOs  2  H2O 

36  00 

Weathered 

Cuprite 

Cu2O 

88  8 

zone 

Melaconite.    . 

CuO 

79  84 

Brochantite  .... 

CuSO4,  3  Cu(OH)2 

62  42 

w  Atacamite.  .  .  . 

Cu(OH)Cl,  Cu(OH)2 

59  45 

Chalcanthite  

CuSO4,  5H2O  

25.4 

The  difference  in  the  nature  of  the  copper  compounds  found  in  the 
weathered  and  unweathered  zones  is  quite  noticeable. 

Most  of  the  copper  ores  now  worked  are  of  low  grade,  that  is  as  low 
as  2  per  cent  copper,  but  they  can  be  profitably  treated  because  of 
the  extent  of  the  operations,  and  the  possibility  of  concentrating  them, 
if  the  ore  minerals  are  sulphides. 

The  presence  of  other  ores  often  increases  the  complexity  of  the 
smelting  process,  but  with  modern  methods  the  several  metals  are 
separated  and  saved,  and  impurities  removed. 


624  ENGINEERING  GEOLOGY 

Copper-ore  bodies  are  extensively  affected  by  weathering.  That 
portion  of  the  ore  body  above  water  level  may  be  either  a  limonitic 
gossan,  from  which  most  of  the  copper  has  been  leached,  or  it  may 
contain  oxidized  ores.  As  a  result  of  the  leaching,  the  copper  may  be 
transferred  below  the  water  level  and  re-deposited;  indeed,  were  it  not 
for  the  process  of  secondary  enrichment  having  taken  place,  many  a 
copper  deposit  in  the  southwest  would  not  be  workable.  In  this  process 
the  copper  is  usually  reprecipitated  as  chalcocite  although  other  sul- 
phides not  infrequently  result;  but  all  occurrences  of  chalcocite  are  not 
secondary  as  was  formerly  supposed,  so  that  it  now  no  longer  serves  as 
a  criterion  of  secondary  enrichment. 

Types  of  copper-ore  bodies.  —  Copper  ores  have  been  formed  at 
different  periods  in  the  geologic  past,  but  the  majority  of  them  show 
an  intimate  association  with  igneous  rocks.  Five  important  types  of 
occurrence  may  be  referred  to,  all  of  which  appear  to  have  been  formed 
by  magmatic  waters,  no  magmatic  segregations  being  known  in  the 
United  States,  but  the  chalcopyrite-pyrrhotite  deposits  of  Sudbury, 
Ontario,  are  of  this  type. 

Contact  metamorphic  deposits.  —  These  are  found  in  crystalline,  usually 
garnetiferous,  limestone,  along  igneous  contacts,  and  are  known  at 
several  points  in  the  west,  including  the  Clifton-Morenci  and  Bisbee 
districts  of  Arizona,  Bingham  Canyon,  Utah,  etc.  These  were  of  some 
importance,  especially  in  former  years,  but  they  have  been  outranked 
by  the  next  type  which  is  often  associated  with  them. 

Disseminated  deposits.  —  Bodies  of  sulphides,  deposited  by  magmatic 
waters,  in  igneous  rocks  or  schists,  either  in  connection  with  the  pre- 
ceding type  or  alone,  form  a  type  which  has  become  of  great  importance 
in  the  West.  The  country  rock  is  more  or  less  fractured,  and  the  low- 
grade  disseminated  ore  is  sometimes  present  in  large  amounts.  Its 
commercial  value  is  due  to  secondary  enrichment,  and  over  it  there 
is  a  leached  capping  of  variable  thickness.  Since  these  ores  often 
occur  in  porphyritic  igneous  rocks,  they  are  sometimes  called  porphyry 
coppers.  This  disseminated  type  is  worked  at  Ely,  Nev.;  Bingham, 
Utah;  Miami  and  Ray,  Ariz.;  Clifton-Morenci,  Ariz.,  etc. 

Vein  deposits.  —  In  some  district  sas  Butte,  Mont.,  the  copper  ore  is 
found  in  fissure  veins,  in  which  it  has  been  deposited  either  by  replace- 
ment or  cavity  filling.  The  wall  rock  is  often  strongly  altered  by 
hydrothermal  metamorphism.  Other  metals  may  be  present  in  variable 
amounts. 

A  modification  of  this  type  is  found  in  the  Michigan  area,  where 
native  copper  occurs  in  amygdaloidal  volcanics,  sandstones,  and  con- 


I 


626 


ENGINEERING  GEOLOGY 


glomerates,  either  as  a  replacement,  or  filling  cavities.  This  occurrence 
is  unique  among  those  of  the  United  States,  but  a  similar  type  is  found 
in  New  Jersey,  and  its  analogue  in  Arctic  Canada. 

Vein  deposits  of  mixed  character,  in  which  the  copper  is  associated 
with  lead,  zinc,  gold  or  silver,  are  worked  at  a  number  of  points  in  the 
Rocky  Mountains.  Copper  veins,  with  or  without  gold,  are  found  at 
several  points  in  the  southern  Appalachians.  The  Virgilina,  Va.,  dis- 
trict is  typical  of  the  former  type,  and  the  Gold  Hill,  N.  C.,  of  the 
latter. 

Lenses  in  schists.  —  Lens-  or  pod-shaped  deposits  of  chalcopyrite, 
with  or  without  pyrite  or  pyrrhotite,  are  found  in  some  schistose  rocks. 
These  deposits,  which  are  usually  of  low  grade,  may  represent  replace- 
ments of  metamorphic  rocks  along  fissures,  replaced  limestone,  or  in 
some  instances  they  are  thought  to  be  metamorphosed  contact-meta- 
morphic  deposits. 

They  are  worked  at  Ducktown,  Tenn.;  and  the  same  type  has  been 
found  at  a  number  of  other  points  in  the  Appalachian  states  from  Ver- 
mont to  Alabama,  but  are  usually  of  low  grade,  owing  to  the  large 
amount  of  pyrrhotite  and  pyrite  and  a  small  percentage  of  chalcopy- 
rite. Similar  occurrences  consisting  of  a  low-grade  mixture  of  chalco- 
pyrite and  pyrite  are  worked  in  Shasta  County,  Cal. 

Production.  —  The  United  States  is  the  leading  copper-producing 
country  of  the  world,  having  in  1912  an  output  of  1,243,268,720  pounds 
valued  at  $205,139,338. 

Montana,  Michigan  and  Arizona  are  the  three  most  important  copper- 
producing  states. 

Lead  and  Zinc  Ores 

These  two  are  frequently  associated  with  each  other,  and  in  the 
Rocky  Mountain  region,  especially,  gold,  silver,  and  copper  may  be 
common  associates. 

Ore  minerals  of  zinc.  —  The  important  ones  are : 


Name. 

Composition. 

Per  cent,  Zn. 

Sphalerite  

ZnS.. 

67.0 

Smithsonite  

ZnCO3  

51.96 

Calamine 

2  ZnO   SiO2,  H2O 

54  20 

Hydroz  incite 

ZnCO3,  2  Zn(OH)2 

60  0 

Zincite 

ZnO                            

80  3 

Willemite 

2  ZnO,  SiO«  

58.5 

Franklinite  .       

(FeMnZn)O,  (FeMn)2O3... 

variable 

ORE  DEPOSITS  627 

The  first  of  these  may  be  either  a  primary  or  secondary  enrichment 
ore,  while  the  following  three  are  found  in  the  weathered  zone.  The 
last  three  are  found  only  at  Franklin  Furnace,  N.  J. 

The  sphalerite  (known  also  as  blende,  jack,  rosin  jack,  or  black  jack) 
is  by  far  the  most  important  ore  of  zinc.  It  is  often  associated  with 
other  sulphides,  especially  galena,  pyrite,  and  marcasite,  but  more 
rarely  chalcopyrite.  Both  smithsonite  and  calamine  may  occur  "in 
the  same  deposit;  they  are  sometimes  of  crystalline  form,  but  more 
often  quite  impure  and  of  crusted  or  earthy  character. 

Ore  minerals  of  lead.  —  These  are  but  few  as  shown  below: 


Name. 

Composition. 

Per  cent,  Pb. 

Galena  

PbS 

86.4 

Cerussite  

PbCO3  

77.5 

Anglesite  

PbSO4  

68.3 

Pyromorphite 

Pb3P2O8+i  PbCl2 

76  36 

Galena  is  the  commonest  lead  mineral  and  may  be  either  primary 
or  due  to  secondary  enrichment.  In  complex  ores  it  frequently  carries 
silver.  The  other  three  minerals  occur  in  the  weathered  zone,  and 
of  these  cerussite  is  the  most  often  found. 

Weathering  of  lead  and  zinc  ores.  —  Sphalerite  weathers  rapidly, 
and  is  leached  out  before  the  galena;  not  that  the  galena  does  not  start 
to  alter  as  soon,  but  because  it  becomes  covered  with  an  insoluble 
weathered  product,  which  protects  the  sulphide.  As  a  result  of  this 
differential  leaching  the  zinc  may  all  be  removed  from  the  upper  part 
of  a  mixed  lead  and  zinc  ore  body.  The  ore  will  consequently  change 
from  lead  above  to  predominant  zinc  below.  However,  both  lead  and 
zinc  ores  may  show  secondary  enrichment. 

Classification  of  lead  and  zinc  ores.  —  On  a  mineralogical  basis 
lead  and  zinc  ores  can  be  divided  into  three  groups  as  follows: 

1.  Lead  and  zinc  ores,  practically  free  from  copper  and  the  precious 
metals. 

2.  Lead  and  zinc  ores,  carrying  more  or  less  gold  and  silver  as  well 
as  some  iron  and  copper. 

3.  Lead-silver  ores. 

In  the  first  group,  lead  and  manganese  are  not  uncommon  impur- 
ities, and  those  of  southwestern  Missouri  carry  small  quantities  of 
cadmium,  calcite,  dolomite,  and  pyrite  or  marcasite  as  common 
gangue  minerals,  and  barite  or  fluorite  may  also  be  present. 


628 


ENGINEERING  GEOLOGY 


The  second  group  is  found  chiefly  in  the  Rocky  Mountains,  and  is 
not  only  of  complex  character,  but  differs  in  form  and  origin  from  the 
eastern  deposits.  Quartz  is  the  common  gangue  mineral,  while  arsenic, 
antimony,  and  iron  are  common  impurities. 

The  third  group  is  confined  to  the  western  states,  and  carries  small 
amounts  of  zinc,  gold,  and  iron,  in  addition  to  the  main  constituents, 
lead  and  silver. 

Mode  of  occurrence  of  lead  and  zinc  ores.  —  Lead  and  zinc  ores 
may  occur  under  several  different  conditions  as  follows: 

1.  As  true  metalliferous  veins,  in  igneous  or  stratified  rocks,  and 
with  or  without  other  metals.     This  type  is  prominent  in  the  Cordilleran 
region. 

2.  Irregular  masses  in  metamorphic  rocks,  as  at  Franklin  Furnace, 
N.  J.     These  supply  zinc  alone. 

3.  As  irregular  masses  or  disseminations,  formed  by  replacement  or 
impregnation   in   limestones    or    quartzites.     Replacement   masses   in 


FIG.  225.  —  Map  showing"  distribution  of  lead  and  zinc  ores.     (After  Ransome, 

Min.  Mag.,  X.) 

quartzite  and  limestone  are  found  at  Leadville,  Col.;  disseminated 
ores  of  lead  in  limestone,  in  the  southeastern  Missouri  district,  and  of 
zinc  with  some  lead  in  limestone,  in  southwestern  Virginia  and  eastern 
Tennessee.  The  disseminated  ores  are  raised  in  tenor  by  mechanical 
concentration. 

4.  Contact  metamorphic  deposits.     The  occurrence  of  lead  and  zinc 
in  these  is  usually  subordinate. 


ORE  DEPOSITS 


629 


5.  In  cavities  not  of  the  fissure-vein  type,  as  the  zinc  ores  of  south- 
western Missouri,  and  the  lead  and  zinc  ores  of  Wisconsin. 

6.  In    residual    clays,    as    in    southwestern    Virginia    and    eastern 
Tennessee. 

Production.  —  In  1912  the  United  States  produced  415,395  short 
tons  of  lead,  valued  at  $37,385,550,  and  323,907  short  tons  of  zinc, 
valued  at  $44,699,166. 


Gold  and  Silver  Ores 

Ore  minerals.  —  Gold  and  silver  are  obtained  from  a  variety  of 
ores,  in  some  of  which  gold  predominates,  in  others  silver,  while  in  still 
a  third  class  the  two  metals  may  be  mixed  with  the  baser  metals,  lead, 
copper,  zinc,  and  iron.  In  some  ores  even  rarer  elements  like  arsenic, 
bismuth,  tellurium,  etc.,  are  present. 

Gold  is  found  in  nature  chiefly  as  native  gold,  or  as  telluride.  In 
the  former  case  it  may  be  visible,  or  mixed  with  pyrite,  chalcopy- 
rite,  sphalerite,  pyrrhotite,  or  arsenopyrite.  Native  gold  may  occur 
in  both  primary  and  secondary  zones,  but  the  telluride  is  always 
primary. 

Silver,  if  in  the  native  form,  may  be  visible,  or  locked  up  mechani- 
cally in  other  sulphides,  especially  galena.  Aside  from  this  both  primary 
and  secondary  ore  minerals  are  found  as  below: 


Name. 

Composition. 

Per  cent,  Ag. 

{Argentite 

Ag2S 

87  1 

Primary 

Pyrargyrite,  ruby  silver 

Ag2S  Sb2S3 

59  9 

or 

Proustite,  light  ruby  silver. 

2  Ag2S,  As2S3 

65  5 

secondary 

Stephanite,  brittle  silver,  black  silver 
f  Cerargyrite,  horn  silver  

5  Ag2S,  Sb2S3  . 
AgCl. 

68.5 
75  3 

Weathered 

1  Bromyrite 

AgBr 

57  4 

zone 

I  Enbolite 

Ag(ClBr) 

64  5 

I  lodyrite 

Ael 

46 

Tetrahedrite  (see  under  copper  ore  minerals)  may  also  carry  silver, 
replacing  some  of  the  copper,  and  its  presence  in  the  ore  is  regarded 
as  a  favorable  indication. 

Occurrence  of  gold  and  silver  ores.  —  Most  of  the  gold  and  silver 
mined  in  the  United  States  is  obtained  from  fissure  veins,  or  similar 
deposits  of  irregular  shape,  and  in  which  the  ores  have  been  deposited 
either  from  solution  in  cavities  or  by  replacement.  Much  gold  and 


630  ENGINEERING  GEOLOGY 

a  little  silver  is  obtained  from  gravel  deposits,  and  some  contact  meta- 
morphic  deposits  are  known.  While  gold  has  been  found  occurring  as 
an  original  constituent  of  igneous  rocks,  this  source  is  not  to  be  regarded 
as  being  of  commercial  value. 

It  can  be  stated  in  general  terms  that  the  mode  of  occurrence  of  these 
two  metals  is  quite  variable,  and  although  the  fissure-vein  type  of 
deposit  predominates,  these  fissures  may  form  in  any  kind  of  rock, 
or  along  the  contact  between  two  different  kinds. 

The  gold-  and  silver-bearing  fissure  veins  include  two  prominent 
types,  viz.:  (1)  Quartz  veins,  and  (2)  propylitic  veins,  characterized 
by  propylitic  (p.  602)  alteration  of  the  wall  rock. 

Quartz-vein  type.  —  This  type  which  is  characterized  by  quartzose 
ores  with  free  gold  and  auriferous  sulphides  extends  from  Lower  Cali- 
fornia, along  the  Pacific  Coast  to  the  Canadian  boundary,  and  is 
also  found  along  the  Alaskan  coast.  The  deposits  of  the  Mother  Lode 
belt  in  California,  and  the  Nevada  City  district  of  the  same  state,  are 
of  this  type,  as  are  also  the  gold  veins  near  Juneau,  Alas.  Other  gold 
quartz  veins,  although  of  older  age,  occur  in  the  Black  Hills,  S.  Dak., 
and  in  the  southern  Appalachian  states,  but  both  these  occurrences 
are  sometimes  more  of  the  nature  of  impregnations  in  quartzose  schists 
than  well-defined  veins. 

Propylitic  veins. — These  represent  an  important  type  associated  with 
lavas  of  Tertiary  age,  the  veins  being  sometimes  entirely  within  the 
volcanic  rocks.  The  ores  are  usually  quartzose,  and  while  either  gold 
or  silver  may  predominate,  the  amounts  of  the  two  metals  may  be  the 
same.  Other  metals  may  be  present,  but  not  necessarily  in  large 
amounts.  The  well-known  mining  camp  of  Cripple  Creek,  Col.  (where 
the  gold  and  silver  are  combined  with  tellurium),  and  the  Goldfield  and 
Tonopah  districts  of  Nevada,  are  of  this  type. 

Auriferous  gravels.  —  These  yield  a  large  percentage  of  the  gold  pro- 
duction of  the  United  States  and  Alaska,  having  yielded  about  23 
million  dollars  worth  in  1911,  but  comparatively  little  silver.  Their 
mode  of  origin  has  already  been  referred  to  on  page  592,  and  they  are 
found  chiefly  in  those  areas  in  which  auriferous  quartz  veins  are  promi- 
nent. Hence,  they  are  somewhat  widely  known  in  the  Cordilleran 
region,  the  Black  Hills,  and  even  in  the  South  Atlantic  states.  Their 
greatest  development,  however,  is  along  the  Pacific  Coast  from  Cali- 
fornia up  to  Alaska. 

In  these  gravels,  the  gold  occurs  in  the  form  of  nuggets,  flakes,  or 
even  small  dust-like  grains.  In  some  cases  the  gravels  have  been 
covered  by  lava  flows.  Where  the  gravels  are  located  on  hill  slopes, 


ORE  DEPOSITS  631 

and  are  not  covered  by  lava  caps,  they  can  be  worked  by  hydraulic 
mining,  but  where  they  occur  in  or  at  the  level  of  modern  stream 
channels,  dredging  is  commonly  resorted  to  for  recovering  the  gold. 
Production.  —  The  United  States  production  of  gold  in  1912  was 
valued  at  $3,451,000,  and  of  silver  at  $39,197,500. 


References  on  Ore  Deposits 

General  works.  —  1.  Farrell,  Practical  Field  Geology,  New  York, 
1912.  (McGraw-Hill  Book  Co.)  2.  Gunther,  Examination  of  Pros- 
pects, New  York,  1912.  (McGraw-Hill  Book  Co.)  3.  Kemp,  Ore 
Deposits  of  United  States  and  Canada,  New  York,  1906.  (McGraw- 
Hill  Book  Co.)  4.  Hies,  Economic  Geology,  3rd  ed.,  New  York,  1910. 
(Macmillan  Co.)  4a.  Lindgren,  Mineral  Deposits,  New  York,  1913. 
(McGraw-Hill  Book  Co.) 

Special  papers  of  importance.  —  5.  Emmons,  S.  F.,  Amer.  Geol. 
Soc.,  Bull.  XV,  p.  1,  1904.  (Theories  of  ore  deposition.)  6.  Emmons, 
S.  F.,  Min.  &  Sci.  Press,  Sept.  22,  1906.  (Forms  of  ore  bodies.)  7. 
Emmons,  W.  H.,  Min.  &  Sci.  Press,  Dec.  4  and  11,  1909.  (Outcrops.) 
8.  Emmons,  W.  H.,  U.  S.  Geol.  Survey,  Bull.  529,  1913.  (Enrichment 
of  sulphide  ores.)  9.  Kemp,  Amer.  Inst.  Min.  Engrs.,  Bull.,  Apr.,  1913. 
(Groundwaters.)  10.  Kemp,  Econ.  Geol.,  I,  p.  207,  1906.  (Problem  of 
metalliferous  veins.)  11.  Kemp,  Contact  metamorphic  deposits,  in 
Types  of  Ore  Deposits,  1911.  12.  Irving,  Econ.  Geol.,  VI,  p.  527,  1911. 
(Replacement  deposits.)  13.  Lindgren,  Econ.  Geol.,  IV,  p.  409,  1909. 
(Metallogenetic  epochs.)  14.  Posepny,  Amer.  Inst.  Min.  Engrs.,  Trans., 
XXIII,  p.  197,  1894.  (Genesis  of  ore  deposits.)  15.  Tolman,  Min.  and 
Sci.  Press,  Jan.  4,  18,  and  25,  1913.  (Secondary  sulphide  enrichment.) 
16.  Van  Hise,  Amer.  Inst.  Min.  Engrs.,  Trans.,  XXX,  p.  27,  1901. 
(Deposition  of  ores.) 

Areal  Reports.  —  To  list  all  of  these,  even  the  important  ones, 
would  be  beyond  the  scope  of  this  book. 

Attention  may  be  called  to  the  fact  that  the  Geological  Surveys  of 
New  York,  New  Jersey,  Virginia,  North  Carolina,  Georgia,  Alabama, 
Michigan,  Arkansas,  Missouri,  Wisconsin,  Minnesota,  Oklahoma,  Colo- 
rado, and  California  have  issued  a  number  of  special  reports. 

In  addition  to  these  a  number  of  bulletins  and  other  special  reports 
have  been  published  by  the  U.  S.  Geological  Survey,  dealing  especially 
with  ore-bearing  districts  in  the  western  United  States. 


APPENDIX  A 

GEOLOGIC  COLUMN 

The  earth's  crust  is  made  up  of  igneous,  sedimentary,  and  metamorphic  rocks. 
The  igneous  rocks  may  represent  in  part  a  portion  of  the  original  crust,  formed  at 
the  time  of  its  early  cooling,  and  in  part  intrusive  or  extrusive  materials  forced  up 
from  below  during  different  periods  in  the  earth's  history. 

The  sedimentary  rocks  are  those  which  have  been  laid  down  on  the  ocean  floor, 
or  on  the  bottom  of  inland  seas  or  lakes,  throughout  the  vast  period  of  geologic 
time.  They  often  contain  the  impressions,  or  even  remains  of  animals  and  plants 
which  lived  in  the  past.  These  imbedded  remains  (fossils)  are  of  great  service  to 
the  geologist  in  determining  the  age  of  the  inclosing  rocks.  The  age  of  the  non- 
fossiliferous  rocks  of  whatever  kind  (igneous,  sedimentary,  and  metamorphic)  is 
determined  where  possible  by  their  structural  relationships  to  other  rocks  of  known 
age. 

The  divisions  and  subdivisions  of  geologic  time  are  not  yet  absolutely  fixed,  and 
the  minor  subdivisions  established,  even  for  one  part  of  a  continent,  may  not  agree 
with  those  of  another  part,  due  in  some  cases  to  the  fact  that  continuous  deposition 
of  sediment  might  be  going  on  in  one  area,  while  in  another,  during  the  same  time, 
the  land  was  above  sea  level,  and  there  was  no  sedimentation.  In  some  cases  the 
thickness  of  the  sedimentary  rocks  deposited  without  break  over  a  given  portion 
of  the  earth's  surface  may  amount  to  twenty  thousand  feet  or  more. 

The  names  applied  to  the  divisions  of  geologic  time  and  to  the  rocks  are  not  the 
same,  but  for  each  division  of  the  one  there  is  a  corresponding  one  of  the  other, 
thus: 

Time  Scale  Rock  Scale 

Era  Group 

Period  System 

Epoch  Series 

Age  Stage 

Thus  we  speak  of  the  Silurian  Period  of  time,  but  the  rocks  of  that  period  are 
referred  to  as  belonging  to  the  Silurian  System. 

We  give  below  a  list  of  the  major  divisions  of  geologic  time  and  their  more  impor- 
tant subdivisions,  arranged  in  their  order  of  formation,  the  youngest  being  at  the 
top.  For  further  details  see  such  books  as  Scott's,  "An  Introduction  to  Geology," 
or  Chamberlin  and  Salisbury's  "Geology,"  Vols.  II  and  III. 

Quaternary..  ..|^ecent 

Cenozoic  I  Pleistocene  or  Glacial. 

f  Pliocene. 

Tertiary |  Miocene. 

I  Eocene. 

Cretaceous I  ^er  or  Cretaceous  proper. 

Mesozoic jurassic  '        &  ™ Comanchean- 

Triassic. 

632 


APPENDIX 


633 


Palaeozoic 


Proterozoic .  . . 
(Algonkian) 


Permian. 
Carboniferous. 


Devonian. 


Silurian , 


Ordovician 


Cambrian 


J  Pennsylvanian  (upper). 
\  Mississippian  (lower). 
[Neodevonian  (upper). 
\  Mesodevonian  (middle). 
I  Paleodevonian  (lower). 
fCayugan  (upper). 
\  Niagaran  (middle). 
lOswegan  (lower), 
f  Cincinnatian  (upper). 
|  Mohawkian  (middle). 
(Canadian  (lower), 
f  Potsdam  (upper). 
|  Acadian  (middle). 
I  Georgian  (lower). 


Keweenawan. 
Upper  Huronian. 
Middle  Huronian. 
Lower  Huronian. 


Archaeozoic  . . .  .Archaean. 


APPENDIX  B 

GEOLOGICAL  SURVEYS 

In  the  preceding  pages  of  this  book  reference  has  been  made,  from  time  to  time, 
to  reports  published  by  State  and  National  Geological  Surveys.  Many  of  these, 
and  others  not  mentioned,  as  well  as  geologic  maps  of  the  states  and  individual 
areas,  can  be  obtained  on  application  to  the  Director,  or  State  Geologist.  The  latter 
can  also  frequently  furnish  engineers  with  information  regarding  local  geologic 
details  in  their  territory;  hence,  we  give  herewith  a  list  of  the  National  and  State 
Geological  Surveys,  together  with  the  name  of  the  official  in  charge  at  the  present 
time. 

United  States 

United  States  Geological  Survey,  Washington,  D.  C.;   G.  O.  Smith,  Director. 

Bureau  of  Mines,  Washington,  D.  C.;  J.  A.  Holmes,  Director. 

Alabama.  —  Geological  Survey  of  Alabama;  E.  A.  Smith,  State  Geologist,  Uni- 
versity. 

Arizona.  —  Geological  Survey  of  Arizona;  C.  H.  Clapp,  Territorial  Geologist, 
Tucson. 

Arkansas.  —  Geological  Survey  of  Arkansas;  N.  F.  Drake,  State  Geologist, 
Fayetteville. 

California.  —  California  State  Mining  Bureau;  F.  McN.  Hamilton,  State  Mineral- 
ogist, San  Francisco. 

Colorado.  —  Colorado  State  Geological  Survey;  R.  D.  George,  State  Geologist, 
Boulder. 

Connecticut.  —  State  Geological  and  Natural  History  Survey;  W.  N.  Rice,  Super- 
intendent, Middletown. 

Florida.  —  Florida  State  Geological  Survey;  E.  H.  Sellards,  State  Geologist, 
Tallahassee. 

Georgia.  —  Geological  Survey  of  Georgia;  S.  W.  McCallie,  State  Geologist, 
Atlanta. 

Illinois.  —  State  Geological  Survey;  F.  W.  DeWolf,  Director,  Urbana. 

Indiana.  —  Department  of  Geology  and  Natural  Resources;  Ed.  Barrett,  State 
Geologist,  Indianapolis. 

Iowa.  —  Iowa  Geological  Survey;  G.  F.  Kay,  State  Geologist,  Iowa  City. 

Kansas.  —  State  Geological  Survey  of  Kansas;  Erasmus  Haworth,  State  Geolo- 
gist, Lawrence. 

Kentucky.  —  Kentucky  Geological  Survey;  J.  B.  Hoeing,  Director,  Frankfort. 

Maine.  —  State  Survey  Commission;   C.  Vey  Holman,  State  Geologist,  Bangor. 

Maryland.  —  State  Geological  and  Economic  Survey;  W.  B.  Clark,  State  Geol- 
ogist, Baltimore. 

Michigan.  —  Michigan  Geological  and  Biological  Survey;  R.  C.  Allen,  State 
Geologist,  Lansing. 

Minnesota.  —  W.  H.  Emmons,  State  Geologist,  University  of  Minnesota,  Minne- 
apolis. 

Mississippi.  —  Geologic,  Economic,  and  Topographic  Survey  of  Mississippi;  E. 
N.  Lowe,  Director,  Jackson. 

634 


APPENDIX  635 

Missouri.  —  Bureau  of  Geology  and  Mines;  H.  A.  Buehler,  Director,  Rolla. 

Nebraska.  —  Nebraska  Geological  Survey;  E.  H.  Barbour,  State  Geologist, 
Lincoln. 

New  Jersey.  —  Geological  Survey  of  New  Jersey;  H.  B.  Kiimmel,  State  Geolo- 
gist, Trenton. 

New  York.  —  Science  Division  (Geological  Survey)  of  the  Educational  Depart- 
ment; J.  M.  Clarke,  State  Geologist,  Albany. 

North  Carolina.  —  North  Carolina  Geological  and  Economic  Survey;  J.  H.  Pratt, 
State  Geologist,  Chapel  Hill. 

North  Dakota.  —  North  Dakota  Geological  Survey;  A.  G.  Leonard,  State  Geol- 
ogist, Grand  Forks. 

Ohio.  —  Geological  Survey  of  Ohio;  J.  A.  Bownocker,  State  Geologist,  Columbus. 

Oregon.  —  Oregon  Bureau  of  Mines  and  Geology;  W.  A.  Parks,  Director,  Cor- 
vallis. 

Pennsylvania.  —  Topographical  and  Geological  Survey  Commission;  R.  R.  Hice, 
State  Geologist,  Beaver. 

Rhode  Island.  —  Natural  Resources  Survey  of  Rhode  Island;  C.  W.  Brown, 
Superintendent,  Providence. 

South  Dakota.  —  Geological  Survey  of  South  Dakota;  E.  C.  Perisho,  State 
Geologist,  Vermillion. 

Tennessee.  —  Tennessee  State  Geological  Survey;  A.  H.  Purdue,  State  Geolo- 
gist, Nashville. 

Vermont.  —  Geological  Survey  of  Vermont;  G.  H.  Perkins,  State  Geologist, 
Burlington. 

Virginia.  —  Virginia  Geological  Survey;  Thomas  L.  Watson,  Director,  Charlottes- 
ville. 

Washington.  —  State  Geological  Survey  of  the  State  of  Washington;  Henry 
Landes,  State  Geologist,  Seattle. 

West  Virginia.  —  West  Virginia  Geological  and  Economic  Survey;  I.  C.  White, 
State  Geologist,  Morgantown. 

Wisconsin.  —  Wisconsin  Geological  and  Natural  History  Survey;  W.  O.  Hotch- 
kiss,  State  Geologist,  Madison. 

Wyoming.  —  Geological  Survey  of  Wyoming;  C.  E.  Jamison,  State  Geologist, 
Cheyenne. 

Canada 

Department  of  Mines,  R.  W.  Brock,  Deputy  Minister;  Geological  Survey  Branch; 

,   Director,  Ottawa;     Mines   Branch,    E.    Haanel,    Director, 

Ottawa. 

Quebec.  —  Department  of  Mines;  Theo.  Denis,  Superintendent  of  Mines,  Quebec. 
Ontario.  —  Bureau  of  Mines;  W.  G.  Miller,  Provincial  Geologist,  Toronto. 
British  Columbia.  —  Bureau  of  Mines;  W.  F.  Robertson,  Provincial  Geologist. 

Mexico 

Geological  Survey  of  Mexico.     J.  G.  Aguilera,  Director,  Mexico  City. 


SUBJECT  INDEX 


Abrasion,  relation  to  weathering,  219. 
Abrasive  resistance    of   building   stone, 
451. 

resistance  of  slate,  486. 
Absorption  of  building  stone,  437. 

of  building  stones  under  pressure,  437. 

of  crushed  stone,  584. 

relation  to  porosity  in  building  stone, 

438. 

Acadian,  633. 
Accessory  minerals,  64. 
Acidity  of  river  waters,  293. 
Acmite,  15. 

Acre  foot,  denned,  252. 
Actinolite,  18. 
Adinole,  208. 
Adobe,  111,  519. 
Aegirite,  15. 
^Eoh'an  rocks,  denned,  47. 

soils,  241. 
Agate,  30. 

Agglomerate,  volcanic,  92. 
Alabaster,  38. 
Albertite,  analysis  of,  571. 

properties  of,  572. 
Albite,  8. 
Algonkian,  106. 
Allotriomorphic  denned,  67. 
Alluvial  plains,  269. 

soils,  241. 
Almandite,  20. 

Alteration  of  rocks,  defined,  200. 
Alunitization,  603. 
Amphibole  group,  17. 
Amygdaloidal  texture,  69. 
Amygdules,  69. 
Analcite,  28. 
Analyses,  of  basalts,  89. 

of  bitumens,  571. 

of  cannel  coal,  532. 

of  clay,  515. 

of  clays  for  Portland  cement,  498. 

of  coals,  535. 

of  coke,  natural,  547. 

of  diorites,  79. 


Analyses,  of  felsites,  87. 

of  gabbro,  80,  82. 

of  gas,  from  producer,  551. 

of  gases,  natural  and  manufactured, 
564. 

of  gneiss,  128. 

of  granite,  75. 

of  gypsum,  507. 

of  hydraulic  lime,  496. 

of  hydraulic  limestone,  496. 

of  igneous  rocks,  average,  63. 

of  lake  waters,  409. 

of  limestone,  476,  493. 

of  maltha,  571. 

of  mine  waters,  593. 

of  natural-cement  rock,  497. 

of  natural  gas,  563. 

of  peat,  529. 

of  plutonic  igneous  rocks,  62. 

of  pyroxenite,  85. 

of  residual  clays  and  rock,  232. 

of  river  waters,  289,  290. 

of  schists,  133. 

of  sedimentary  rocks,  composite,  98. 

of  serpentine,  143. 

of  slag,  503. 

of  slate,  138. 

of  soapstone,  143,  145. 

of  syenite,  77. 

of  talc,  143,  145. 

of  tars,  coke  oven,  550. 

of  volcanic  ash,  503. 

of  volcanic  fragmental  rocks,  93. 

of  volcanic  glass,  90. 
Analysis,  elementary,  of  coal,  533. 

proximate,  of  coal,  533. 
Anamorphic  zone,  204. 
Andesine,  8.. 
Andesite,  89. 
Andradite,  20. 
Angle  of  pull,  353. 
Angle  of  rest,  353. 
Angle  of  slide,  353. 
Anglesite,  43,  627. 
Anhedron,  defined,  2. 


637 


638 


SUBJECT  INDEX 


Anhydrite,  39. 

in  gypsum  deposits,  505. 

relation  to  tunneling,  114. 

(rock)  occurrence,  113. 
Anorthite,  8. 
Anorthosite,  80. 
Anthracite,  532. 

analyses  of,  535. 

sections  in  Pennsylvania,  539. 
Anticline  defined,  148. 
Anticlinorium  defined,  151. 
Apatite,  39. 

in  iron  ore,  616. 
Apex  of  vein,  605. 
Aphanitic  texture,  67. 
Aplite,  75. 

Aqueducts,  effects  of  faulting  on,  183. 
Aqueous  rocks,  denned,  47. 
Aquifer,  denned,  317. 

source  of  water,  322. 
Aragonite,  36. 
Archaean,  633. 
Archaeozoic,  633. 
Arfvedsonite,  18. 
Argentite,  629. 
Arkose,  106. 
Artesian  province,  defined,  317. 

slope,  315. 

water,  aquifer,  317. 

water,  capacity  of  rocks,  315. 

water,  cavities  holding,  315. 

water,  change  from  salt  to  fresh,  322. 

water,  collecting  area,  317. 

water,  corrosion  of  casing,  320. 

water,  definition  of,  314. 

water,  depletion  of  supply,  321. 

water,  depths  at  which  found,  322. 

water,  factors  affecting  supply,  320. 

water  in  buried  channels,  323. 

water  in  crystalline  rocks,  324. 

water  in  glacial  drift,  322. 

water  in  limestones,  319. 

water  in  sand,  318. 

water  in  sandstone,  318. 

water  in  stratified  rocks,  318. 

water,  interference  of  wells,  321. 

water,  irregularities  of  supply,  320. 

water,  requisite  conditions  for,  317. 

water,  see  Underground  Water,  295. 

water,  several  aquifers  in  same  section, 
320. 

water,  source  of,  322. 

water,    yield   from   crystalline   rocks, 
326. 


Artesian  water,  yield  of  wells,  322. 

well,  315. 

wells,  see  Wells. 
Asbestos,  27. 
Ash  beds,  56. 
Asphalt,  origin  of,  566. 
Asphaltene  defined,  572. 
Asphaltic  limestone,  575. 
Asphalt,  Trinidad,  analysis  of,  571. 

Trinidad,  properties  of,  573. 

varieties  of,  572. 
Asymmetrical  fold,  defined,  154. 
Atacamite,  622. 
Augen  gneiss,  129. 

texture,  125. 
Augite,  15. 

diorite,  79. 

syenite,  78. 
Auxiliary  fault,  169. 
Axial  plane,  of  folds,  148. 
Axis,  of  folds,  148. 
Azurite,  622. 

Banded  structure  (metamorphic),  125. 

Bar  defined,  380. 

Barite  as  concretions,  199. 

Barrier  beach,  370. 

Bars,  at  river  mouth,  277. 

relation  to  rivers,  385. 
Basalt,  as  building  stone,  469. 

porosity  of,  438. 

porphyry,  87,  91. 

properties  and  occurrence,  87. 
Base  level  defined,  256. 
Batholith,  139. 
Bathylith,  see  Batholith. 
Beach  defined,  368. 
Bedding  defined,  96. 
Bedford  limestone,  474,  476. 
Bedded  vein,  605. 
Beds,  depth  of  measured,  163. 
Belt  of  cementation,  204. 

of  weathering,  204. 
Bends  in  rivers,  259. 
Berea  sandstone,  471. 

sandstone,  pyrite  in,  435. 
Biotite,  12. 

granite,  75. 
Bitumens,  570. 

references  on,  576. 
Bituminous  coal,  analyses  of,  535. 

coal,  coking,  530. 

coal,  properties  of,  530. 

rocks,  573. 


SUBJECT  INDEX 


639 


Bituminous  rocks,  distribution  of,  574. 

Black  waxy  soil,  580. 

Blind  joints,  191. 

Block  coal,  557. 

Bluestone,  life  of,  432. 

Body  current,  in  lakes,  403. 

Bog  lime,  119. 

Bogs,  relation  to  roads,  577. 

Boiler  scale,  338. 

Boils,  in  rivers,  261. 

Bombs,  volcanic,  92. 

Bonanzas,  605. 

Bornite,  622. 

Boss,  55. 

Bottom-set  beds,  274. 

Boulder  clay,  421. 

Boulets,  551. 

Breakers,  zone  of,  361. 

Breccia,  eruptive,  102. 

fault,  100,  169. 

friction,  100. 

talus,  99. 

use  of,  102. 

volcanic,  57,  92,  102. 

zone,  169. 
Breccias,  99. 
Brenner  railroad,  343. 
Brick  clay,  519. 

manufacture  of,  521. 

properties  of,  523. 
Bridge  piers  affected  by  erosion,  256. 

piers,  slipping  ot,  349. 
Briquetting  of  coal,  551. 
Brochantite,  622. 
Bromyrite,  629. 
Brown  coal,  see  Lignite. 

ore,  616. 
Brownstone,  mica  in,  433. 

life  of,  432. 
Buhrstone,  135. 
Building  stone,  abrasive  resistance,  451. 

absorption,  437. 

absorption,  relation  to  porosity,  438. 

atmospheric  gases,  effect  of,  on,  455. 

basalt,  469. 

chemical  composition,  456. 

chert  in,  433. 

color  as  affecting  selection,  429. 

contraction  of,  450. 

cost  of,  429. 

crushing  strength,  439. 

diabase,  469. 

durability  of,  431. 

elasticity,  modulus  of,  451. 


Building  stone,  estimated  life  of,  432. 

expansion,  coefficient  of,  450. 

expansion  when  heated,  450. 

factors  governing  selection,  429. 

fire  resistance,  446. 

flint  in,  433. 

frost  resistance,  453. 

gabbro,  469. 

granites,  457.  • 

igneous  rocks,  457. 

injurious  minerals  in,  433. 

joints,  effect  of,  430. 

kinds  of  rock  used,  429. 

life  of,  432. 

life,  prolongation  of,  435. 

limestone,  473. 

magnetite  in,  32. 

marbles,  478. 

mica  in,  14,  433. 

microscopic  examination,  456. 

mineral  composition  as  affecting  dur- 
ability, 432. 

onyx  marble,  482. 

ophicalcite,  483. 

ophiolite,  483. 

permanent  swelling,  450. 

physical  properties,  437. 

pores,  character  of,  439. 

pores,  relation  to  freezing,  439. 

porosity  of,  438. 

properties  of,  429. 

pyrite  in,  435. 

pyroxene  in,  17. 

quarry  water  in,  432. 

quartzite,  470. 

references  on,  490. 

rhyolite,  469. 

sandstone,  470. 

saturation  coefficient,  437. 

serpentine,  482. 

slate,  483. 

stratification,  effects  of,  431. 

structural  features  of,  430. 

structure  in  relation  to  weathering, 
431. 

syenite,  469. 

texture,  effect  on  durability,  431. 

trachyte,  469. 

transverse  strength,  443. 

tremolite  in,  435. 

volcanic  rocks,  469. 

water  absorbed  under  different  con- 
ditions, 439. 

whitewash  on,  435. 


640 


SUBJECT  INDEX 


Bumps  in  coal  mines,  537. 
Buried  channels,  rivers,  280. 
Burning  brick,  523. 
Bytownite,  8. 

Cables  broken  by  faulting,  185. 
Calamine,  626. 
Calc  sinter,  121. 
Calcareous  tufa,  121. 
Calcite,  35. 

uses,  36. 
Cambrian,  633. 
Camptonite,  79. 
Canadian  epoch,  633. 
Cannel  coal,  532. 
Canyons,  formation  of,  280. 
Carbonaceous  rocks,  121. 
Carbonates  in  river  water,  292. 
Carbonation  as  a  weathering  process,  228. 
Carbonettes,  551. 
Carboniferous,  633. 
Carbonite,  547. 
Carlsbad  twins,  9. 
Carrara  marble,  438. 
Cayugan  epoch,  633. 
Catskill  aqueduct,  183,  282,  308,  426. 
Cave  onyx,  121. 
Cementation,  belt  of,  204. 

index,  503. 
Cement,  calcareous,  493. 

cementation  index,  503. 

changes  in  burning,  500. 

collos,  503. 

distribution  of  raw  materials,  504. 

grappier,  496. 

hydraulic,  495. 

natural,  497. 

natural,  raw  materials,  497. 

-plaster,  508. 

Portland,  497. 

production,  505. 

puzzolan,  501. 

references  on,  509. 

silicate,  495. 

slag,  501. 

tests  required,  503. 
Cerargyrite,  629. 
Cenozoic,  632. 
Centrochnal  folds,  151. 
Cerussite,  43,  627. 
Chalcanthite,  622. 
Chalcedony,  30. 
Chalcocite,  622. 
Chalcopyrite,  42. 


Chalcopyrite  as  an  ore  mineral,  622. 
Chalk,  119. 

Chemical  tests  of  minerals,  7. 
Chert,  30. 

as  concretions,  199. 

for  roads,  582. 

in  building  stone,  433. 

in  limestones,  476. 

mode  of  occurrence,  115. 

uses,  115. 

Chickamauga  limestone,  153. 
Chimney  ore  deposit,  605. 
Chlorine  in  river  water,  291. 
Chlorite,  27. 
Chrysocolla,  622. 
Chrysotile,  27. 
Cincinnatian,  633. 
Cippolino  marble,  life  of,  433. 
Clay,  analyses  of,  515. 

brick  manufacture,  521. 

chemical  properties,  514. 

color,  513. 

difference  from  shale,  108. 

distribution  in  United  States,  525. 

estuarine,  517. 

engineering  uses  of,  519. 

flood  plain,  517. 

for  brick,  519. 

for  railroad  ballast,  524. 

for  road  foundations,  577. 

for  road  material,  525,  580. 

for  sewer  pipe,  524. 

fusibility  of,  511. 

glacial,  517. 

in  dam  foundations,  285. 

kinds  of,  519. 

lake,  517. 

marine,  517. 

occurrence  of,  516. 

plasticity  of,  510. 

properties  of,  510. 

references  on,  525. 

residual,  516. 

residual,  denned,  232. 

slates,  488. 

slides,  306,  344,  348. 

shrinkage  of,  511. 

specific  gravity,  513. 

tensile  strength,  511. 

transported,  517. 

uses  of,  519. 
Classification  of  igneous  rocks,  70. 

of  metamorphic  rocks,  126. 

of  sedimentary  rocks,  97. 


SUBJECT  INDEX 


641 


Cleavability  of  slate,  484. 
Cleavage,  cause  of,  194. 

close-joint,  191. 

definition,  190. 

false,  191. 

flow,  191. 

fracture,  191. 

in  minerals,  4. 

in  slates,  483. 

original,  190. 

secondary,  190. 

slaty,  191. 

slip,  191. 

strain-slip,  191. 
Clinton  ore,  616,  618. 
Clinochlore,  27. 
Clinometer,  147. 
Closed  fault,  166. 

folds,  154. 

Close-joint  cleavage,  191. 
Coal,  analyses  of,  532,  535. 

anthracite,  532. 

Appalachian  region,  556. 

ash,  536. 

bituminous,  530. 

bone  denned,  551. 

briquetting  of,  551. 

brown,  529. 

calorific  power  of,  544. 

cannel,  532. 

classification  of,  540. 

coke,  530. 

coke,  artificial,  546. 

coke,  natural,  547. 

composition  of,  533. 

concretions  in,  199. 

distribution  in  United  States,  554. 

Eastern  Interior  region,  557. 

gas  escape  from,  554. 

geologic  distribution,  555. 

kinds  of,  527. 

lignite,  529. 

Michigan  field,  559. 

Northern  Interior  field,  559. 

origin  of,  543. 

Pacific  Coast  region,  560. 

peat,  527. 

proximate  analysis,  533. 

references  on,  560. 

Rocky  Mountain  region,  559. 

semianthracite,  532. 

semibituminous,  532. 

slack,  551. 

subbituminous,  530. 


Coal,  sulphur  in,  536. 

use  of,  in  producers,  550. 

volatile  matter  in,  534. 

Western  Interior  region,  559. 
Coal  beds,  associated  rocks,  536. 

faulting  of,  539. 

folding  of,  539. 

splits  in,  538. 

structural  features,  536. 

variations  in,  537. 

variation  in  quality,  538. 
Coal  blossom,  536. 
.  Coalettes,  551. 

Coal  mine  drainage,  into  rivers,  293. 
Coal  mines,  bumps  in,  537. 

faults  in,  184. 

Coast  line,  United  States,  393. 
Coke,  530. 

analyses  of,  546. 

laboratory  tests  of,  547. 

making  of,  546. 

natural,  547. 

tar  from,  550. 

tar  from,  analyses,  550. 
Collos  cement,  503. 
Collecting  area,  artesian  water,  denned, 

317. 

Colluvial  deposits,  241. 
Colmesneil,  Tex.,  515. 
Color,  exotic,  of  minerals,  5. 

natural,  of  minerals,  5. 

of  clay,  513. 

of  minerals,  5. 

of  slate,  486. 

Comagmatic  area,  denned,  70. 
Comanchean  period,  632. 
Competent  beds,  193. 
Complementary  rocks,  69. 
Compression  joints,  166,  194. 
Conformity,  195. 
Concretions,  form  of,  198. 

material  forming,  199. 

occurrence  of,  198. 

origin  of,  199. 

uses  and  effects  of,  199. 
Conglomerate,  102. 

for  roads,  587. 

volcanic,  92,  103. 

use  of,  103. 

Connate  wrater  denned,  295. 
Consanguinity  of  igneous  rocks,  70. 
Contact-metamorphic  ore  deposits,  600. 
metamorphic  deposits,  copper,  624. 
metamorphic  minerals,  207. 


642 


SUBJECT  INDEX 


Contact  metamorphism,  204,  206. 

zone,  207. 
Copper,  native,  622. 

ores,  ore  minerals,  622. 

ores,  production  of,  626. 

ores,  types  of  ore  bodies,  624. 
Coquina,  121,  474. 
Coral  rock,  121. 
Corrasion  by  rivers,  253. 
Cortlandite,  82. 
Corundum,  31. 
Covellite,  622. 
Crater  lakes,  401. 
Creep  defined,  343. 
Crenothrix,  in  water,  339. 
Cretaceous,  632. 
Crinoidal  limestone,  121. 
Crossings,  in  rivers,  259. 
Crushed  stone,  see  Road  materials,  583. 
Crushing  strength,  bed  and  edge,  441. 

of  building  stone,  439. 

of  building  stone,  frozen,  442. 

of  building  stone,  intermittent  pres- 
sure, 442. 

of  building  stone,  wet  and  dry,  441. 
Crustification,  603. 
Crystal  axes,  2. 

form,  of  minerals,  5. 

systems,  2. 
Culm,  analysis  of,  535. 

defined,  557. 

Cumberland  smithing  coal,  557. 
Cumulose  deposits,  241. 
Cuprite,  622. 
Cut-off  in  rivers,  defined,  259. 

in  rocks,  defined,  462. 

Dakota  epoch,  422. 

sandstone,  334. 

Dams,  effect  on  water  table,  302. 
Dam  failures,  307. 

foundation,   relation  to  underground 
water,  307. 

foundations,  284. 

foundations,  in  glacial  drift,  425. 

foundations,  in  tuffs,  59. 

leakage  of,  308. 

Decay  of  rocks,  summary  of,  231. 
Decomposition  defined,  216. 
Dehydration    as    a  weathering  [process, 

225. 
Deltas,  channels  across,  271. 

channels  of,  improvement,  273. 

conditions  of  formation,  274. 


Deltas,  extent  of,  274. 

formation  of,  271. 

form  of,  271. 

fossil,  275. 

relation  to  engineering  work,  275. 

structure  of,  273. 

Deoxidation  as  a  weathering  process,  228. 
Deposition,  by  rivers,  253. 
Devitrification,  90. 
Devonian,  633. 
Diabase  as  building  stone,  469. 

porosity  of,  438. 
Diatomaceous  earth,  115. 

uses  of,  115. 

Differentiation  in  igneous  rocks,  69. 
Dikes,  50. 

complementary,  69. 

in  granite  quarries,  464. 
Diopside,  15. 
Diorite-gneiss,  129. 

porosity  of,  438. 

properties  and  occurrence,  78. 
Dip  defined,  147. 

fault,  169. 

fault,  defined,  171. 

joints,  166. 

-slip,  defined,  172. 

-slip  fault,  defined,  174. 

throw,  173. 

variations  in,  148. 
Disintegration,  defined,  216. 
Disseminated  ore-deposit,  606. 

ore-deposits,  copper,  624. 
Distributive  fault,  174. 
Dolomite,  36,  119. 

weathering  of,  239. 
Dolomitization,  119. 
Drag  folds  defined,  151. 
Drainage  by  wells,  312. 

by  wells,  applications  of,  314. 

lines,  along  faulting,  171. 
Drift-dam  lakes,  400. 
Drift,  see  Glacial  drift. 
Drilling,  effect  of  faults  on,  183. 
Drying  brick,  523. 
Dry-press  process,  522. 
Dunes,  see  Sand  dunes. 
Dunite,  82. 
Dynamo-metamorphism,  204. 

Earthquakes,  effect  on  engineering  work, 

184. 

relation  to  faults,  184. 
Earth's  crust,  average  composition,  593. 


SUBJECT   INDEX 


643 


Earth's  crust,  zones  of,  192. 

Eddies  in  rivers,  261. 

Eddy,  suction,  261. 

Eggettes,  551. 

Elements  in  earth's  crust,  1. 

Embankments,  erosion  of,  263. 

Embolite,  629. 

Enargite,  622. 

Enstatite,  15. 

Eocene,  632. 

Eolian  deposits,  109. 

Epidote,  22. 

Erosion,  by  rivers,  253. 

by  waves,  363. 

depth  of,  256. 

effect  on  folds,  154. 

factors  governing  rate  of,  254. 

of  river  banks,  262. 

structures  due  to,  195. 
Eruptive  breccia,  102. 
Eskers,  422. 
Estrich  gyps,  508. 
Extrusive  rocks,  56. 
Excavation  deformation,  354. 

Fahlband,  606. 
Falls,  see  Waterfalls. 
False  cleavage,  191,  483. 
Fan  structure,  151. 
Fault  breccia,  100. 

dip,  169. 

line,  169. 

plane,  168. 

scarps,  171,  177. 

space,  168. 

strike,  169. 

surface,  168. 

terms,  168. 
Faults,  breccia  zone,  169. 

classes  of,  171. 

classified  by  movement,  174. 

criteria  of,  169. 

defined,  168. 

determination  of,  185. 

effect  on  outcrop,  177. 

flaws,  175. 

footwall,  151. 

geologic  effects  of,  178. 

gouge,  169. 

hade  of,  169. 

hanging  wall,  169. 

heave,  173. 

in  sedimentary  rocks,  171. 

normal,  174. 


Faults,  offset,  174. 

overthrust,  175. 

relation  to  cables,  184. 

relation  to  coal  mining,  184. 

relation  to  earthquakes,  184. 

relation  to  engineering  work,  181. 

relation  to  folds,  180. 

relation  to  ore-deposits,  184. 

repeating  of  beds  by,  178. 

reverse,  174. 

rotatory,  177. 

shear  zone,  169. 

shift,  172. 

shifting  of  beds  by,  178. 

significance,  168. 

slip,  171. 

strike,  171. 

throw,  173. 

topographic  effects,  177. 

translatory,  177. 
Fayalite,  21. 
Feel  of  minerals,  7. 
Feldspars,  description  of,  7. 

in  building  stone,  11. 

occurrence,  11. 
Feldspathoids,  12. 
Felsite  porphyry,  86,  91. 

properties  and  occurrence,  86. 
Felsitic  texture,  67. 
Fire  clay,  519. 

Fire  tests  of  building  stone,  447. 
Fissility,  191. 
Fissure  eruptions,  57. 

springs,  306. 

veins,  603. 
Flagstone,  106. 
Flaws,  175. 
Flint,  as  concretions,  199. 

described,  30. 

in  building  stone,  433. 

mode  of  occurrence,  115. 
Float  ore,  612. 
Flood  plains,  269. 

material  of,  275. 
Flood-plain  terraces,  275. 
Floods,  artificial  regulation  of,  283. 

cause  of,  282. 

from  glaciers,  418. 

regulation,  natural,  283. 
Flowage  zone,  192. 
Flow  cleavage,  191. 
Flowing  well,  defined,  315. 
Folds,  147. 

anticline,  148. 


644 


SUBJECT  INDEX 


Folds,  anticlinorium,  151. 

asymmetrical,  154. 

axis  defined,  148. 

axial  plane  of,  148. 

cause  of,  192. 

centroclinal,  151. 

close,  154. 

criteria  for  determining  origin,  193. 

drag,  151. 

effect  on  mining,  157. 

effect  on  quarry  work,  157. 

effect  on  tunneling,  155. 

erosion  effect  on,  154. 

field  measurements  of,  158. 

geanticline,  151. 

geosyncline,  151. 

inclined,  154. 

isocline,  151. 

kinds  of,  148. 

limbs  of,  148. 

monoclines,  149. 

open,  154. 

overturned,  154. 

parallel,  151. 

parts  of,  148. 

pitch  of,  148. 

quaquaversal,  150. 

recumbent,  154. 

relation  to  engineering  work,  155. 

relation  to  faults,  180. 

similar,  151. 

symmetrical,  154. 

synclines,  149. 

synclinorium,  151. 

tilted,  154. 

upright,  154. 
Foliates,  125. 

Foliation  in  metamorphic  rocks,  125. 
Footwall,  fault,  169. 

vein,  605. 
Fore-set  beds,  274. 
Formation,  defined,  97. 
Forsterite,  21. 

Foundations,  effect  of  underground  water 
on,  312. 

of  dams,  284. 
Fracture  cleavage,  19 1, 

of  minerals,  6. 

zone,  192. 

Fragmental  texture,  69. 
Franklinite,  626. 
Freeport  coal,  558. 
Freestone,  106. 
Friction  breccia,  100. 


Frost  resistance  of  building  stone,  453. 
resistance,  determination  of,  454. 
effect  on  rocks,  219. 

Gabbro  as  building  stone,  469. 
-diorite,  79. 
for  roads,  587. 
-gneiss,  129. 
orbicular,  465. 

properties  and  occurrence,  80. 
Galena,  43,  627. 
Ganges  delta,  274. 
Gangue  minerals,  590. 
Garnet,  20. 

occurrence  of,  21. 
Gases,  atmospheric,   effect  on  building 

stone,  455. 

Gas  producers,  use  of  coal  in,  550. 
Geanticline  defined,  151. 
Geologic  column,  632. 
Geological  surveys,  addresses  of,  634. 
Georgian,  633. 
Geosyncline,  defined,  151. 
Geyserite,  30,  115. 
Gibbsite,  31. 
Gilsonite,  analysis  of,  571. 

properties  of,  572. 

Glacial  deposits,  buried  valleys,  424. 
by  water,  422. 
economic  materials  in,  426. 
eskers,  422. 
groundwater  in,  425. 
dam  foundations  in,  425. 
kames,  422. 
nature  of,  421. 
outwash  plain,  422. 
valley  train,  422. 
drift,  artesian  water  in  322. 
distribution,  422. 
lakes  in,  397. 
relation  to  drilling,  423. 
thickness  of,  423. 
topography  of,  423. 
epoch,  632. 
lakes,  destruction  due  to,  417. 

floods  caused  by,  417. 
soils,  241 

Glaciation,  effect  on  quarrying,  426. 
past,  422. 

relation  to  engineering,  424. 
Glaciers,  advancing,  effects  of,  417. 
alpine,  415. 
continental,  415. 
deposits  by,  420. 


SUBJECT  INDEX 


645 


Glaciers,  erosion  by,  418. 

general  features  of,  415. 

motion  of,  415. 

origin,  414. 

piedmont,  415. 

polar,  415. 

references  on,  426. 

transportation  by,  420. 

types  of,  415. 

valley,  415. 

Glance  pitch,  properties  of,  573. 
Glassy  texture,  67. 
Gneiss,  definition,  127. 

distribution  of,  464. 

frost  tests  of,  455. 

life  of,  432. 

mineral  composition,  128. 

properties  and  occurrence,  127. 

uses  of,  129. 

varieties  of,  128. 
Gneissic  structure,  192. 
Gold  gravels,  630. 

ores,  occurrence  of,  629. 

ores,  ore  minerals,  629. 

production  of,  631. 

prophylitic  veins,  630. 

quartz-vein  deposits,  630. 
Gorge,  defined,  280. 
Gouge,  169,  605. 
Grahamite,  analysis  of,  571. 

properties  of,  572. 
Grain  in  slate,  484. 
Granite  absorption,  459. 

absorption  of,  437. 

classification,  462. 

color  of,  459. 

Cordilleran  area,  468. 

crushing  strength,  441,  443,  459. 

cut-off,  462. 

definition,  457. 

dikes  in,  462. 

distribution  of,  464. 

elasticity  of,  459. 

elasticity,  modulus  of,  451. 

fire  resistance  of,  459. 

flexibility  of,  459. 

for  roads,  587. 

frost  resistance  of,  454. 

frost  tests  of,  455. 

gneiss,  77,  129. 

hardway  of,  462. 

inclusions  of,  464. 

knots  in,  462. 

life  of,  432. 


Granite,  Minnesota- Wisconsin  area,  468. 

permanent  swelling,  450. 

porosity  of,  438. 

properties  and  occurrence,  74. 

properties  of,  457. 

rift  of,  462. 

run  in,  462. 

sheets,  462. 

southwestern  area,  468. 

specific  gravity  of,  457. 

structure,  462. 

texture,  459. 

transverse  strength,  444,  445. 

uses  of,  464. 
Granitoid  texture,  67. 
Grano-diorite,  79. 
Granulite,  129. 
Graphic  granite,  75. 
Grappier  cement,  496. 
Gravel  for  roads,  581. 

for  road  foundations,  577. 

for  roads,  tests  of,  581. 

in  dam  foundations,  285. 
Graywacke,  106. 
Greenstone  schist,  133. 
Greisen,  598. 
Greisenization,  603. 
Grit,  106. 
Grossularite,  20. 
Groundwater,  composition  of,  337. 

fluctuations  of,  301. 

movement  of,  298,  299. 

province,  defined,  330. 

underflow  in  valleys,  299. 
Gullies,  formation  of,  277. 
Gumbo  clay,  519. 

for  roads,  580. 
Gypsite,  505. 
Gypsum,  37. 

analyses  of,  507. 

as  concretions,  199. 

calcination  of,  507. 

distribution  of,  508. 

earth,  505. 

rock,  occurrence  of,  115. 

plasters,  505,  508. 

properties  and  occurrence,  505. 

references  on,  509. 

rock,  505. 

solution  caverns  in,  230,  240. 

weathering  of,  240. 

Hade  of  fault,  169. 
Hanging  valleys,  420. 


646 


SUBJECT  INDEX 


Hanging  wall,  fault,  169. 

wall,  vein,  605. 
Harbor  improvements,  381. 
Hardness,  of  minerals,  3. 

of  road  material,  584. 

scale,  3. 
Hardpan,  331. 
Hardway  denned,  462. 
Harzburgite,  82. 

Heat  as  a  metamorphic  agent,  203. 
Heave  applied  to  faults,  173. 
Hematite,  33,  616. 

as  iron  ore,  618. 
Heulandite,  28. 
Hexagonal  system,  2. 
Hook  denned,  380. 
Hornblende,  18. 

diorite,  79. 

granite,  75. 

syenite,  78. 
Hornblendite,  84. 
Hornfels,  207,  208. 
Horn  silver,  629. 
Hornstone,  115,  208. 
Hot  springs,  304. 
Huronian,  633. 
Hurricane  fault,  177. 
Hydatogenesis,  598. 
Hydration,  weathering  effect  of,  225. 
Hydraulic  cements,  495. 

lime,  495. 

lime,  analyses  of,  496. 

limestone,  121. 

lime,  varieties  of,  497. 

limestone,  analyses  of,  496. 
Hydro-metamorphism,  204. 
Hydromica  schists,  140. 
Hydrozincite,  626. 
Hypersthene,  15. 

Ice,  erosion  work  of,  263. 

gorges,  in  rivers,  284. 
Ice  ramparts,  401. 
Idiomorphic  minerals,  67. 
Igneous  rocks,  analyses  of,  62. 

artesian  water  in,  324. 

ash  beds,  56. 

average  composition  of,  63. 

basalt,  87. 

batholiths,  56. 

boss,  55. 

chemical  composition  of,  60. 

classification  of,  70. 

complementary,  69. 


Igneous  rocks,  composition  of,  59. 

defined,  47. 

description  of,  74. 

differentiation  of,  69. 

dikes,  50. 

diorite,  78. 

essential  minerals  in,  64. 

extrusive,  56. 

felsite,  86. 

gabbro,  80. 

glassy,  89. 

granite,  74. 

intrusive,  48. 

intrusive  sheets,  51. 

joints  in,  166. 

laccoliths,  53. 

mineral  composition,  63. 

mineralizers,  65. 

obsidian,  90. 

occurrence  and  origin,  47. 

order  of  crystallization,  65. 

original  minerals,  64. 

peridotite,  82. 

phonolite,  86. 

pitchstone,  90. 

plutonic,  48k 

porphyritic,  91. 

pyroclastic,  56,  92. 

pyroxenite,  84. 

pumice,  90. 

rhyolite,  86. 

trachytes,  86. 

secondary  minerals,  64. 

sills,  51. 

stocks,  55. 

syenite,  77. 

texture  of,  66. 

tuffs,  56. 

volcanic,  56. 

volcanic  breccias,  57. 

volcanic,  described,  85. 

weathering  of,  236. 
Ilmenite,  32. 

Impsonite,  analysis  of,  571. 
Inclined  fold  defined,  154. 
Inclusions  in  granite,  464. 
Incompetent  beds,  193. 
Industrial  wastes,  disposal  of  by  wells, 

314. 

Infusorial  earth,  115. 
Inliers  defined,  197. 
Intrusive  rocks,  48. 

sheets,  51. 
lodyrite,  629. 


SUBJECT  INDEX 


647 


Iron-ore  minerals,  616. 

production,  622. 
Iron  ores,  615. 

form  of  deposits,  618. 

Lake  Superior  district,  619. 
Irrigation  canals,  sediment  in,  267. 
Isocline  defined,  151. 
Isometric  system,  2. 
Itacolumite,  135. 

Jasper,  30,  115. 
Jaspilite,  115. 
Joints,  164. 

blind,  191. 

cause  of,  194. 

classification  of,  166. 

columnar,  167. 

compression,  166,  194. 

denned,  164. 

dip  of,  166. 

effect  on  quarrying,  167. 

effect  on  slides,  167. 

effect  on  water  supply,  167. 

horizontal,  166. 

in  building  stone,  430. 

in  igneous  rocks,  166. 

in  sedimentary  rocks,  166. 

in  slate  quarries,  484. 

major,  166. 

minor,  166. 

position  of,  166. 

relation  to  engineering  work,  167. 

relation  to  ore-deposits,  168. 

relation  to  reservoirs,  167. 

strike  of,  166. 

systems,  164. 

tension,  194. 
Jolly  balance,  6. 
Jurassic,  632. 
Juvenile  water,  594. 

Kames,  422. 
Kaolin,  515,  519. 
Kaolinite,  24. 
Kaolinization,  11. 
Katamorphic  zone,  204. 
Kersantite,  79. 
Keweenawan,  633. 
Kilns  for  brick,  523. 
Kirunavaara,  Swe.,  591. 
Knots  in  granite,  462. 

Labradorite,  8. 
Laccoliths,  53. 


Lafarge  cement,  496. 
Lake  currents,  403. 

filling,  by  deltas,  410,  411. 

level,  relation  to  water  table,  397. 

levels,  variations  in,  403. 

outlet,  cutting  down  of,  410. 
Lakes,  areas  of  different  ones,  397. 

beaches  across  inlets,  397. 

body  current,  403. 

composition  of  water,  408. 

crater,  401. 

crustal  movement,  399. 

currents  in,  403. 

definition,  396. 

drift-dam,  400. 

due  to  accident,  399. 

evaporation  of,  408. 

extinct,  413. 

filling  of,  410. 

glacial-dam,  401. 

glacial,  marginal,  417. 

ice  action  on,  401. 

landslide,  348,  400. 

lava-dam,  400. 

normal  development  type,  397. 

obliteration  of,  408. 

original  consequent,  396. 

overturning,  405. 

oxbow,  259,  397. 

periodic,  410. 

playas,  410. 

references  on,  413. 

relation  to  engineering  work,  396. 

relation  to  water  table,  412. 

seiches  in,  404. 

sink-hole,  399. 

stagnation,  405. 

temperature  of,  404. 

types  of,  396. 

waves  on,  401. 

working  of,  405. 

waters  of,  401. 
Lamination  defined,  96. 
Landslide  lakes,  400. 
Landslides  along  supping  planes,  349. 

Canada,  347. 

Cascade  Mts.,  Wash.,  348. 

classification  of,  342. 

clay,  344. 

creep,  343. 

definition,  342. 

effect  on  tunnels,  343. 

Frank,  Alberta,  350. 

Hudson  River  Valley,  348. 


648 


SUBJECT  INDEX 


Landslides,  references  on,  356. 

relation  to  engineering,  353. 

rock  falls,  350. 

rockstreams,  349. 

slope  angles  to  adopt,  356. 

slopes  to  minimize  sloughing,  355. 
Lapilli,  92. 
Lava-dam  lakes,  400. 
Lava,  flows,  56,  57. 

for  building  stone,  59. 
Law  of  rock  resistance,  234. 
Lead  ores,  classification  of,  627. 

mode  of  occurrence,  628. 
Lead  production,  629. 
Lepidolite,  12,  13. 
Lepidomelane,  12. 
Levees,  formation  of,  263. 
Lherzolite,  84. 
Lignite,  analyses  of,  535. 

properties  of,  529. 
Lime,  distribution  of  raw  materials,  504. 

hydraulic,  495. 

in  clay,  514. 

properties  of,  494. 

references  on,  509. 

selenitic,  497. 
Limes,  493. 
Limestone,  absorption  of,  437,  476. 

analyses  of,  493. 

artesian  water  in,  319. 

burning  of,  494. 

color,  474. 

chemical  composition  of,  476. 

contact  metamorphosed,  207. 

crushing  strength  of,  441,  442,  443,  476, 
477. 

crystalline,  properties  of,  140. 

distribution  in  United  States,  478. 

durability  of,  474. 

elasticity,  modulus  of,  451. 

fire  resistance  of,  476. 

frost  resistance  of,  454. 

frost  tests  of,  455. 

hardness  of,  474. 

life  of,  432. 

mineral  impurities,  474. 

permanent  swelling  of,  450. 

porosity  of,  438,  477. 

properties  and  occurrence  of,  116. 

sink  holes  in,  309. 

solubility  of,  118. 

specific  gravity  of,  477. 

structural  features  of,  473. 

texture  of,  474. 


Limestone,  transverse  strength  of,  444, 
445,  447. 

variation  in  composition,  118. 

varieties,  119. 

weathering  of,  239,  474. 
Limonite,  34,  616. 

as  iron  ore,  621. 
Lithographic  limestone,  121. 
Lode,  605. 
Loess,  519. 

properties  and  occurrence,  109. 
Luster,  in  minerals,  4. 

Mack's  cement,  508. 
Magmatic  differentiation,  70. 

water,  295. 

Magnesia,  in  clay,  514. 
Magnetism,  of  minerals,  7. 
Magnetite,  32,  616,  618. 

gneiss,  131. 

in  building  stone,  32. 
Major  joints,  166. 
Malachite,  622. 
Maltha,  analysis  of,  571. 

properties  of,  573. 
Mammoth  coal  seam,  538. 
Manjak,  573. 
Map,,  geologic,  209. 

geologic,  construction  of,  211. 
Maps,  topographic,  209. 
Marble,  absorption  of,  "437,  480,  482. 

amphibole  in,  19. 

as  building  stone,  478. 

color  of,  478. 

composition  of,  140. 

crushing  strength  of,  441,  480. 

distribution  of,  481. 

durability  of,  480. 

frost  tests  of,  455. 

general  properties  of,  141. 

life  of,  432. 

mica  in,  433. 

mineral  composition  of,  480. 

permanent  swelling  of,  450. 

porosity  of,  438. 

properties  of,  140. 

sonorousness  of,  482. 

specific  gravity  of,  480. 

structure  of,  478. 

texture  of,  478. 

transverse  strength  of,  445,  480. 

tremolite  in,  435. 

uses  of,  482. 
Marcasite,  42. 


SUBJECT  INDEX 


649 


Marcasite,  in  building  stone,  435. 

in  slate,  486. 
Margarite,  31. 
Marl,  119. 

used  for  Portland  cement,  505. 
Meanders,  257. 
Measurement  of  depth  of  beds,  163. 

of  folds,  158. 

of  streams,  units  of,  252. 
Melaconite,  622. 
Melanite,  20. 
Mesodevonian,  633. 
Mesozoic,  129,  632. 

Metamorphic    rocks,    agents    of    meta- 
morphism,  122. 

artesian  water  in,  324. 

chemical  composition,  124. 

classification  of,  126. 

criteria  of  origin,  125. 

crystalline  schists,  132. 

defined,  47. 

gneiss,  127. 

limestones,  crystalline,  140. 

marbles,  140. 

mineral  composition,  124. 

ophicalcite,  142. 

phyllite,  137. 

quartzite,  135. 

serpentine,  142. 

slate,  137. 

soapstone,  142. 

structure,  124. 

texture,  124. 

varieties  of  structure,  125. 
Metamorphism,  agents  of,  203. 

defined,  199,  200. 

kinds  of,  204. 

regional,  208. 

see  Cleavage. 

zones  of,  204. 
Metasomatism,  596. 
Meteoric  water,  absorption  of,  297. 

water  defined,  295. 

water,  disposal  of,  296. 

water,  evaporation  of,  297. 
Miarolitic,  69. 
Micaceous  sandstone,  106. 
Mica  diorite,  79. 

group,  12. 

effect  on  building  stone,  14. 

in  building  stones,  433. 

in  marbles,  480. 

peridotite,  84. 
Mineral,  definition  of,  1. 


Mineralizers  defined,  65. 

Minerals,  coefficients  of  expansion,  218. 

descriptions  of,  7. 

injurious,  433. 

properties  of,  3. 
Miner's  inch  defined,  252. 
Minette,  78. 

Mining,  relation  to  folds,  157. 
Minor  joints,  166. 
Miocene,  632. 
Mississippian,  633. 
Mohawkian,  633. 
Monocline  defined,  149. 

faulted,  179. 
Monoclinic  system,  2. 
Monzonite,  77,  79. 
Moraine,  ground,  421. 

lateral,  421. 

medial,  421. 

terminal,  421. 
Moraines,  glacial,  420. 
Multiple  fault,  169. 
Muscovite,  12. 

granite,  75. 

Narrows  in  river  valleys,  278. 
Natrolite,  28. 
Natural  cements,  497. 

gas,  analyses  of,  563. 

gas,  distribution  in  United  States,  570 

gas,  occurrence  of,  564. 

gas,  origin  of,  566. 

gas,  properties  of,  563. 

gas,  references  on,  575. 

gas,  sands,  structure  of,  566. 

gas,  waste  of,  564. 

gas,  well  pressure,  565. 
Neodevonian,  633. 
Nephelite,  12. 

syenite,  78. 
Net  shift  defined,  172. 

slip  defined,  171. 
Niagaran,  633. 
Norite,  80,  81. 
Normal  fault,  173,  174. 
Novaculite,  115. 

Oblique  fault  denned,  171. 

slip  faults,  174. 
Obsidian,  90. 
Odor  of  minerals,  7. 
Offset  defined,  174. 
Oil  pool,  565. 

sand,  565. 


650 


SUBJECT  INDEX 


Oil,  see  Petroleum. 
Oligoclase,  8. 
Olivine,  21. 

gabbro,  80. 
Onyx,  30. 

marble,  121,  482. 
Oolitic  limestone,  121. 
Open  fault,  168. 

folds,  154. 
Ophicalcite,  27,  483. 

properties  and  occurrence,  143. 
Ophiolite,  27,  483. 
Ordovician,  633. 
Ore  bodies,  change  with  depth,  611. 

definition,  590. 

forms  of,  603. 
Ore  deposition,   physical  conditions  of, 

598. 
Ore  deposits,  alunitic  alteration,  603. 

Appalachian  province,  614. 

at  surface,  602. 

bonanzas,  605. 

Coastal  Plain,  614. 

contact  metamorphic,  600. 

contemporaneous,  591. 

Cordilleran  region,  615. 

deep  seated,  601. 

definition,  590. 

distribution  in  United  States,  612. 

epigenetic,  592. 

formation  by  magmatic  water,  594. 

formed  at  shallow  depths,  601. 

gangue  minerals,  590. 

gouge,  605. 

Great  Plains,  615. 

greisenization,  603. 

hydrothermal  alterations,  602. 

iron,  615. 

lead,  626. 

magmatic  segregation,  591. 

metasomatism,  596. 

mode  of  concentration,  593. 

outcrop  of,  612. 

ore  minerals,  590. 

origin  of,  591. 

Piedmont  Plateau,  614. 

pneumatolytic,  598. 

Prairie  Plains,  614. 
Ore  minerals,  589. 

shoots,  606,  607. 
Ores,  deposition  in  cavities,  595. 

precipitation  from  solution,  595. 

primary,  607. 

propylitization,  602. 


Ores,  references  on,  631. 

relation  to  faults,  184. 

relation  to  folds,  157. 

relation  to  joints,  168. 

replacement,  596. 

role  of  meteoric  water,  594. 

secondary,  607. 

secondary  sulphide  zone,  610. 

sericitization,  603. 

shoots,  606. 

silicification,  603. 

source  of  water  forming,  594. 

subsequent,  592. 

syngenetic,  591. 

weathering  of,  607. 

zinc,  626. 

zone  of  primary  sulphides,  612. 

zone  of  weathering,  608. 

zones  in,  607. 

Organisms,  relation  to  weathering,  221. 
Original  minerals,  64. 
Orthoclase,  8. 
Orthorhombic  system,  2. 
Oswegan,  633. 
Ottrelite  schist,  133. 
Outcrop,  forms  of,  determination,  188. 
Outcrops,  affected  by  faulting,  177. 

shifting  by  faulting,  177. 
Outliers  defined,  197. 
Outwash  plain,  422. 
Overlap  defined,  196. 
Overthrust  fault,  175. 
Overturned  f o  d  defined,  154. 
Ox-bow  defined,  259. 

lakes,  259,  397. 

Oxidation  as  a  weathering  process,  227. 
Ozokerite,  analysis  of,  571. 

Paleodevonian,  633. 
Palaeozoic,  633. 
Paragonite  schist,  133. 
Parallel  folds  defined,  151. 
Paving  blocks,  588. 

-brick  clay,  519,  521. 
Peachbottom  slate,  488. 
Peat,  analyses  of,  529,  535. 

deposits,  nature  of,  527. 

properties  of,  529. 
Pegmatite,  75 

uses  of,  75. 
Peneplain,  257. 

defined,  278. 
Pennsylvanian,  633. 
Perched  water  table,  303  313. 


SUBJECT  INDEX 


651 


Peridotite,  properties  and  occurrence,  82. 

Perknite,  85. 

Perlite,  90. 

Permanent  streams.  248. 

swelling  of  stone,  450. 
Permian,  633. 
Perpendicular  slip,  172. 

throw,  173. 

Petrographic  province,  70. 
Petroleum,  Appalachian  field,  567. 

base  of,  562. 

California  field,  569. 

distribution  in  United  States,  567. 

Gulf  field,  569. 

Illinois  f  eld,  569. 

Mid-Continental  field,  569. 

nitrogen  in,  562. 

occurrence  of,  564. 

Ohio-Indiana  field,  569. 

origin  of,  566. 

pool,  565. 

properties  of,  562. 

references  on,  575. 

sand,  565. 

sands,  structure  of,  566 

sulphur  in,  562. 

well  pressure,  565. 

wells,  yield  of,  566. 
Phenocrysts,  6,  67. 
Phlogopite,  12. 
Phonolite,  86. 
Phosphate  rock,  121. 
Phosphorite,  121. 
Phyllite,  definition  of,  137. 

properties  and  occurrence  of,  140. 
Piracy  river,  278. 
Pisolite,  121. 

Pitch  of  folds  defined,  148. 
Pitchstone,  90. 
Placer  deposits,  592. 
Plagioclasite,  81. 

Plants,  relation  to  weathering,  221. 
Plaster  of  Paris,  38. 
Plasters,  sec  Gypsum. 
Plasticity  of  clay,  511. 
Playas,  410. 
Pleistocene,  632. 
Pliocene,  632. 
Plucking  by  glaciers,  420. 
Plutonic  rocks,  48. 
Pneumatolysis,  598. 
Pocahontas  steam  coal,  Va.,  557. 
Poikilitic  texture,  84. 
Pollution  by  drainage  wells,  314. 


Pollution  of  rivers,  291. 
Ponds,  drainage  of,  by  wells,  313. 
Porphyritic  texture,  67. 
Porphyry,  porosity  of,  438. 
Porosity  formula,  438. 

of  building  stone,  438. 
Portland  cement,  497. 

calculation  of  mixture,  500. 

economic  considerations,  501. 

limestone  used  in,  499. 

magnesia  in,  499. 

raw  materials,  498. 

silica  in,  499. 

sulphur  in,  499. 
Posts  in  slate  defined,  484. 
Potholes  in  rivers,  264. 
Potsdam  formation,  633. 
Pudding  stone,  103. 
Puddle,  clay  used  for,  525. 
Pugmill,  521. 
Pumice,  90. 
Pumiceous  texture,  69. 
Puzzolan  cements,  501. 
Pressed-brick  clay,  519. 
Pressure  eddy,  262. 
Propylitization,  602. 
Proterozoic,  633. 
Proustite,  629. 
Pseudophenocrysts,  125. 
Pseudoporphyritic,  125. 
Pyrargyrite,  629. 
Pyrite,  41,  616. 

as  concretions,  199. 

in  building  stone,  435. 

in  limestones,  476. 

in  marbles,  480. 

in  slate,  486. 
Pyroclastic  rocks,  56. 
Pyromorphite,  627. 
Pyrope,  20. 
Pyroxene  in  building  stone,  17. 

group,  15. 
Pyroxenite,  84. 
Pyrrhotite,  42. 

hi  building  stone,  435. 

Quaquaversal  defined,  150. 
Quarrying,  relation  to  glaciation,  426. 

relation  to  folds,  157. 

relation  to  joints,  167. 
Quarry  water,  432. 
Quartz,  29. 

varieties,  30. 

diorite,  78. 


652 


SUBJECT  INDEX 


Quartzite,  definition  of;  135. 

for  roads,  587. 

properties  and  occurrence,  135. 

schist,  135. 

uses  of,  137. 

varieties  of,  135. 

see  also  Sandstone. 
Quartz  monzonite,  79. 
Quaternary,  632. 

Railroad  ballast  of  burned  clay,  524. 
Railway  cuts,  landslides  in,  353. 

embankments,   effect  of  underground 

water  on,  312. 
Rainfall,  in  United  States,  295. 

relation  to  run-off,  245. 

see  Meteoric  water. 
Rapids  in  rivers,  264. 
Recent,  632. 

Recumbent  fold  defined,  154. 
Reeds  in  sandstone,  109. 
References  on  rocks,  146. 
Regional  metamorphism,  204,  208. 
Regolith,  241. 

defined,  298. 
Replacement,  596. 
Repressing  brick,  522. 
Reservoir  failures,  307,  309. 

foundations,  307. 
Reservoirs,  relation  to  joints,  167. 
Residual  clay,  defined,  232. 

ore  deposits,  606. 
Reverse  fault,  174. 
Rhyolite,  86. 

as  building  stone,  469. 
Ribbons  in  slate,  139. 
Ridge  road,  413. 
Rift  defined,  462. 
River  basin  defined,  250. 

crossings,  259. 

measurement,  units  of,  252. 
Rivers,  244. 

alluvial  plains,  269. 

amount  of  sediment  carried  by,  266. 

analyses  of  waters,  290. 

bars  at  mouth  of,  277. 

bends  in,  259. 

buried  channels,  280,  424. 

canyons,  280. 

corrosion  by,  254. 

cross-section,  268. 

cross-section,  change  of,  268. 

currents,  261. 

dam  foundations,  284. 


Rivers,  deltas,  271. 
deposition  by,  269. 
dissolved  material,  230. 
drainage  forms,  277. 
eddies,  261. 
erosion  of  banks,  262. 
erosion  by,  253. 
floods  of,  282. 
flood-plain  terraces,  275. 
gorge,  280. 
ice  erosion  on,  263. 
ice  gorges,  284. 
levees,  263. 

meandering,  characters  of,  257. 
measurement  of,  252. 
outlets  of,  277. 
peneplain,  278. 
permanent,  248. 
piracy,  278. 
potholes,  264. 
rapids  in,  264. 
references  on,  294. 
relation  to  bars,  385. 
scour,  259. 

scour  and  section,  268. 
size  of  particles  carried  by,  267. 
shoals,  259. 
slope  of,  268. 

slope  related  to  section,  268. 
temporary,  248. 
terraces,  275. 

tidal,  improvement  of,  388. 
transportation  work  of,  264. 
tributaries,  effect  on  slope,  269. 
valleys,  280. 
velocity  of,  266. 
water,  carbonates  in,  292. 
water,  chlorine  in,  291. 
water,  composition,  286. 
water,  composition,  relation  to  country 

rock,  288. 
water  falls,  264. 
work  performed  by,  253. 
water,  salt  in.  291. 
water,  sulphuric  acid  in,  292. 
Road    construction,    relation   to   glacial 

streams,  418. 
foundations,  across  valleys,  578. 

drainage  of,  579. 

effect  of  rock  structure,  578. 

kind  of  rock,  577. 

slope  of  cuts,  578. 

material,  broken  stone,  582. 

burned  clay,  525. 


SUBJECT  INDEX 


653 


Road  construction  material,  chert,  582. 

clay,  580. 

conglomerate,  587. 

crushed  stone,  properties  of,  583. 

crushed  stone,  significance  of  tests, 
585. 

gabbro,  587. 

granite,  587. 

gravel,  580. 

gravel,  requirements  of,  581. 

limestone,  588. 

paving  blocks,  588. 

quartzite,  587. 

rocks  used  for,  579. 

schist,  587. 

shale,  588. 

slate,  587. 

economic  considerations,  588. 

tests  of,  586. 

trap  rock,  587. 

volcanic  rocks,  587. 
Rock  cleavage,  190. 
definition  of,  46. 

-forming  minerals,  descriptions  of,  7. 
gypsum,  38. 

structure,  relation  to  roads,  578. 
Rocks,  as  source  of  metals,  593. 
bituminous,  573. 
coefficients  of  expansion  of,  218. 
contraction  of,  216. 
expansion  of,  216. 
for  road  foundations,  577. 
hydrated,  225. 

igneous,  as  building  stone,  457. 
igneous,  defined,  47. 
igneous,  weathering  of,  236. 
importance  to  engineer,  46. 
metamorphic  defined,  22. 
metamorphism  of,  147,  199. 
need  of  correct  identification,  46. 
occurrence,  etc.,  46. 
porosity  of,  316. 

relation  to  dam  foundations,  285. 
sedimentary,  defined,  47. 
sedimentary,    weathering    resistance, 

238. 

see  Igneous. 
see  Sedimentary. 
structural  features,  147. 
trap,  for  roads,  587. 
varieties  of,  47. 
water  capacity  of,  315. 
weathering  of,  215. 
Rock  streams,  349. 


Roof  structure  in  folds,  150. 
Rosiwal  method,  456. 
Rotatory  fault,  177. 
Run  defined,  462. 
Run-off  defined,  244. 

factors  controlling,  244,  297. 

of  different  watersheds,  246. 

relation  to  rainfall,  245. 

St.  Croix  sandstone,  333. 
St.  Peter  sandstone,  333. 
Salinity,  definition  of,  288. 
Salt  in  river  water,  291. 

occurrence  of,  126. 

relation  to  chlorine  in  well  water,  114. 
Sand,  artesian  water  in.  318. 

in  dam  foundations,  285. 

mineral  composition  of,  111. 

residual,  232. 

wind-blown,   weathering  effects,   221. 
Sand  dunes,  formation  and  occurrence, 
111. 

distribution  of,  113. 

fixing  of,  113. 

lakes  in,  397. 
Sandstone,  444. 

absorption  of,  437,  471,  472. 

artesian  water  in,  318. 

cement  of,  371. 

color  of,  471. 

contact  metamorphosed,  208. 

crushing  strength  of,  441,  442,  443, 471, 
472. 

distribution  of,  473. 

durability  of,  472. 

elasticity,  modulus  of,  451. 

fire  resistance  of,  472. 

frost  resistance  of,  454. 

frost  tests  of,  455. 

geologic  distribution  of,  473. 

hardness  of,  471. 

permanent  swelling,  450. 

porosity  of,  438. 

properties  of,  470. 

properties  and  occurrence  of,  103. 

reedy,  109. 

relation  to  dam  foundations,  105. 

specific  gravity  of,  472. 

structural  features  of,  470. 

texture  of,  470. 

transverse  strength  of,  446,  472. 

uses  of,  105. 

varieties  of,  106. 

weathering  of,  238. 


654 


SUBJECT  INDEX 


Sandstone  deposits,  variation  in,  108. 

Satin  spar,  38. 

Saturation  coefficient,  437. 

Saxonite,  82. 

Schist,  for  roads,  587. 

uses,  134. 

Schistose  structure,  125. 
Schistosity,  191. 
Schists,  132. 

varieties  of,  132. 
Scoriaceous  texture,  69. 
Scott's  cement,  497. 
Scour  in  rivers,  259. 
Scove  kiln,  523. 
Sculping  slate,  485. 
Secondary  minerals,  64. 
Second-foot  denned,  252. 
Section,  geologic,  construction  of,  211. 
Sedentary  soils,  241. 
Sediment  transported    by    rivers,    264, 

266. 
Sedimentary  rocks,  anhydrite,  114. 

artesian  water  in,  318. 

bog  lime,  119. 

breccias,  99. 

carbonaceous  rocks,  121. 

cementation,  95. 

cement,  color  of,  96. 

cement,  durability  of,  96. 

cement,  quantity  of,  96. 

chalk,  119. 

chemically  formed,  113. 

classification  of,  99. 

composition  of,  97. 

conglomerate,  102. 

defined,  47. 

diatomaceous  earth,  115. 

dolomite,  119. 

faults  in,  171. 

flint,  115. 

general  properties  of,  94. 

geyserite,  115. 

gypsum,  113. 

introduction,  93. 

iron  ores,  116. 

jasper,  115. 

joints  in,  166. 

limestone,  116,  118. 

loess,  109. 

mechanically  formed,  99. 

phosphate  rock,  121. 

repeated  by  faults,  178. 

salt,  114. 

sand  dunes,  111. 


Sedimentary  rocks,  sandstone,  103. 

shale,  106. 

siliceous  deposits,  114. 

size  of  particles,  94. 

structure,  96. 

texture,  94. 

weathering  of,  238. 

wind  deposits,  109. 
Seepage  springs,  304. 
Seiches  defined,  404. 
Selenite,  38. 
Selenitic  lime,  497. 
Selvage,  605. 
Semianthracite,  532. 

coal,  analyses  of,  535. 
Semibituminous  coal,  532. 

analyses  of,  535. 
Septarium,  199. 
Sericite,  13. 

schist,  133. 
Sericitization,  603. 
Serpentine,  as  building  stone,  482. 

mineral,  26. 

life  of,  433. 

porosity  of,  438. 

(rock)  properties  and  occurrence,  144. 
Sewer-pipe,  clays  used,  524. 

manufacture  of,  524. 

clay,  519. 
Shale,  commercial  value,  108. 

defined,  517. 

deposits,  variation  in,  108. 

for  roads,  588. 

properties  and  occurrence,  106. 

relation  to  tunnels,  108. 

contact  metamorphosed,  207. 

slides  of,  348. 

varieties  of,  108. 

weathering  of,  239. 
Shear  zone,  169. 
Sheets  defined,  462. 

joint,  166. 
Shell  marl,  119. 

limestone,  121. 
Shift  applied  to  faults,  172. 
Shoals  in  rivers,  259. 
Shore  currents,  363,  394. 

currents,  transportation  by,  368. 

drift,  368. 
Siderite,  616. 

as  concretions,  199. 

as  iron  ore,  621. 

in  slate,  486. 
Silicates,  minerals,  7. 


SUBJECT  INDEX 


655 


Siliceous  deposits,  formation  and  occur- 
rence, 114. 

sinter,  115. 
Silicification,  603. 
Sill,  51. 

Silurian,  118,  633. 
Silver  ores,  occurrence  of,  629. 

ores,  ore  minerals,  629. 
Silver,  production  of,  631. 
Similar  folds  defined,  151. 
Sink  holes,  in  limestone,  309. 

-hole  lakes,  399. 
Slag  cement,  501. 
Slate,  abrasive  resistance  of,  486. 

absorption  of,  437. 

classification  of,  488. 

cleavability  of,  483,  484. 

color  of,  486. 

composition  of,  138. 

contact  metamorphosed,  207. 

corrodibility  of,  486. 

cross  fracture,  485. 

definition  of,  137. 

distribution  of,  488. 

elasticity  of,  486. 

false  cleavage  of,  483. 

for  roads,  587. 

grain  of,  484. 

in  coal,  538. 

joints  in,  484. 

mineral  impurities,  486. 

porosity  of,  438. 

posts  defined,  484. 

properties  and  occurrence  of,  139. 

pyrite  in,  486. 

quarrying,  488. 

ribbons,  483. 

slip  cleavage,  483. 

sonorousness  of,  484. 

strength  of,  486. 

structural  features  of,  483. 

tests  of,  487. 

toughness  of,  486. 

varieties  of,  138. 

weathering  of,  239. 

waste,  uses  of,  488. 
Slatiness,  191. 
Slaty  cleavage,  191. 

structure,  125. 
Slickensides,  169. 
Slides,  see  Landslides. 
Slip,  as  applied  to  faults,  171. 

cleavage,  191. 
Smithsonite,  626. 


Snow  field,  formation  of,  414. 
Snow  line  defined,  414. 
Soapstone,  25. 

properties  and  occurrence,  145. 

uses  of,  146. 
Sodalite,  12. 
Soft-mud  process,  521. 
Soil  areas,  242. 

province,  243. 

series,  243. 

type,  243. 
Soils,  classification  of,  241. 

composition  of,  242. 

definition  of,  241. 

formation  of,  241. 

Solution  as  a  weathering  process,  230. 
Solutions  as  metamorphic  agents,  203. 
Specific  gravity  of  clay,  513. 

of  granite,  457. 

of  limestone,  477. 

of  marble,  480. 

of  minerals,  6. 
Specular  hematite,  33. 
Sphalerite,  43,  626. 
Spit  defined,  372. 
Splint  coal,  557. 
Spring  defined,  303. 
Springs,  artesian,  304. 

fissure,  306. 

gravity,  304. 

hot,  304. 

in  glacial  drift,  305. 

in  limestone,  305. 

pollution  of,  305. 

source  of  water,  303. 

tubular,  305. 

value  for  water  supply,  306. 
Stagnation  in  lakes,  405. 
Stalactites,  121. 
Stalagmites,  121. 
Static  metamorphism,  204. 
Staurolite,  23. 
Steatite,  25. 
Step  fold,  180. 
Stephanite,  629. 
Stiff-mud  process,  521. 
Stilbite,  28. 

Stock,  ore  deposit,  606. 
Stocks  of  igneous  rock,  55. 
Strain-slip  cleavage,  191. 
Stratification,  hi  building  stone,  431. 

defined,  96. 

Stratified  rocks,  faults  in,  171. 
Stratigraphic  throw,  173. 


656 


SUBJECT  INDEX 


Stratum  defined,  97. 
Streak  of  minerals,  4. 
Stream  flow,  244. 

formation,  248. 

measurement,  252. 
Strike  defined,  148. 

fault,  169. 

variations  in,  148. 

fault,  defined,  171. 

joints,  166. 
Strike-shift,  173. 
Strike-slip,  defined,  172. 

fault,  174. 

Structural  features  of  rocks,  147. 
Subbituminous  coal,  analyses  of,  535. 

difference  from  other  coals,  530. 

properties  of,  530. 
Suction  eddy,  261,  262. 
Sulphuric  acid  waters,  292. 
Surface  waters,  244. 
Surveys,  geological,  addresses  of,  634. 
Swamps  drained  by  wells,  313. 

relation  to  railroad  work,  411. 

sinking  of  tracks  in,  411. 
Syenite,  as  building  stone,  469. 

porosity  of,  438. 

properties  and  occurrence,  77. 
Syenite»gneiss,  78-129. 
Symmetrical  fold  defined,  154. 
Syncline,  faulted,  180. 

defined,  149. 
Synclinorium  defined,  151. 

Talc,  25. 

Talus  breccia,  99. 

Taste  of  minerals,  7. 

Temperature  changes,  relation  to  weath- 
ering, 216. 

Temporary  streams,  248. 

Tennantite,  622. 

Tension  joints,  194. 

Terrace,  wave  cut,  366. 

Tertiary,  632. 

Tetragonal  system,  2. 

Tetrahedrite,  622,  629. 

Texture  of  building  stone,  431. 
of  igneous  rocks,  66. 
sizes  of  grain,  67. 

Thermo-metamorphism,  204. 

Thickness  of  beds,  measurement  of,  158. 

Thorofare,  380. 

Throw  defined,  173. 

Tides,  effect  of,  on  wells,  301. 
on  water  table,  301. 


Till  defined,  421. 

Tilted -folds,  154. 

Titanium  in  iron  ore,  616. 

Tonalite,  79. 

Topography,  affected  by  faulting,  177. 

mature,  280. 

shore,  due  to  waves,  368. 

young  and  old,  279. 
Top-set  beds,  274. 
Touch  of  minerals,  7. 
Toughness  of  road  material,  584. 
Tourmaline,  23. 
Trace-slip  defined,  172. 
Trachyte,  86. 

as  building  stone,  469. 

porosity  of,  438. 
Translatory  fault,  177. 
Transported  soils,  242. 
Transverse    strength  of  building  stone, 
443. 

formula  for,  444. 
Trap  rock  for  road  material,  587. 
Travertine,  121. 
Tremolite,  18. 

in  building  stone,  435. 

in  marbles,  480. 
Trenton  epoch,  118. 
Triassic,  632. 
Triclinic  system,  2. 
Trinidad  lake  asphalt,  573. 
Tripolite,  115. 
Troctolite,  81. 
Tubular  springs,  305. 
Tufa,  calcareous,  defined,  121. 

porosity  of,  438. 

See  Calcareous  tufa. 
Tuffs,  56,  92. 

absorption  of,  437. 

for  building  stone,  59,  469. 

for  dam  foundations,  59. 
Tunneling,  effect  of  faults  on,  181. 

effect  of  underground  water  on,  312. 

relation  to  folding,  155. 

relation  to  glacial  deposits,  424. 
Tunnels,  affected  by  landslides,  343. 

relation  to,  anhydrite,  114. 
Twinning  axis,  3. 

in  crystals,  2. 

plane,  3. 

Uintaite,  properties  of,  572. 
Unakite,  75. 

Unconformity  defined,  195. 
Underflow,  299. 


SUBJECT  INDEX 


657 


Underflow,  factors  influencing,  300. 
Undertow,  363. 
Upper  Huronian,  633. 
Upright  fold  defined,  154. 
Uralite,  19. 
Uralitization,  81. 

Underground  water,  Applachian  Moun- 
tain Province,  332. 

artesian,  conditions  of  flow,  317. 

artesian  water,  314. 

Atlantic  Coastal  Plain  province,  331. 

chlorides  in,  339. 

color  of,  340. 

composition  of,  337. 

connate  water,  295. 

corrosion  by,  338. 

effect  on  foundations,  312. 

effect  of  mineral  ingredients,  338. 

glacial  drift  province,  331. 

Great  Basin  Province,  335. 

groundwater,  298. 

groundwater,  amount  in  rocks,  315. 

hardness  of,  338. 

High  Plains  Province,  334. 

magmatic  water,  295. 

meteoric,  295,  296. 

miscellaneous  effects  of,  306. 

Mississippi-Great  Lakes  Basin,  333. 

movement,  rate  of,  317. 

nitrates  in,  339. 

nitrites  in,  339. 

Pacific  Provinces,  336. 

Piedmont  Plateau  Province,  332. 

potable  water,  339. 

Provinces  of  United  States,  330. 

rainfall,  295. 

references  on,  340. 

relation  of  rock  material  and  dissolved 
matter,  337. 

relation  to  dam  foundations,  307. 

relation  to  embankments,  312. 

relation  to  reservoir  foundations,  307. 

relation  to  tunneling,  312. 

rocks,  capacity  of,  315. 

Rocky  Mountain  Province,  335. 

springs,  303. 

suspended  matter  in,  339. 

underflow,  299. 

water  table,  298. 

Weathered  Rock  Province,  331. 

wells,  drainage  by,  312. 

Valleys,  formation  by  rivers,  277. 
glacial,  section  of,  420. 
hanging,  420. 


Valleys,  pre-Glacial,  282. 

relation  to  roads,  578. 

river,  section  of,  420. 

young,  279. 
Valley  train,  422. 
Vanadium  in  clay,  514. 
Vein,  apex,  605. 

bedded,  605. 

bitumens,  571. 

deposits  of  copper,  624. 

material,  603. 
Veins,  603. 

conjugate,  605. 

footwall,  605. 

hanging  wall,  605. 
Vein  system,  605. 
Verde  antique,  143. 
Vertical  faults,  175. 

shift,  173. 

Vesicular  texture,  69. 
Vitrification  of  clay,  511. 
Vitrophyre,  90,  91. 
Volcanic  ash,  92. 

blocks,  92. 

breccia,  57,  102. 

glasses,  properties  and  occurrence,  89. 

igneous  rocks,  described,  85. 

rocks,  56. 

rocks  as  building  stone,  469. 

rocks  for  roads,  587. 

tuffs,  92. 

Water,  analyses,  method  of  stating,  288. 
as  agent  in  ore  formation,  594. 
available,  in  rocks,  315. 
boiler  scale,  338. 
combined,  in  rocks,  315. 
effect  of  when  frozen,  218. 
free,  in  rocks,  315. 
hardness  of,  338. 
hot,  effect  on  rocks,  602. 
juvenile,  594. 
lake,  composition  of,  408. 
magmatic,  in  metamorphism,  203. 
magmatic,  in  ore  formation,  594. 
measurement,  for  duty,  250. 
measurement,  for  supply,  250. 
meteoric,  depth  of  circulation,  595. 
meteoric,  hi  ore  formation,  594. 
falls,  264. 

falls,  interference  with  navigation,  264. 
falls,  relation  to  engineering  work,  264. 
power,  relation  to  glaciation,  426. 
river,  analyses  of,  290. 
river,  composition  of,  286. 


658 


SUBJECT   INDEX 


Water,  river,  need  of  analysis,  286. 

river,  relation  of  composition  to  rock, 
288. 

river,    source   of    mineral   matter  in, 
286. 

river,  statement  of  analyses,  286. 

r61e  in  metamorphism,  203. 
Watershed  denned,  245. 
Water  table,  affected  by  pumping,  302. 

defined,  298. 

fluctuation  of,  300. 
artificial  causes,  302. 
natural  causes,  301. 

lowering  of,  effect  on  lakes,  412. 

perched,  303,  313. 

relation  to  barometric  pressure,  301. 

relation  to  reservoir  level,  302. 

relation  to  surface,  298. 

relation  to  temperature,  302. 

relation  to  tides,  301. 
Water,   underground,    see    Underground 

water. 

Wave  action,  along  United  States  coast, 
393. 

beaches,  368. 

engineering  problems,  381. 

hooks,  380. 

on  lakes,  401. 

spits,  372. 

vertical  range,  365. 
Wave  motion,  theory  of,  359. 
Wave  phase,  360. 
Waves,  barrier  beach  by,  370. 

beach  formed  by,  368. 

cause  of,  359. 

cliffs  cut  by,  366. 

coast  line  due  to,  366. 

depth  of,  359. 

erosion  by,  363. 

force  of,  362,  364. 

of  oscillation,  362. 

of  translation,  362. 

recession  of  coast  due  to,  365. 

references  on,  394. 

relation  to  engineering,  358. 

sea  cliff,  366. 

shore  currents,  363. 

shore  drift,  390. 

storms,  362. 

terrace  cut  by,  366. 

topography  due  to,  366. 

undertow,  363. 
Weathering,  215. 

abrasive  action,  219. 

belt  of,  204. 


Weathering,  carbonation,  228. 

chemical  agents,  223. 

defined,  216. 

dehydration,  225. 

deoxidation,  228. 

depth  of,  234. 

desilication,  228. 

frost  action,  218. 

hydration,  225. 

importance  of,  215. 

losses  in  rocks  due  to,  232. 

mechanical  agents,  216. 

mineral  resistance,  234. 

of  building  stone,  431. 

of  dolomites,  239. 

of  gypsum,  240. 

of  lead  and  zinc  ores,  627. 

of  limestones,  239. 

of  ores,  607. 

of  sedimentary  rocks,  238. 

organisms,  effects  of,  221. 

oxidation,  227. 

relation  to  engineering  work,  234. 

relation  to  structure,  234. 

resistance,  igneous  rocks,  236. 

solution,  230. 
Wehrlite,  84. 
Well,  artesian,  defined,  315. 

flowing,  defined,  315. 

pressure,  oil,  565. 
Wells,  blowing,  328. 

breathing,  328. 

cause  of  irregularities  in,  329. 

drainage  by,  312. 

freezing  of,  328. 

fluctuations  of  head,  327. 

irregularities  in  behavior,  327. 

pollution  of,  314. 

roiliness  of  water,  328. 

variation  in,  302. 
Wet  pan,  521. 

Whitewash,  on  building  stone,  435. 
Willemite,  626. 
Wind  deposits,  109. 
Wurtzilite,  analysis  of,  571. 

Yellow  gravel,  for  roads,  581. 
Youghiogheny  gas  coal,  557. 

Zeolites,  28. 
Zincite,  626. 
Zinc  ores,  classification  of,  627. 

mode  of  occurrence,  628. 
Zinc  production,  629. 
Zone  of  fracture,  192. 


LOCALITY   INDEX 


Absecon  Beach,  N.  J.,  366. 
Adirondack  Mountains,  N.  Y.,  80,  81, 

144,  246,  423,  469,  591,  614,  618. 
Adria,  Italy,  274. 
Adriatic  Sea,  274. 
Akron,  N.  Y.,  504. 
Alabama,  134,  247,  290,  332,  340,  464, 

473,  476,  478,  482,  536,  570,  580,  614, 

626. 

Alabama  River,  Ala.,  247. 
Alabaster,  Mich.,  507. 
Alaska,  185,  203,  401,  417,  592,  630. 
Albany,  N.  Y.,  614. 
Albemarle  County,  Va.,  143,  232. 
Alberta,  258,  350,  416,  577. 
Allegheny  River,  283,  290,  292,  293. 
Allen  glacier,  418. 
Alps,  151. 

Amador  County,  Cal.,  128. 
Amazon  River,  277,  385. 
Amberg,  Wis.,  451,  468. 
Anglesea,  N.  J.,  366. 
Animas  River,  Colo.,  246. 
Anne  Arundel  County,  Md.,  201. 
Appalachians,  115,  129,  134,  137,  149, 

181,  184,  215,  227,  540. 
Appling,  Ga.,  232. 
Aransas  Pass,  Tex.,  385. 
Arbuckle  Mountains,  Okla.,  468. 
Ardmore,  Okla.,  575. 
Arizona,  247,  335,  340,  470,  472,  559, 

567,  615,  624,  626. 
Arkansas,  115,  271,  445,  446,  469,  488, 

490,  535,  544,  559. 
Arkansas  River,  267,  290. 
Aroostock  County,  Me.,  87. 
Arrow  Lake,  B.  C.,  357. 
Arvonia,  Va.,  487,  488. 
Ashland,  Wis.,  441. 
Ashokan  dam,  95,  109. 
Assig,  Bohemia,  232. 
Asulkan  Glacier,  B.  C.,  416,  419. 
Athabasca  Lake,  397. 
Athelstane,  Wis.,  443. 
Athens,  Tex.,  518. 


Atlantic  City,  N.  J.,  370. 
Atlantic  Coast,  113,  393. 
Atlantic  Ocean,  359. 
Augusta,  Ga.,  247. 
Ausable  Chasm,  N.  Y.,  107. 
Ausable  River,  N.  Y.,  373. 
Austin,  Tex.,  477. 
Austinville,  Va.,  597. 

Bahamas,  112. 

Bakersfield,  Cal.,  246. 

Baltimore,  Md.,  62,  446,  469. 

Baltimore  County,  Md.,  82,  85. 

Barre,  Vt.,  466. 

Bass  Island,  Wis.,  451. 

Beaver,  Ark.,  445. 

Beaverdam  Creek,  Va.,  279. 

Beaver  Valley,  Utah,  341. 

Bedford,  Ind.,  445,  477,  478,  498. 

Ben  Hur,  Va.,  153. 

Benson,  Vt.,  488. 

Berea,  O.,  472,  473. 

Berkshire  Hills,  Mass.,  332. 

Berkshire  Valley,  N.  J.,  128. 

Berlin,  Wis.,  443. 

Bermudas,  112. 

Bethel,  Vt.,  466. 

Bighorn  Basin,  Wyo.,  341. 

Bingham,  Utah,  606,  615. 

Bingham  Canyon,  Utah,  600,  619,  624. 

Binghamton,  N.  Y.,  247. 

Birch  Lake,  Minn.,  80. 

Birmingham,  Ala.,  290,  592,  618. 

Bisbee,  Ariz.,  624. 

Bitter  Root  VaUey,  Mont.,  400. 

Black  Hills,  S.  Dak.,  87,  334,  464,  615. 

Blacksburg,  Va.,  535,  544. 

Black  Sea,  287. 

Black  Warrior  River,  Ala.,  247. 

Block  Island,  393. 

Blue  Ridge,  Va.,  279. 

Bohemia,  232. 

Boise,  Ida.,  246. 

Boise  River,  Ida.,  246. 

Bonanza,  Ark.,  544. 


659 


660 


LOCALITY  INDEX 


Bonneville,  Lake,  Utah,  408,  413. 

Boston,  Mass.,  103. 

Botourt  County,  Va.,  153. 

Boulder,  Colo.,  165. 

Bowling  Green,  Ky.,  445,  477. 

Bowling  Green,  Mo.,  441,  442. 

Boyd  County,  Ky.,  575. 

Brahmaputra  delta,  274. 

Brainerd,  Minn.,  290. 

Brandberget,  Norway,  62,  85. 

Branford,  Conn.,  445. 

Brazos  River,  Tex.,  268. 

Breckenridge,  Mo.,  441,  442. 

Breckenridge  County,  Ky.,  575. 

Brewster  County,  Tex.,  615. 

Bridgeport,  Wis.,  477. 

Brigantine  Inlet,  N.  J.,  382. 

British  Columbia,    170,   181,  398,   416, 

419. 

British  Isles,  57. 
Brookwood,  Ala.,  548. 
Brownville,  Me.,  487;  488. 
Buckhorn  district,  Okla.,  575. 
Burlington,  Wis.,  443,  451. 
Burns,  Kan.,  507. 
Bush  Creek,  Colo.,  62. 
Butte,  Mont.,  184,  593,  605,  624. 
Butte  County,  Cal.,  62,  79. 
Buttes,  Ariz.,  250. 

Cabin  Creek,  Ark.,  446. 

Cache  la  Poudre  River,  289. 

Caddo  field,  La.,  569. 

Caen,  France,  477. 

Caen  stone,  474,  476. 

Cahaba  River,  290. 

Cairo,  Miss.,  248,  263. 

California,  115,  145,  183,  246,  255,  289, 
290,  300,  335,  340,  366,  376,  397,  401, 
464,  468,  472,  482,  483,  490,  498,  535, 
544,  560,  562,  566,  567,  569,  570,  573, 
575,  592,  615,  617. 

Camak,  Ga.,  232. 

Canaan,  N.  H.,  466. 

Canaan,  Conn.,  493. 

Canada,  57,  80,  81,  82,  114,  129,  145, 
256,  290,  422,  498. 

Canadian  Pacific  Railway,  181. 

Cannelburg,  Ind.,  532. 

Canyon  City,  Colo.,  290. 

Canton,  Mo.,  477. 

Cape  Cod,  Mass.,  221,  393. 

Cape  Henry,  Va.,  110. 

Cape  Verde  Islands,  89. 


Carroll  County,  Ark.,  445. 

Carson  River,  Nev.,  246. 

Carter  County,  Ky.,  575. 

Carterville,  111.,  544. 

Carthage,  Mo.,  441,  442,  477,  482. 

Cascade  Mountains,  Wash.,  336,  348. 

Castle  Rock,  Colo.,  470. 

Cayuga  Lake,  N.  Y.,  281,  397,  410,  418. 

Catawba  River,  S.  C.,  247. 

Cathedral  Mountain,  B.  C.,  181. 

Cedar  Rapids,  la.,  320. 

Cerillos,  N.  Mex.,  533.  547,  560. 

Champlain  Lake,  N.  Y.,  373,  397,  409. 

Chapman  Quarries,  Pa.,  487. 

Chatham,  Va.,  232. 

Chattanooga,  Tenn.,  247,  248. 

Chesapeake  Bay,  301. 

Chester,  Mass.,  466. 

Chesterfield  County,  Va.,  128. 

China,  109. 

Christiansburg,  Va.,  202. 

Chuckanut,  Wash.,  446. 

Cincinnati,  O.,  283. 

Clagamas  River,  Ore.,  50,  308. 

Clarion  County,  Pa.,  535. 

Clarksburg,  W.  Va.,  535. 

Clealum  Ridge,  Wash.,  349. 

Clearfield  County,  Pa.,  557. 

Clifton,  Ariz.,  606,  624. 

Coastal  Plain,  197. 

Coast  Range,  336. 

Cochituate,  Lake,  Mass.,  405. 

Cockeysville,  Md.,  480. 

Cceur  d'Alene,  Ida.,  598,  605,  615. 

Coffeen,  111.,  535,  544. 

Cold  water,  Mich.,  498. 

Colfax  County,  N.  Mex.,  87. 

Colima  volcano,  Mexico,  58. 

Colorado,  165,  221,  246,  247,  255,  280, 
290,  340,  400,  464,  470,  472,  482, 
533,  535,  544,  547,  567,  572,  601, 
615. 

Columbia,  Mo.,  442. 

Columbia  River,  Ore.,  391. 

Columbia,  S.  C.,  466. 

Columbia,  Tenn.,  247. 

Columbus,  Miss.,  247. 

Columbus,  Mont.,  472. 

Columbus,  O.,  247. 

Colusa,  Cal.,  472. 

Colville,  Wash.,  480. 

Como,  Lake,  Mont.,  400. 

Compton,  N.  J.,  466. 

Concord,  N.  H.,  290,  466. 


LOCALITY  INDEX 


661 


Connecticut,  326,  332,  340,  445,  446,  450, 

466,  469,  472,  493. 
Connelsville,  Pa.,  557. 
Coplay,  Pa.,  497. 
Copper  River,  Alas.,  401. 
Copper  River  Ry.,  Alaska,  417. 
Cordilleran  Region,  129. 
Cordova,  Ala.,  247. 
Corundum  Hill,  N.  C.,  82. 
Coweta,  Ga.,  232. 
Crater  Lake,  Ore.,  397,  401. 
Crawford,  Tex.,  477. 
Crazy  Mountains,  Mont.,  87. 
Crested  Butte,  Colo.,  533,  535,  544,  560, 

574. 

Cripple  Creek,  Colo.,  601,  615,  630. 
Crittenden  County,  Ky.,  62. 
Crockett,  Tex.,  535. 
Cromwell,  Conn.,  446. 
Crotch  Island,  Me.,  466. 
Croton  River,  N.  Y.,  230. 
Crystal  Falls,  Mich.,  80. 
Crystal  Lake,  Colo.,  400. 
Cumberland  Gap,  Ky.,  532. 
Cumberland,  Md.,  290,  493,  497,  504. 
Curless,  Mo.,  442. 
Custer,  Colo.,  593. 

Dalles,  Ore.,  113. 

Danube  River,  230,  264,  266,  267,  270, 

277. 

De  Kalb,  N.  Y.,  445,  480. 
Delaware  and  Hudson  canal,  119. 
Del  Norte,  Colo.,  470. 
Denver,  Colo.,  246,  321. 
Desplaines  River,  291. 
Devils  Lake,  N.  Dak.,  409. 
Dexter,  Kan.,  563. 
Dillon,  Kan.,  507. 
District  of  Columbia,  225,  232. 
Dorset,  Vt.,  480. 
Duck  Creek,  Wis.,  443,  451. 
Duck  River,  Tenn.,  247. 
Duck  town,  Tenn.,  626. 
Duluth,  Minn.,  469. 
Dunnville,  Wis.,  441,  443,  451,  472. 
Dunsbach  Ferry,  N.  Y.,  247. 
Durango,  Colo.,  246. 
Dutch  Gap,  Va.,  259. 

Eagle  Mountain,  Va.,  153. 
East  Longmeadow,  Mass.,  446,  472. 
Edmonson  Countv,  Ky.,  575. 
Egypt,  221. 


Elberton,  Ga.,  232. 

Elizabeth,  Pa.,  290. 

Elizabethtown,  N.  Y.,  80. 

Elk  Mountains,  Colo.,  62. 

Ellsworth,  Pa.,  546. 

El  Paso,  Tex.,  248. 

Ely,  Nev.,  615,  624. 

Empire,  Nev.,  246. 

Encampment  district,  Wyo.,  605. 

Enterprise,  Cal.,  79. 

Erie,  Lake,  397,  399*,  403,  404,  409. 

Esopus  Creek,  N.  Y.,  426. 

Essex  County,  N.  Y.,  80. 

Everglades,  Fla.,  396. 

Fairfax  County,  Va.,  143,  232. 

Fairhaven,  Vt.,  487,  488. 

Faith,  N.  C.,  22. 

Fall  Creek  gorge,  N.  Y.,  281. 

Fall  River,  Mass.,  466. 

Fayette  County,  Pa.,  557. 

Ferris,  Tex.,  515. 

Field,  B.  C..  136,  170,  181,  343. 

Fishkill,  N.  Y.,  270. 

Fitzwilliam,  N.  H.,  466. 

Five-mile  Beach,  N.  J.,  366. 

Flagstaff,  Ariz.,  472. 

Florida,  121,  314,  340,  362,  390,  393,  396, 

397,  399,  474,  535. 
Fort  Dodge,  la.,  507. 
Fort  Scott,  Kan.,  497. 
Fort  Smith,  Ark.,  446, 
Fountain  City,  Wis.,  451. 
Fourche  Mtn.,  Ark.,  77,  232. 
Fox  Islands,  Me.,  466. 
France,  223. 

Frank,  Alberta,  104,  350,  351. 
Franklin,  Cal.,  183. 
Franklin  Furnace,  N.  J.,  627,  628. 
Franklin*  Hill,  Cal.,  89. 
Fredericksburg,  Va.,  466. 
Fredonia,  Kan.,  563. 

Gadsden,  Ala.,  623. 

Gainesville,  Fla.,  314. 

Gallup,  N.  Mex.,  535. 

Geneva,  Lake,  Switzerland,  411. 

Geneva,  Lake,  Wis.,  397. 

George,  Lake,  N.  Y.,  397,  400. 

Georgetown,  Ky.,  314. 

Georgia,   145,  247,  314,  340,  402,  445, 

466,  480,  482,  483,  496,  498,  614. 
Giant's  Causeway,  8. 
Gila  River,  250. 


662 


LOCALITY  INDEX 


Gila  Valley,  Ariz.,  340. 

Glacier  Bay,  Alaska,  415. 

Glendive,  Mont.,  535. 

Glenwood  Springs,  Colo.,  560. 

Golden,  Colo.,  58. 

Goldfield,  Nev.,  602,  615. 

Gold  HiU,  N.  C.,  626. 

Goodenough  Lake,  B.  C.,  409. 

Grand  Rapids,  Mich.,  507. 

Grand  Forks,  N.  Dak.,  246. 

Granite  Heights,  Wife.,  451. 

Graniteville,  Mo.,  468. 

Granville,  N.  Y.,  488. 

Grayson  County,  Ky.,  575. 

Great  Falls,  Mont.,  341. 

Great  Plains,  341. 

Great  Lakes,  283. 

Great  Bear  Lake,  N.  W.  Ty.,  397. 

Great  Lakes,  418. 

Great  Lakes  Region,  129. 

Great  Salt  Lake,  Utah,  397,  408,  409. 

Green  Mountains,  Vt.,  144,  332. 

Green  River,  Utah,  246. 

Green  River,  Wyo.,  246,  250. 

Greenville,  Cal.,  143. 

Greenville,  Ga.,  232. 

Greenville,  Miss.,  271. 

Greenwich,  Conn.,  466. 

Greystone,  N.  C.,  466. 

Gulf  of  Mexico,  394. 

Guthrie  Center,  la.,  515. 

Kale's  Bar,  Tennessee  River,  308. 
Halifax,  Mass.,  535. 
Hallowell,  Me.,  458,  466. 
Hampton,  N.  Y.,  488. 
Hannibal,  Mo.,  441,  442,  447. 
Hardwick,  Vt.,  463,  466. 
Harford  County,  Md.,  239. 
Harrisburg,  Pa.,  250. 
Haverstraw,  N.  Y.,  348. 
Hawaii,  57,  93. 

Hay  den  Mountain,  Colo.,  351. 
Haystack  Mountain,  Me.,  87. 
Hector,  B.  C.,  181. 
Helena,  Ark.,  271. 
Hempstead  Reservoir,  302. 
Henry  Mountains,  Utah,  54. 
Hereford  Inlet,  N.  J.,  366. 
Herkimer  County,  N.  Y.,  232. 
Herndon,  Cal.,  246. 
Hitchcock,  Tex.,  322. 
Hoang-Ho  delta,  274. 
Hocking  district,  O.,  557. 


Hoogly  River,  India,  388. 

Hopatcong,  Lake,  N.  J.,  397. 

Hopyard,  Va.,  201. 

Poughton,  Wis.,  451,  472. 

Hudson,  N.  Y.,  290. 

Hudson  River,  N.  Y.,  230,  247,  276,  290. 

Hudson  River  Valley,  275,  280,  282,  424. 

Humboldt  River,  Nev.,  246. 

Hungary,  264. 

Huntington,  Ark.,  535. 

Huron,  Lake,  397,  403,  409. 

Iceland,  57. 

Idaho,  89,  121,  246,  335,  340,  605,  615. 

Idaho  Springs,  Colo.,  593. 

Illinois,  247,  290,  334,  422,  477,  535,  544, 

557,  562,  567. 

Illinois  River,  111.,  247,  290,  291,  292. 
Imperial  Valley,  Cal.,  255. 
India,  57. 
Indiana,  314,  340,  422,  445,  450,  477, 

478,  498,  505,  532,  569. 
Inverness,  N.  S.,  380. 
lola,  Kan.,  563. 
Iowa,  248,  333,  340,  422,  440,  441,  507, 

508,  515,  559. 

Iron  Mountain,  Mich.,  247. 
Iron  Mountain,  Wyo.,  591,  618. 
Iron  Springs,  Utah,  600,  619. 
Ironton,  Ala.,  617. 
Irrawaddy  River,  266. 
Isle  La  Motte,  Vt.,  445. 
Ithaca,  N.  Y.,  281. 
Ivanhoe,  Va.,  235. 

Jackson,  Miss.,  290. 
James  River,  Va.,  259,  290. 
Janesville,  Wis.,  307. 
Jellico  basin,  Tenn.,  557. 
Johnson  City,  Tenn.,  309. 
Johnstown,  Pa.,  535. 
Joliet,  111.,  477. 
Joplin,  Mo.,  44,  442. 
Jordan  River,  341. 
Juneau,  Alaska,  630. 

Kaibab  fault,  Utah,  181. 
Kampsville,  111.,  290. 
Kanawha,  W.  Va.,  557. 
Kanawha  River,  Va.,  248,  283. 
Kansas,  114,  290,  296,  334,  340,  505,  507, 

508,  509,  544,  559,  569,  570. 
Kansas  City,  Mo.,  441. 


LOCALITY  INDEX 


663 


Kentucky,  314,  320,  340,  445^  477,  532, 

544,  567,  569. 
Kern  River,  Cal.,  246. 
Kerrville,  Tex.,  477. 
Kettle  River,  Minn.,  472,  473. 
Keweenaw  Point,  Mich..  615. 
Kicking  Horse  River,  B.  C.,  181. 
Kings  River,  Cal.,  246. 
Kiskimetas  River,  293. 
Kit-tanning,  Pa.,  290. 
Knob  Lick,  Tex.,  468. 
Kootenay  Lake,  B.  C.,  398,  411. 
Kootenay  River,  B.  C.,  411. 

Lafayette,  Colo.,  535. 

Lahontan,  Lake,  Nev.,  408. 

Lake  City,  Colo.,  348. 

Lake  City,  Fla.,  314. 

Lake  Sanford,  N.  Y.,  618. 

Lake  Superior  region,  134,  137,  139,  208. 

Laramie  River,  Wyo.,  246. 

Lassen  County,  Cal.,  93. 

Lawrenceburg,  Ky.,  320. 

Leadville,  Colo.,  400,  597,  615,  628. 

Lebanon,  N.  H.,  466. 

Lee,  Mass.,  450. 

Leete  Island,  Conn.,  466. 

Lehigh,  N.  Dak.,  535,  544. 

Lehigh  Valley,  Pa.,  504. 

Le  Puy,  France,  88. 

Lester,  Ark.,  535. 

Lexington,  Ga.,  232,  466. 

Lexington,  Va.,  117. 

Lievre  River,  Que.,  345. 

Litchfield  County,  Me.,  62. 

Lithonia,  Ga.,  232. 

Little  Egg  Harbor,  N.  J.,  382. 

Little  Rock,  Ark.,  445,  469. 

Live  Oak,  Fla.,  314. 

Logan  County,  Ky.,  575. 

Logan  River,  Utah,  246. 

Loire  River,  France,  267. 

Long  Island,  N.  Y.,  341,  393,  399. 

Los  Angeles,  Cal.,  183,  336,  505. 

Louise,  Lake,  Alberta,  101,  416. 

Louisiana,  114,  340,  569,  570. 

Louisville,  Ky.,  497,  504. 

Luossavaara,  Swe.,  591. 

Lynchburg,  Va.,  131. 

Mackenzie  delta,  274. 

Maine,  325,  329,  340,  393,  445,  458,  459, 

464,  466,  487,  488,  544. 
Manasquan  Inlet,  N.  J.,  383,  386,  387. 


Manchester,  Va.,  128,  226. 
Mankato,  Minn.,  497. 
Marlboro,  N.  H.,  466. 
Marlow,  Okla.,  507. 
Marquette,  Mich.,  472,  477. 
Martha's  Vineyard,  Mass.,  374. 
Maryland,  115,  201,  239,  258,  290,  301, 

450,  460,  466,  469,  480,  482,  483,  488, 

496,  504,  544,  556,  557. 
Mason  City,  la.,  515. 
Massachusetts,  332,  393,  399,  405,  446, 

450,  466,  472,  473,  483,  535. 
Massillon,  O.,  557. 
Maumee  River,  O.,  247. 
McGregor,  la.,  320. 
Medford,  Mass.,  232. 
Medicine  Hat,  Alberta,  258. 
Medina,  N.  Y.,  472. 
Mediterranean,  359. 
Memphis,  Tenn.,  257. 
Mendota,  Lake,  Wis.,  397. 
Menominee  River,  Mich.,  247. 
Merrimac  River,  Me.,  290. 
Mexico,  59,  93,  265,  276,  410. 
Miami,  Ariz.,  624. 
Miami,  Mo.,  442. 
Miccosukee,  Lake,  Fla.,  399. 
Michigan,  114,  194,  247,  314,  323,  340, 

412,  472,  477,  493,  498,  505,  507,  508, 

567,  615,  626. 

Michigan,  Lake,  397,  403,  409. 
Miles,  Mont.,  544. 
Milford,  Mass.,  450,  466. 
Millbridge,  Me.,  445. 
Millville,  W.  Va.,  290. 
Milwaukee,  Wis.,  165,  475,  497,  502,  504, 

515. 

Mineville,  N.  Y.,  423. 
Minneapolis,  Minn.,  314. 
Minnesota,  82,  193,  248,  290,  314,  329, 

332,  334,  422,  473,  445,  458,  459,  464, 

468,  469,  472,  490. 
Minnetonka,  Lake,  Minn.,  409. 
Mississippi,  230,  247,  248,  271,  290. 
Mississippi  delta,  274. 
Mississippi   River,   248,  257,  259,  261, 

263,  266,  267,  269,  271,  272,  277,  290, 

340,  385,  392. 

Mississippi  Valley,  109,  318,  615. 
Missouri,  248,  438,  440,  441,  442,  444, 

454,  464,  468,  472,  477,  515,  559,  614, 

627,  628. 

Missouri  River,  267,  269,  341. 
Mohawk  River,  N.  Y.,  247. 


664 


LOCALITY  INDEX 


Mokelumne  River,  Cal.,  128. 

Mono,  Lake,  Cal.,  397,  409. 

Monongahela  River,  283,  290,  293. 

Monroe,  N.  Y.,  217. 

Monson,  Me.,  450,  487,  488. 

Montana,  184,  341,  400,  470,  472,  535, 

543,  544,  567,  626. 
Montello,  Wis.,  443,  451,  468. 
Montezuma,  N.  Y.,  493. 
Montgomery,  Ala.,  614. 
Montreal,  Can.,  290. 
Montville,  N.  J.,  143. 
Moosehead  Lake,  Me.,  409. 
Morenci,  Ariz.,  624. 
Moriah,  N.  Y.,  27. 
Morro  Bay,  Cal.,  376. 
Mother  Lode,  Cal.,  615,  630. 
Mt.  Airy,  N.  C.,  466. 
Mount  Ascutney,  Vt,  62. 
Mount  Blanc,  151. 
Mount  Ogden,  B.  C.,  170,  181. 
Mount  Stephen,  B.  C.,  170,  181,  343. 
Mount  Vernon,  Ky.,  445. 
Muskogee,  Okla.,  569. 

Nanaimo,  B.  C.,  217. 

Nantucket,  Mass.,  393. 

Nashville,  Tenn.,  307. 

Navesink  Highlands,  N.  J.,  378. 

Nebraska,  290,  329,  334,  341. 

Neuse  River,  290. 

Nevada,  246,  408,  602,  624. 

Nevada  City,  Cal.,  630. 

Newaygo,  Mich.,  493. 

New  Bedford,  Mass.,  466. 

New  Brunswick,  Can.,  509,  572. 

New  Cambria,  Kan.,  290. 

New  England,  332,  341,  416. 

New  Hampshire,  290,  445,  459,  466. 

New  Jersey,  365,  370,  378,  381,  383,  393, 
466,  469,  483,  493,  581,  618. 

New  Mexico,  246, 290,  341,  505,  533,  535, 
547,  559,  560,  567. 

New  Orleans,  La.,  248,  274,  295. 

Newton,  N.  J.,  448. 

New  York,  82,  114,  183,  240,  247,  270, 
275,  276,  281,  290,  307,  332,  341,  348, 
397,  400,  418,  422,  423,  424,  445,  450, 
466,  471,  472,  478,  480,  483,  488,  493, 
496,  498,  504,  505,  507,  508,  570,  588, 
591,  614,  618. 

New  York  City,  282. 

New  Zealand,  82. 

Niagara  River,  248,  254. 


Niger  delta,  274. 

Nile  River,  230,  266,  267,  277,  385. 

Niota,  111.,  477. 

North  Blanchard,  Me.,  488. 

North  Carolina,  247,  290,  296,  332,  441, 

466,  473,  515. 
North  Dakota,  246,  329,  334,  535,  544, 

551. 

Northfield,  Vt.,  488. 
North  Garden,  Va.,  232. 
North  Jay,  Me.,  463,  466. 
North  Platte,  Neb.,  290. 
North  Platte  River,  290. 
Norway,  82. 
Nova  Scotia,  380,  509. 

Oahu,  93. 

Obsidian  Cliff,  91. 

Ocala,  Fla.,  314. 

Oconomowoc,  Lake,  Wis.,  397. 

Odessa,  Minn.,  128. 

Oglesby,  Ga.,  229,  232,  466. 

Ohio,  247,  296,  339,  341,  422,  450,  471, 

472,  473,  505,  507,  508,  515,  539,  556, 

557,  567,  569,  570,  618. 
Ohio  River,  269. 
Ohio  Valley,  283. 
Okechobee,  Lake,  Fla.,  397. 
Oklahoma,  240,  334,  464,  468,  509,  569, 

572,  575. 

Oldman  River,  Alta.,  350. 
Olive  Bridge  dam,  N.  Y.,  426. 
Olympia,  Wash.,  446. 
Oneida,  Lake,  N.  Y.,  397. 
Onondaga,  N.  Y.,  507. 
Ontario,  440,  441,  444,  591. 
Ontario,  Lake,  397,  399,  403,  413. 
Open  Lake,  Mich.,  82. 
Oregon,  59,  87,  145,  246,  308,  335,  341, 

391,  397,  399,  400,  401,  560. 
Orizaba,  Mex.,  276. 
Orlando,  Fla.,  314,  535. 
Orleans,  Nev.,  246. 
Ortonville,  Minn.,  468. 
Oswego,  N.  Y.,  247,  307. 
Oswego  River,  N.  Y.,  247. 
Ouray,  Colo.,  351. 

Owens  Lake,  Cal.,  183,  336,  397,  409. 
Owen  Sound,  Can.,  498. 
Owens  Valley,  Cal.,  335,  336,  340. 
Oxford  Furnace,  N.  J.,  493. 

Pacific  Coast,  113,  394. 

Palisades  of  Hudson  River,  52,  81. 

Panama  Canal,  185,  348. 


LOCALITY  INDEX 


665 


Paola,  Kan.,  569. 

Patuxent  River,  Md.,  258. 

Pawlet,  Vt.,  488. 

Pearl  River,  290. 

Pecos  River,  290. 

Pennsylvania,  118,  157,  205,  248,  290, 
293,  307,  329,  332,  472,  473,  481,  482, 
483,  487,  488,  504,  505,  535,  536,  538, 
543,  544,  556,  557,  569,  614,  618. 

Penokee,  Mich.,  232. 

Penrhyn,  Pa.,  205,  481. 

Peoria,  III.,  247. 

Ferris,  Calif.,  461. 

Petersburg,  Va.,  466. 

Philadelphia,  Pa.,  128. 

Piedmont  region,  141,  154,  168. 

Pike's  Peak,  Colo.,  89. 

Pinto  Mountain,  Tex.,  89. 

Pittsburgh,  Pa.,  248,  283,  563,  558. 

Pittsfield,  III,  563. 

Platte  River,  289. 

Pleasant  Valley,  New  Mex.,  87. 

Plumas  County,  Cal.,  89. 

Pocahontas  district,  Va.,  557. 

Po  delta,  274. 

Point  of  Rocks,  Va.,  247. 

Polk  County,  Ark.,  488. 

Po  River,  266. 

Portage  County,  Wis.,  412. 

Port  Angeles,  Wash.,  307. 

Port  Deposit,  Md.,  460,  466. 

Portland,  Conn.,  450,  472. 

Port  Monmouth,  N.  J.,  378. 

Port  Wing,  Wis.,  443,  472. 

Potomac  River,  Va.,  247,  266,  279,  290. 

Potsdam,  N.  Y.,  472. 

Poultney,  Vt.,  488. 

Presque  Isle,  Wis.,  443,  451,  472. 

Proctor,  Vt.,  481. 

Pyramid  Lake,  Nev.,  409. 

Pyrenees,  202,  220. 

Quebec,  Can.,  144. 
Quincy,  Mass.,  450,  466. 
Quinnesec  Falls,  Wis.,  128. 
Quitman,  Ga.,  314. 

Rainy  Lake  Region,  Ont.,  80. 
Raleigh,  N.  C.,  290. 
Randolph,  Va.,  247. 
Rappahannock  River,  Va.,  201. 
Ray,  Ariz.,  624. 
Raymond,  Cal.,  468. 


Red  Bluff,  Cal.,  246. 

Red  River,  N.  Dak.,  246. 

Redstone,  N.  H.,  466. 

Rhode  Island,  466. 

Rhone  River,  230,  266,  267,  411. 

Richmond,  Va.,  62,  259,  290,  327,  461, 

466,  547. 
Rico,  Colo.,  596. 

Rio  Grande  River,  246,  248,  266. 
Rio  Grande  valley,  N.  M.,  341. 
Rion,  S.  C.,  466. 
Riverside  County,  Cal.,  468. 
Roanoke  River,  Va.,  247. 
Roanoke,  Va.,  306. 
Rock  Hill,  S.  C.,  247. 
Rocklin,  Calif.,  468. 
Rockmart,  Ga.,  498. 
Rockport,  Mass.,  450,  466. 
Rockville,  Minn.,  445. 
Rocky   Mountains,   56,   318,   334,   335, 

336,  414. 
Rolla,  Mo.,  442. 
Rondout,  N.  Y.,  493. 
Rondout  Valley,  N.  Y.,  425. 
Ronkonkoma,  Lake,  N.  Y.,  397. 
Rosendale,  N.  Y.,  497,  504. 
Roswell,  N.  M.,  341. 
Rowe,  Mass.,  143. 
Rusk,  Tex.,  515. 
Russellville,  Ark.,  535. 
Rutland,  Vt.,  445,  480. 

Sacramento,  Cal.,  290. 

Sacramento  River,  Cal.,  246,  290. 

St.  Augustine,  Fla..  364. 

St.  Cloud,  Minn.,  458,  468.  , 

St.  Croix  Falls,  Wis.,  247. 

St.  Croix  River,  Wis.,  247. 

St.  John's  River,  Fla.,  390. 

St.  Joseph,  Ark.,  445. 

St.  Lawrence  County,  N.  Y.,  466. 

St.  Lawrence  River,  277,  248,  290. 

St.  Louis,  Mo.,  248,  442,  477,  515. 

St.  Paul,  Minn.,  248. 

Salem  Neck,  Mass.,  77. 

Salina,  Kan.,  -507. 

Saline  River,  Kan.,  290. 

Salineville,  O.,  515. 

Salisbury,  N.  C.,  250,  466. 

Salton  Sink,  Cal.,  255. 

Saltville,  Va.,  507. 

Sandusky,  O.,  507. 

Sandy  Hook,  N.  J.,  378,  381. 

San  Francisco,  Cal.,  183,  184,  446. 


666 


LOCALITY  INDEX 


Sanger,  Cal.,  246. 

San  Joaquin  River,  246. 

Sanpete  Valley,  Utah,  341. 

Santa  Cruz,  Cal.,  575. 

Saskatchewan  River,  Alberta,  256,  258. 

Savannah  River,  Ga.,  247. 

Scioto  River,  O.,  247. 

Scranton,  Pa.,  535,  544. 

Searles,  Ala.,  545. 

Seine  River,  277. 

Selkirk  Mountains,  414. 

Selma,  Ala.,  247. 

Seneca  Lake,  N.  Y.,  397,  410. 

Servia,  270. 

Sevier  Valley,  Utah,  341. 

Seward,  Alaska,  185. 

Seyssel,  France,  575. 

Shasta  County,  Cal.,  87,  626. 

Shelby  County,  Ala.,  619. 

Shenandoah  River,  Va.,  290. 

Shrewsbury  River,  N.  J.,  378. 

Sierra  Nevadas,  56,  139. 

Siluria,  Ala.,  476. 

Sioux  City,  la.,  248. 

Sioux  Falls,  Minn.,  445. 

Sitka,  Alaska,  185. 

Slate  River,  Va.,  170. 

Slumgullion  slide,  Colo.,  348. 

Smith's  Basin,  N.  Y.,  493. 

Smith's  Landing,  N.  Y.,  498. 

Snag  Lake,  Cal.,  401. 

Soldier  Creek,  Ky.,  575. 

South  Britain,  Conn.,  89. 

South  Carolina,  121,  247,  466. 

South  Dakota,  334,  341,  493,  615. 

South  Dover,  N.  Y.,  480. 

South  Husent  Creek,  Cal.,  79. 

South  Platte  River,  Colo.,  246. 

Sparta,  Ga.,  466. 

Stassfurt,  Ger.,  114. 

Staunton,  Va.,  309. 

Steamboat  Springs,  Nev.,  115,  117. 

Stone  Mountain,  Ga.,  55,  466. 

Stone  Mountain,  N.  C.,  226. 

Stony  Creek,  Conn.,  466. 

Straight  Creek,  Ky.,  544. 

Sturgeon  Bay,  Wis.,  443,  477. 

Sudbury,  Can.,  42,  591. 

Suisun,  Cal.,  498. 

Superior,  Lake,  397,  403,  409. 

Susquehanna  River,  N.  Y.,  247,  250,  284. 

Sweden,  82,  591. 

Sydney,  N.  S.,  107. 

Syracuse,  Ind.,  498. 


Tahoe,  Lake,  Cal.,  397,  409. 

Tallahassee,  Fla.,  295. 

Tarboro,  N.  C.,  247. 

Tar  River,  N.  C.,  247. 

Tate,  Ga.,  445,  480. 

Teil,  France,  496. 

Temagami,  Ont.,  131. 

Temiskaming,  Lake,  Ont.,  399. 

Tenino,  Wash.,  446,  472. 

Tennessee,  102,  121,  247,  248^  314,  341, 

480,  482,  567,  569,  614,  628,  629. 
Tennessee  River,  Tenn.,  247,  248. 
Tesla,  Cal.,  535,  544. 
Tetschen,  Bohemia,  89. 
Texas,  268,  334,  335,  341,  385,  464,  468, 

477,  509,  515,  535,  562,  566,  569,  573, 

615. 

Texcoco  Lake,  Mex.,  410. 
Thames  River,  230,  277. 
Thirlmere  aqueduct,  119. 
Thun,  Lake,  Switz.,  407. 
Thurston,  Md.,  488. 
Tivoli,  Italy,  475. 
Toluca,  Mex.,  401,  402. 
Tomasopo  Canyon,  Mex.,  265. 
Tombigbee  River,  Miss.,  247. 
Tongore  dam,  New  York,  426. 
Tonopah,  Nev.,  184,  602,  615,  630. 
Torbrook,  N.  S.,  592. 
Trinidad,  Colo.,  472,  560. 
Tristan  d'Acunha,  S.  Atlantic,  93. 
Troy,  N.  H.,  445. 
Truckee  River,  Nev.,  246. 
Tuckahoe,  N.  Y.,  480. 
Tulameen  River,  B.  C.,  82. 
Tulare  Lake,  409. 

Turtle  Mountain,  Alta.,  346,  350,  351. 
Twin  Lakes,  Colo.,  400. 

Umatilla,  Ore.,  246. 

Umatilla  River,  Ore.,  246. 

Umpqua  River,  Ore.,  400. 

Unga  Island,  Alaska,  89. 

United  States,    groundwater  provinces, 

330. 

Uruguay  River,  266. 
Utah,  121,  181.  246,  335,  341,  397,  567, 

600,  615,  624. 
Utah  Lake,  341. 
Utica,  111.,  497,  504. 
Uva,  Wyo.,  246. 
Uvalde  County,  Tex.,  89. 

Valdez,  Alaska,  185,  417. 


LOCALITY  INDEX 


667 


Vancouver  Island,  B.  C.,  480.  - 

Vorwohle,  Ger.,  575. 

Veedersburg,  Ind.,  520. 

Vermont,  297,  445,  450,  466,  480,  482, 
483,  486,  487,  488,  626. 

Vicksburg,  Miss.,  248. 

Victoria  Glacier,  B.  C.,  416. 

Vineyard  Haven,  Mass.,  393. 

Virgilina,  Va.,  599,  626. 

Virginia,  102,  114,  115,  145,  153,  157, 
170,  193,  201,  220,  232,  247,  259, 
279,  290,  297,  306,  314,  341,  466, 
487,  488,  505,  507,  508,  535,  544, 
547,  606,  614,  618,  628,  629. 

Virginia  City,  Nev.,  615. 

Vista,  Nev.,  246. 

Volbonne,  Pyrenees,  85. 

Wabana  Island,  N.  F.,  592. 

Walker  Lake,  Nev.,  409. 

Waltonville;  Pa.,  472. 

Wapsipinicon  River,  323. 

Warner  Lakes,  Ore.,  399. 

Warren  County,  Ky.,  575. 

Warrensburg,  Mo.,  441,  442,  472. 

Warsaw,  N.  Y.,  472. 

Washington,  89,  145,  307,  335,  341,  446, 

472,  480,  560. 

Washington  County,  Pa.,  557. 
Waterville,  O.,  247. 
Wausau,  Wis.,  468. 
Wauwatosa,  Wis.,  441,  443. 
Webster,  N.  C.,  143,  515. 
Westerly,  R.  I.,  466. 
Westernport,  Md.,  544. 
West  Mineral,  Kan.,  544. 
West  Monson,  Me.,  488. 
Westmoreland  County,  Pa.  557. 


West  Pawlet,  Vt.,  487. 

West  Point,  N.  Y.,  276. 

West  Virginia,  149,  290,  293,  535,  551, 

556,  557,  570,  572. 
Wetheredville,  Md.,  80. 
Weyers  Cave,  Va.,  120. 
White  Mountains,  N.  H.,  296. 
Wichita  Mountains,  Tex.,  468. 
Wilkesbarre,  Pa.,  538. 
Wilkes  County,  N.  C.,  226. 
Williston,  N.  Dak.,  531,  544. 
Wilmington,  N.  C.,  322. 
Winchester,  Cal.,  222. 
Windsor,  Vt.,  466. 
Winnipeg,  Lake,  Can.,  397. 
Winnipegosis,  Lake,  Can.,  397. 
Wisconsin,  165,  247,  307,  314,  332,  333, 

341,  397,  403,  412,  438,  440,  441,  443, 

444,  451,  454,  464,  468,  472,  475,  477, 

504,  515,  618,  629. 
Wise  County,  Va.,  535. 
Woodbury,  Vt.,  466. 
Woodstock,  Md.,  205,  229,  466. 
Wyoming,  82,  121,  246,  283,  341,  509, 

567,  591,  619. 

Yadkin  River,  N.  C.,  250. 
Yakima  County,  Wash.,  341. 
Yankton,  S.  Dak.,  493. 
Yare  River,  England,  385. 
Yellowstone  Lake,  Mont.,  409. 
Yellowstone  Park,  79,  87,  91,  93,  115, 

304. 

York,  Me.,  544. 
Yosemite  Valley,  Cal.,  55. 
Yukon  delta,  274. 
Yuma,  Ariz.,  247. 

Zuni  River  dam,  308. 


AUTHOR'S   INDEX 


Abbott,  M.  L.,  274. 
Adams,  F.  D.,  192. 
Adams,  L.  H.,  509,  560. 
Allan,  J.  A.,  183. 
Allanson-Winn,  R.  G.,  394. 
ArgaU,  G.,  597. 
Ashley,  G.  H.,  589. 
Atwood,  W.  W.,  404. 
Aubury,  L.,  490. 

Babb,  C.  C.,  266. 

Baker,  E.  R.,  492. 

Bartlett,  W.  H.  C.,  218. 

Bassler,  R.,  117. 

Bastin,  E.  S.,  125,  561,  589. 

Bayley,  W.  S.,  325. 

Beck,  R.,  609. 

Berkey,  C.  P.,  109,  119,  183,  308,  423, 

426,  427. 

Beyer,  S.  W.,  491. 
Black,  W.  M.,  394. 
Blanchard,  A.  H.,  589. 
Blatchley,  W.  S.,  520,  589. 
Bleininger,  A.  V.,  509. 
Bourry,  E.,  526. 
Branner,  J.  C.,  225,  475. 
Bristol,  W.  A.,  492. 
Brock,  R.  W.,  357. 
Brown,  W.  M.,  490. 
Buckley,  E.  R.,  219,  243,  316,  403,  438, 

440,  441,  442,  444,  451,  454,  491,  492, 

589. 

Buckman,  H.  O.,  234,  243. 
Buehler,  H.  A.,  491. 
Burchard,  E.  F.,  491,  492. 

Calkins,  F.  C.,  341. 

Campbell,  M.  R.,  541,  542,  544,  530,  560. 

Capps,  S.  H.,  340. 

Chamberlin,  T.  C.,  Ill,  213,  248,  257, 

274,  294,  317,  320,  394,  411,  422,  424, 

426,  632. 

Chittenden,  H.  S.,  245,  283,  294. 
Choisy,  M.,  221. 
Clapp,  F.  G.,  326,  340,  341,  575. 


Clarke,  F.  W.,  cited,  1,  63,  64,  97,  108, 
141,  146,  218,  286,  290,  291,  294,  409, 
560,  575,  589,  592,  593. 

Clement,  J.  K.,  560. 

Collier,  A.  J.,  560. 

Cushman,  A.  B.,  83. 

Dachnowski,  A.,  561. 

Dale,  T.  N.,  138,  208,  459,  484,  488,  490, 

491,  492. 
Daly,  R.  A.,  356. 
Dana,  E.  S.,  cited,  44. 
Dana,  J.  D.,  213,  274,  356. 
Darton,  N.  H.,  340,  341,  491. 
Davis,  C.  A.,  412,  413,  527,  561. 
Davis,  W.  M.,  213,  274,  396. 
Dawson,  G.  M.,  400. 
Day,  D.  T.,  568. 
Delesse,  A.,  316,  317. 
Derby,  O.  A.,  225. 
Dickinson,  H.  T.,  106,  491. 
Diller,  J.  S.,  232. 
Dole,  R.  B.,  340. 
Dowling,  D.  B.,  560. 
Drinker,  H.  S.,  343,  356. 
Drowne,  H.  B.,  589. 
Du  Buat,  267. 

Ebelman,  M.,  232. 
Eckel,  E.  C.,  490,  498,  501,  509. 
Egleston,  T.,  221. 
Eldridge,  G.  H.,  574,  575. 
Ellis,  E.  E.,  318,  319,  326,  340. 
Ells,  R.  W.,  345,  576. 
Emmons,  S.  F.,  631. 
Emmons,  W.  H.,  631. 
Eno,  F.  H.,  509. 

Fairchild,  H.  L.,  413. 

Farrell,  J.  H.,  213,  631. 

Fenneman,  N.  M.,  360,  394,  397,  403, 

413. 

Fernald,  R.  H.,  560. 
Finlay,  G.  L,  146. 
Fisher,  C.  A.,  341. 


669 


670 


AUTHOR'S  INDEX 


Fitzgerald,  D.,  405. 

Foerster,  M.,  438. 

Ford,  W.  E.,  cited,  44. 

Forster,  C.  L.,  490. 

Frazer,  J.  C.  W.,  560. 

Frazer,  P.,  540,  560,  532. 

Free,  E.  E.,  113. 

Fuller,  M.  L.,  297,  298,  302,  304,  305, 
306,  312,  313,  314,  315,  316,  318,  319, 
322,  325,  327,  328,  329,  330,  340,  341. 

Gaillard,  D.  D.,  362. 

Gardiner,  J.  H.,  491. 

Gary,  M.,  452,  453. 

Geikie,  A.,  213,  275,  316,  356,  394,  417. 

Geikie,  J.,  213. 

Gilbert,  G.  K.,  54,  184,  221,  366,  368, 

370,  380,  394,  408. 
Glenn,  L.  C.,  294,  340,  341. 
Goldthwaite,  J.  W.,  370,  372,  404. 
Gordon,  C.  H.,  491. 
Gosling,  E.  B.,  576. 
Gould,  C.  N.,  341,  491. 
Grasty,  J.  S.,  224. 
Greeley,  W.  M.,  294. 
Gregory,  H.  E.,  340. 
Grimsley,  G.  P.,  492. 
Griswold,  W.  T.,  566. 
Grout,  F.  F.,  560. 
Gulliver,  F.,  394. 
Gunther,  C.  G.,  631. 

Haanel,  B.  F.,  561. 

Hamlin,  H.,  340. 

Harcourt,  364. 

Harder,  E.  C.,  606,  621. 

Harker,  A.,  cited,  65,  146. 

Harris,  G.  D.,  340. 

Haskins,  C.  N.,  560. 

Hatch,  F.  H.,  146. 

Haupt,  L.  M.,  367,  383,  386,  387,  394. 

Haworth,  E.,  340. 

Hayes,  C.  W.,  158,  159,  163,  185,  213, 

565. 

Headden,  W.  P.,  289. 
Heim,  A.,  152,  154,  182,  342,  349,  351, 

357. 

Heinz,  H.  J.,  294. 
Hertle,  J.  C.,  576. 
Hilgard,  E.  W.,  243. 
Hirschwald,  J.,  437,  438,  443,  454,  490. 
Hitchcock,  A.  S.,  113. 
Hobbs,  W.  H.,  184. 
Hofer,  H.,  575. 


Hoffman,  E.  J.,  560. 

Holmes,  J.  A.,  560. 

Hook,  J.  S.,  101. 

Hopkins,  T.  C.,  490,  491. 

Hoskins,  L.  M.,  213. 

Hovey,  E.  O.,  401. 

Howe,  E.,  357. 

Hoyt,  J.  C.,  297. 

Hubbard,  P.,  550,  560. 

Humphreys,  R.  L.,  274,  490,  509. 

Iddings,  J.  P.,  44,  146. 
Irving,  J.  D.,  631. 
Irving,  R.  D.,  232. 

Jackson,  D.  D.,  291,  294. 
Johnson,  A.  N.,  589. 
Johnson,  D.  W.,  340,  341. 
Johnston,  J.  K.,  558. 
Julien,  A.  A.,  432. 

Keilhack,  K.,  340. 

Keith,  A.,  491. 

Kellogg,  R.  S.,  113. 

Kemp,  J.  F.,  60,  91,  146,  456,  631. 

Keyes,  C.  R.,  539. 

King,  F.  H.,  316. 

Kirchoffer,  W.  G.,  341. 

Knight,  W.  C.,  492. 

Kraus,  E.  H.,  cited,  44. 

Kreisinger,  H.,  561. 

Kummel,  H.  B.,  371. 

Lakes,  A.,  490. 

Landes,  H.,  341,  589. 

Lane,  A.  C.,  323,  404,  340,  559. 

Laney,  F.  B.,  441,  491,  599. 

Lawson,  A.  C.,  183,  213. 

Lee,  W.  T.,  336,  340,  341. 

Leighton,  H.,  294,  589. 

Leith,  C.  K.,  151, 182,  191,  193,  200,  213. 

Leverett,  F.,  340. 

Lewis,  J.  V.,  491. 

Lindgren,  W.,  184,  631. 

Lord,  E.  C.,  589. 

Luquer,  L.  Mel.,  455. 

Lyell,  C.,  357. 

Marston,  A.,  440,  491. 
Martin,  L.,  185,  401,  417,  418. 
Matson,  G.  C.,  340. 
Matthews,  E.  B.,  491. 
McCallie,  S.  W.,  340,  490. 
McConnell,  R.  G.,  357. 


AUTHOR'S  INDEX 


671 


McCourt,  W.  E.,  447,  448,  490,  561. 

McDonald,  D.  F.,  357. 

McGee,  W.  J.,  294. 

MerriU,  F.  J.  H.,  589. 

MerriU,  G.  P.,  45,  103,  143,  146,  225, 

232,  239,  242,  243,  316,  441,  490,  491, 

526. 

Miller,  W.  G.,  356. 
Milne,  J.,  184. 
Moldenke,  R.,  560. 
Moses,  A.  J.,  44,  213. 
Munn,  M.  J.,  537,  566. 
Murphy,  E.  C.,  250,  252,  294. 

Nettleton,  E.  S.,  300. 

Newell,  F.  H.,  245,  250,  252,  294. 

Nichols,  W.  R.,  413. 

Norton,  W.  H.,  340. 

Nystrom,  E.,  561. 

Orton,  Jr.,  E.,  491. 
Ovitz,  F.  K.,  561. 

Paige,  S.,  610. 

Palmer,  A.  W.,  291,  294. 

Parker,  E.  W.,  560. 

Parks,  A.  W.,  440,  444. 

Parmelee,  C.  W.,  561. 

Parr,  S.  W.,  540,  560. 

Parsons,  C.  L.,  44. 

Patton,  H.  B.,  357. 

Peck,  F.  B.,  118. 

Peckham,  S.  F.,  571,  576. 

Peppel,  S.,  491. 

Perkins,  G.  H.,  404,  492. 

Phillips,  A.  H.,  44. 

Pirsson,  L.  V.,  cited,  5,  44,  70,  72,  84, 

98,  127,  146,  213,  399. 
Pishel,  M.,  560. 
Pope,  G.  S.,  560. 
Porter,  H.  C.,  561. 
Posepny,  F.,  631. 
Purdue,  A.  H.,  308,  490. 
Pratt,  J.  H.,  589. 
Prouty,  W.  F.,  589. 

Rafter,  G.  W.,  294. 

Randall,  D.  T.,  561. 

Ransome,  F.  L.,  213,  613,  628. 

Reade,  T.  M.,  230. 

Redwood,  B.,  575. 

Reid,  H.  F.,  213,  589. 

Renwick,  W.  G.,  490. 

Rice,  G.  S.,  356,  357. 

Richardson,  C.,  341,  576,  571,  572,  575. 


Rickard,  T.  A.,  596. 

Ries,  H.,  45;  76,  104,  107,  110,  117,  121, 

130,  136,  165,  170,  490,  492,  526,  560, 

561,  575,  631. 
Rogers,  A.  F.,  45. 
Rogers,  H.  D.,  532. 
Rosenbusch,  H.,  65. 
Rowe,  J.  P.,  491. 
Russell,  I.  C.,  230,  252,  294,  340,  341, 

348,  357,  404,  408,  413. 

Salisbury,  R.  D.,  Ill,  213,  248,  257,  274, 
294,  317,  394,  411,  422,  424,  426,  428, 
632. 

Sanborn,  M.  F.,  394. 

Sanford,  S.,  302,  304,  321,  327,  340,  341. 

Schmid,  H.  de',  49. 

Schuyler,  J.  D.,  400. 

Scott,  W.  B.,  52,  213,  632. 

Seipp,  H.,  490. 

Sellards,  E.  H.,  340,  413. 

Shaw,  E.  W.,  537. 

Shedd,  S.,  492. 

Shepard,  E.  M.,  341. 

Siebenthal,  C.  E.,  491. 

Slichter,  C.  S.,  298,  300,^317,  333,  340. 

Slifer,  H.  J.,  411. 

Sloan,  E.,  491,  589. 

Smith,  C.  D.,  560. 

Smith,  E.  A.,  340,  490. 

Smith,  G.  O.,  341. 

Smith,  Jr.,  H.,  413. 

Smock,  J.  C.,  491. 

Smyth,  Jr.,  C.  H.,  232,  413,  596. 

Snider,  L.  C.,  589. 

Spurr,  J.  E.,  184. 

Stabler,  H.,  294. 

Stevenson,  T.,  362,  364,  395. 

Stevenson,  J.  J.,  544,  560. 

Stuntz,  S.  C.,  113. 

Tarr,  R.  S.,  185,  413,  418. 

Taylor,  C.  H.,  341,  491. 

Thomas,  B.  F.,  256,  257,  260,  261,  262, 
263,  264,  272,  273,  277,  283,  286,  294, 
297,  301,  385,  388,  390,  391,  394. 

Thompson,  491. 

Todd,  J.  E.,  341,  491. 

Tolman,  C.  F.,  185,  213,  608,  631. 

Turner,  H.  W.,  357. 

Van  Hise,  C.  R.,  124,  127,  132,  152,  155, 
180,  192,  196,  213,  225,  232,  243,  317, 
607,  631. 


672 


AUTHOR'S  INDEX 


Van  Ingen,  G.,  446. 
Veatch,  A.  C.,  301,  303,  341. 
Vedel,  395. 
Vernon-Harcourt,  L.  F.,  388. 

Walther,  J.,  221. 

Watson,  T.  L.,  74,  110,  131,  153,  170, 

232,  243,  341,  441,  490,  491,  492. 
Watt,  D.  A.,  256,  257,  260-264,  272,  273, 

277,  283,  286,  294,  297,  301,  385,  388, 

390,  391,  394. 


Weed,  W.  H.,  605,  609. 

Weidman,  S.,  412. 

Wheeler,  H.  A.,  395,  526. 

White,  D.,  560,  547. 

Wilder,  F.,  531. 

Williams,  I.  A.,  491. 

Williams,  J.  F.,  490. 

Willis,  B.,  151,  180,  213,  279. 

Wilson,  A.  W.  G.,  413. 

Wright,  C.  L.,  490,  552,  553,  561. 


U.  C.  BERKELEY  LIBRARIES 


CDSlbDSDIE 


294723 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


